From 5deb1cb0e473aa7018fb72b633081acd6bc34b83 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 19 Feb 2026 01:45:32 +0100
Subject: [PATCH 01/22] H-5839: Extract Monaco setup into dedicated module with
 async loading

Replace the monolithic useMonacoGlobalTypings hook with a monaco/ module
that loads Monaco and @monaco-editor/react asynchronously, disables all
built-in TS workers, and provides a custom completion provider. CodeEditor
uses React 19 Suspense with use() to show a loading placeholder until
Monaco is ready.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../src/hooks/use-monaco-global-typings.ts    | 380 ------------------
 .../{components => monaco}/code-editor.tsx    |  67 +--
 .../petrinaut/src/monaco/context.ts           |  12 +
 .../petrinaut/src/monaco/provider.tsx         | 108 +++++
 libs/@hashintel/petrinaut/src/petrinaut.tsx   |  36 +-
 .../differential-equation-properties.tsx      |   4 +-
 .../PropertiesPanel/place-properties.tsx      |  46 +--
 .../PropertiesPanel/transition-properties.tsx |  11 +-
 8 files changed, 194 insertions(+), 470 deletions(-)
 delete mode 100644 libs/@hashintel/petrinaut/src/hooks/use-monaco-global-typings.ts
 rename libs/@hashintel/petrinaut/src/{components => monaco}/code-editor.tsx (63%)
 create mode 100644 libs/@hashintel/petrinaut/src/monaco/context.ts
 create mode 100644 libs/@hashintel/petrinaut/src/monaco/provider.tsx

diff --git a/libs/@hashintel/petrinaut/src/hooks/use-monaco-global-typings.ts b/libs/@hashintel/petrinaut/src/hooks/use-monaco-global-typings.ts
deleted file mode 100644
index 9dbfe496bde..00000000000
--- a/libs/@hashintel/petrinaut/src/hooks/use-monaco-global-typings.ts
+++ /dev/null
@@ -1,380 +0,0 @@
-import { loader } from "@monaco-editor/react";
-import type * as Monaco from "monaco-editor";
-import { use, useEffect, useState } from "react";
-
-import type {
-  Color,
-  DifferentialEquation,
-  Parameter,
-  Place,
-  Transition,
-} from "../core/types/sdcpn";
-import { EditorContext } from "../state/editor-context";
-import { SDCPNContext } from "../state/sdcpn-context";
-
-interface ReactTypeDefinitions {
-  react: string;
-  reactJsxRuntime: string;
-  reactDom: string;
-}
-
-/**
- * Fetch React type definitions from unpkg CDN
- */
-async function fetchReactTypes(): Promise<ReactTypeDefinitions> {
-  const [react, reactJsxRuntime, reactDom] = await Promise.all([
-    fetch("https://unpkg.com/@types/react@18/index.d.ts").then((response) =>
-      response.text(),
-    ),
-    fetch("https://unpkg.com/@types/react@18/jsx-runtime.d.ts").then(
-      (response) => response.text(),
-    ),
-    fetch("https://unpkg.com/@types/react-dom@18/index.d.ts").then((response) =>
-      response.text(),
-    ),
-  ]);
-
-  return { react, reactJsxRuntime, reactDom };
-}
-
-/**
- * Convert a transition to a TypeScript definition string
- */
-function transitionToTsDefinitionString(
-  transition: Transition,
-  placeIdToNameMap: Map<string, string>,
-  placeIdToTypeMap: Map<string, Color | undefined>,
-): string {
-  const input =
-    transition.inputArcs.length === 0
-      ? "never"
-      : `
-    {${transition.inputArcs
-      // Only include arcs whose places have defined types
-      .filter((arc) => placeIdToTypeMap.get(arc.placeId))
-      .map((arc) => {
-        const placeTokenType = `SDCPNPlaces['${arc.placeId}']['type']['object']`;
-        return `"${placeIdToNameMap.get(arc.placeId)!}": [${Array.from({ length: arc.weight }).fill(placeTokenType).join(", ")}]`;
-      })
-      .join(", ")}
-    }`;
-
-  const output =
-    transition.outputArcs.length === 0
-      ? "never"
-      : `{
-    ${transition.outputArcs
-      // Only include arcs whose places have defined types
-      .filter((arc) => placeIdToTypeMap.get(arc.placeId))
-      .map((arc) => {
-        const placeTokenType = `SDCPNPlaces['${arc.placeId}']['type']['object']`;
-        return `"${placeIdToNameMap.get(arc.placeId)!}": [${Array.from({ length: arc.weight }).fill(placeTokenType).join(", ")}]`;
-      })
-      .join(", ")}
-    }`;
-
-  return `{
-    name: "${transition.name}";
-    lambdaType: "${transition.lambdaType}";
-    lambdaInputFn: (input: ${input}, parameters: SDCPNParametersValues) => ${transition.lambdaType === "predicate" ? "boolean" : "number"};
-    transitionKernelFn: (input: ${input}, parameters: SDCPNParametersValues) => ${output};
-  }`;
-}
-
-/**
- * Generate TypeScript type definitions for SDCPN types
- */
-function generateTypesDefinition(types: Color[]): string {
-  return `declare interface SDCPNTypes {
-    ${types
-      .map(
-        (type) => `"${type.id}": {
-        tuple: [${type.elements.map((el) => `${el.name}: ${el.type === "boolean" ? "boolean" : "number"}`).join(", ")}];
-        object: {
-          ${type.elements
-            .map(
-              (el) =>
-                `${el.name}: ${el.type === "boolean" ? "boolean" : "number"};`,
-            )
-            .join("\n")}
-        };
-        dynamicsFn: (input: SDCPNTypes["${type.id}"]["object"][], parameters: SDCPNParametersValues) => SDCPNTypes["${type.id}"]["object"][];
-      }`,
-      )
-      .join("\n")}
-  }`;
-}
-
-/**
- * Generate TypeScript type definitions for SDCPN places
- */
-function generatePlacesDefinition(places: Place[]): string {
-  return `declare interface SDCPNPlaces {
-    ${places
-      .map(
-        (place) => `"${place.id}": {
-        name: ${JSON.stringify(place.name)};
-        type: ${place.colorId ? `SDCPNTypes["${place.colorId}"]` : "null"};
-        dynamicsEnabled: ${place.dynamicsEnabled ? "true" : "false"};
-      };`,
-      )
-      .join("\n")}}`;
-}
-
-/**
- * Generate TypeScript type definitions for SDCPN transitions
- */
-function generateTransitionsDefinition(
-  transitions: Transition[],
-  placeIdToNameMap: Map<string, string>,
-  placeIdToTypeMap: Map<string, Color | undefined>,
-): string {
-  return `declare interface SDCPNTransitions {
-      ${transitions
-        .map(
-          (transition) =>
-            `"${transition.id}": ${transitionToTsDefinitionString(transition, placeIdToNameMap, placeIdToTypeMap)};`,
-        )
-        .join("\n")}
-    }`;
-}
-
-/**
- * Generate TypeScript type definitions for SDCPN differential equations
- */
-function generateDifferentialEquationsDefinition(
-  differentialEquations: DifferentialEquation[],
-): string {
-  return `declare interface SDCPNDifferentialEquations {
-    ${differentialEquations
-      .map(
-        (diffEq) => `"${diffEq.id}": {
-        name: ${JSON.stringify(diffEq.name)};
-        typeId: "${diffEq.colorId}";
-        type: SDCPNTypes["${diffEq.colorId}"];
-      };`,
-      )
-      .join("\n")}
-  }`;
-}
-
-function generateParametersDefinition(parameters: Parameter[]): string {
-  return `{${parameters
-    .map(
-      (param) =>
-        `"${param.variableName}": ${param.type === "boolean" ? "boolean" : "number"}`,
-    )
-    .join(", ")}}`;
-}
-
-/**
- * Generate complete SDCPN type definitions
- */
-function generateSDCPNTypings(
-  types: Color[],
-  places: Place[],
-  transitions: Transition[],
-  differentialEquations: DifferentialEquation[],
-  parameters: Parameter[],
-  currentlySelectedItemId?: string,
-): string {
-  // Generate a map from place IDs to names for easier reference
-  const placeIdToNameMap = new Map(
-    places.map((place) => [place.id, place.name]),
-  );
-  const typeIdToTypeMap = new Map(types.map((type) => [type.id, type]));
-  const placeIdToTypeMap = new Map(
-    places.map((place) => [
-      place.id,
-      place.colorId ? typeIdToTypeMap.get(place.colorId) : undefined,
-    ]),
-  );
-
-  const parametersDefinition = generateParametersDefinition(parameters);
-  const globalTypesDefinition = generateTypesDefinition(types);
-  const placesDefinition = generatePlacesDefinition(places);
-  const transitionsDefinition = generateTransitionsDefinition(
-    transitions,
-    placeIdToNameMap,
-    placeIdToTypeMap,
-  );
-  const differentialEquationsDefinition =
-    generateDifferentialEquationsDefinition(differentialEquations);
-
-  return `
-declare type SDCPNParametersValues = ${parametersDefinition};
-
-${globalTypesDefinition}
-
-${placesDefinition}
-
-${transitionsDefinition}
-
-${differentialEquationsDefinition}
-
-// Define Lambda and TransitionKernel functions
-
-declare type SDCPNTransitionID = keyof SDCPNTransitions;
-
-${
-  currentlySelectedItemId
-    ? `type __SelectedTransitionID = "${currentlySelectedItemId}"`
-    : `type __SelectedTransitionID = SDCPNTransitionID`
-};
-
-declare function Lambda<TransitionId extends SDCPNTransitionID = __SelectedTransitionID>(fn: SDCPNTransitions[TransitionId]['lambdaInputFn']): void;
-
-declare function TransitionKernel<TransitionId extends SDCPNTransitionID = __SelectedTransitionID>(fn: SDCPNTransitions[TransitionId]['transitionKernelFn']): void;
-
-
-// Define Dynamics function
-
-type SDCPNDiffEqID = keyof SDCPNDifferentialEquations;
-
-${
-  currentlySelectedItemId
-    ? `type __SelectedDiffEqID = "${currentlySelectedItemId}"`
-    : `type __SelectedDiffEqID = SDCPNDiffEqID`
-};
-
-declare function Dynamics<DiffEqId extends SDCPNDiffEqID = __SelectedDiffEqID>(fn: SDCPNDifferentialEquations[DiffEqId]['type']['dynamicsFn']): void;
-
-
-// Define Visualizer function
-
-type SDCPNPlaceID = keyof SDCPNPlaces;
-
-${
-  currentlySelectedItemId
-    ? `type __SelectedPlaceID = "${currentlySelectedItemId}"`
-    : `type __SelectedPlaceID = SDCPNPlaceID`
-};
-
-declare function Visualization<PlaceId extends SDCPNPlaceID = __SelectedPlaceID>(fn: (props: { tokens: SDCPNPlaces[PlaceId]['type']['object'][], parameters: SDCPNParametersValues }) => React.JSX.Element): void;
-
-  `.trim();
-}
-
-/**
- * Configure Monaco TypeScript compiler options
- */
-function configureMonacoCompilerOptions(monaco: typeof Monaco): void {
-  const ts = monaco.typescript;
-
-  ts.typescriptDefaults.setCompilerOptions({
-    target: ts.ScriptTarget.ES2020,
-    allowNonTsExtensions: true,
-    moduleResolution: ts.ModuleResolutionKind.NodeJs,
-    module: ts.ModuleKind.ESNext,
-    noEmit: true,
-    esModuleInterop: true,
-    jsx: ts.JsxEmit.ReactJSX,
-    allowJs: false,
-    checkJs: false,
-    typeRoots: ["node_modules/@types"],
-  });
-
-  ts.javascriptDefaults.setCompilerOptions({
-    target: ts.ScriptTarget.ES2020,
-    allowNonTsExtensions: true,
-    noEmit: true,
-    allowJs: true,
-    checkJs: false,
-  });
-
-  ts.typescriptDefaults.setDiagnosticsOptions({
-    noSemanticValidation: false,
-    noSyntaxValidation: false,
-  });
-}
-
-/**
- * Global hook to update Monaco's TypeScript context with SDCPN-derived typings.
- * Should be called once at the app level to avoid race conditions.
- */
-export function useMonacoGlobalTypings() {
-  const {
-    petriNetDefinition: {
-      types,
-      transitions,
-      parameters,
-      places,
-      differentialEquations,
-    },
-  } = use(SDCPNContext);
-
-  const { selectedResourceId: currentlySelectedItemId } = use(EditorContext);
-
-  const [reactTypes, setReactTypes] = useState<ReactTypeDefinitions | null>(
-    null,
-  );
-
-  // Configure Monaco and load React types once at startup
-  useEffect(() => {
-    void loader.init().then((monaco: typeof Monaco) => {
-      // Configure compiler options
-      configureMonacoCompilerOptions(monaco);
-
-      // Fetch and set React types once
-      void fetchReactTypes().then((rTypes) => {
-        setReactTypes(rTypes);
-
-        // Set React types as base extra libs - this is done only once
-        monaco.typescript.typescriptDefaults.setExtraLibs([
-          {
-            content: rTypes.react,
-            filePath: "inmemory://sdcpn/node_modules/@types/react/index.d.ts",
-          },
-          {
-            content: rTypes.reactJsxRuntime,
-            filePath:
-              "inmemory://sdcpn/node_modules/@types/react/jsx-runtime.d.ts",
-          },
-          {
-            content: rTypes.reactDom,
-            filePath:
-              "inmemory://sdcpn/node_modules/@types/react-dom/index.d.ts",
-          },
-        ]);
-      });
-    });
-  }, []); // Empty deps - run only once at startup
-
-  // Update SDCPN typings whenever the model changes
-  useEffect(() => {
-    if (!reactTypes) {
-      return; // Wait for React types to load first
-    }
-
-    void loader.init().then((monaco: typeof Monaco) => {
-      const sdcpnTypings = generateSDCPNTypings(
-        types,
-        places,
-        transitions,
-        differentialEquations,
-        parameters,
-        currentlySelectedItemId ?? undefined,
-      );
-
-      // Create or update SDCPN typings model
-      const sdcpnTypingsUri = monaco.Uri.parse(
-        "inmemory://sdcpn/sdcpn-globals.d.ts",
-      );
-      const existingModel = monaco.editor.getModel(sdcpnTypingsUri);
-
-      if (existingModel) {
-        existingModel.setValue(sdcpnTypings);
-      } else {
-        monaco.editor.createModel(sdcpnTypings, "typescript", sdcpnTypingsUri);
-      }
-    });
-  }, [
-    reactTypes,
-    types,
-    parameters,
-    places,
-    transitions,
-    differentialEquations,
-    currentlySelectedItemId,
-  ]);
-}
diff --git a/libs/@hashintel/petrinaut/src/components/code-editor.tsx b/libs/@hashintel/petrinaut/src/monaco/code-editor.tsx
similarity index 63%
rename from libs/@hashintel/petrinaut/src/components/code-editor.tsx
rename to libs/@hashintel/petrinaut/src/monaco/code-editor.tsx
index 2c0a62d349b..8ccef730cb4 100644
--- a/libs/@hashintel/petrinaut/src/components/code-editor.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/code-editor.tsx
@@ -1,10 +1,10 @@
 import { css, cva } from "@hashintel/ds-helpers/css";
 import type { EditorProps, Monaco } from "@monaco-editor/react";
-import MonacoEditor from "@monaco-editor/react";
 import type { editor } from "monaco-editor";
-import { useCallback, useRef } from "react";
+import { Suspense, use, useCallback, useRef } from "react";
 
-import { Tooltip } from "./tooltip";
+import { Tooltip } from "../components/tooltip";
+import { MonacoContext } from "./context";
 
 const containerStyle = cva({
   base: {
@@ -24,31 +24,34 @@ const containerStyle = cva({
   },
 });
 
+const loadingStyle = css({
+  display: "flex",
+  alignItems: "center",
+  justifyContent: "center",
+  gap: "2",
+  height: "full",
+  color: "fg.muted",
+  bg: "bg.subtle",
+  fontSize: "base",
+});
+
 type CodeEditorProps = Omit<EditorProps, "theme"> & {
   tooltip?: string;
 };
 
-/**
- * Code editor component that wraps Monaco Editor.
- *
- * @param tooltip - Optional tooltip to show when hovering over the editor.
- *                  In read-only mode, the tooltip also appears when attempting to edit.
- */
-export const CodeEditor: React.FC<CodeEditorProps> = ({
-  tooltip,
+const CodeEditorInner: React.FC<CodeEditorProps> = ({
   options,
-  height,
   onMount,
   ...props
 }) => {
-  const isReadOnly = options?.readOnly === true;
+  const { Editor } = use(use(MonacoContext));
+
   const editorRef = useRef<editor.IStandaloneCodeEditor | null>(null);
 
   const handleMount = useCallback(
-    (editorInstance: editor.IStandaloneCodeEditor, monaco: Monaco) => {
+    (editorInstance: editor.IStandaloneCodeEditor, monacoInstance: Monaco) => {
       editorRef.current = editorInstance;
-      // Call the original onMount if provided
-      onMount?.(editorInstance, monaco);
+      onMount?.(editorInstance, monacoInstance);
     },
     [onMount],
   );
@@ -67,19 +70,35 @@ export const CodeEditor: React.FC<CodeEditorProps> = ({
     ...options,
   };
 
+  return (
+    <Editor
+      theme="vs-light"
+      height="100%"
+      options={editorOptions}
+      onMount={handleMount}
+      {...props}
+    />
+  );
+};
+
+export const CodeEditor: React.FC<CodeEditorProps> = ({
+  tooltip,
+  options,
+  height,
+  ...props
+}) => {
+  const isReadOnly = options?.readOnly === true;
+
   const editorElement = (
     <div className={containerStyle({ isReadOnly })} style={{ height }}>
-      <MonacoEditor
-        theme="vs-light"
-        height="100%"
-        options={editorOptions}
-        onMount={handleMount}
-        {...props}
-      />
+      <Suspense
+        fallback={<div className={loadingStyle}>Loading editor...</div>}
+      >
+        <CodeEditorInner options={options} height={height} {...props} />
+      </Suspense>
     </div>
   );
 
-  // Regular tooltip for non-read-only mode (if tooltip is provided)
   if (tooltip) {
     return (
       <Tooltip
diff --git a/libs/@hashintel/petrinaut/src/monaco/context.ts b/libs/@hashintel/petrinaut/src/monaco/context.ts
new file mode 100644
index 00000000000..3075ef10660
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/monaco/context.ts
@@ -0,0 +1,12 @@
+import type { EditorProps } from "@monaco-editor/react";
+import type * as Monaco from "monaco-editor";
+import { createContext } from "react";
+
+export type MonacoContextValue = {
+  monaco: typeof Monaco;
+  Editor: React.FC<EditorProps>;
+};
+
+export const MonacoContext = createContext<Promise<MonacoContextValue>>(
+  null as never,
+);
diff --git a/libs/@hashintel/petrinaut/src/monaco/provider.tsx b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
new file mode 100644
index 00000000000..b09b0644aea
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
@@ -0,0 +1,108 @@
+import type * as Monaco from "monaco-editor";
+
+import type { MonacoContextValue } from "./context";
+import { MonacoContext } from "./context";
+
+interface LanguageDefaults {
+  setModeConfiguration(config: Record<string, boolean>): void;
+}
+
+interface TypeScriptNamespace {
+  typescriptDefaults: LanguageDefaults;
+  javascriptDefaults: LanguageDefaults;
+}
+
+/**
+ * Disable all built-in TypeScript language worker features.
+ * Syntax highlighting (Monarch tokenizer) is retained since it runs client-side.
+ */
+function disableBuiltInTypeScriptFeatures(monaco: typeof Monaco) {
+  // The `typescript` namespace is marked deprecated in newer type definitions
+  // but the runtime API still exists and is the only way to control the TS worker.
+  const ts = monaco.languages.typescript as unknown as TypeScriptNamespace;
+
+  const modeConfiguration: Record<string, boolean> = {
+    completionItems: false,
+    hovers: false,
+    documentSymbols: false,
+    definitions: false,
+    references: false,
+    documentHighlights: false,
+    rename: false,
+    diagnostics: false,
+    documentRangeFormattingEdits: false,
+    signatureHelp: false,
+    onTypeFormattingEdits: false,
+    codeActions: false,
+    inlayHints: false,
+  };
+
+  ts.typescriptDefaults.setModeConfiguration(modeConfiguration);
+  ts.javascriptDefaults.setModeConfiguration(modeConfiguration);
+}
+
+function registerCompletionProvider(monaco: typeof Monaco) {
+  monaco.languages.registerCompletionItemProvider("typescript", {
+    provideCompletionItems(model, position) {
+      const word = model.getWordUntilPosition(position);
+      const range = {
+        startLineNumber: position.lineNumber,
+        endLineNumber: position.lineNumber,
+        startColumn: word.startColumn,
+        endColumn: word.endColumn,
+      };
+
+      // eslint-disable-next-line no-console
+      console.log("Completion requested", {
+        position: { line: position.lineNumber, column: position.column },
+        word: word.word,
+        range,
+      });
+
+      return {
+        suggestions: [
+          {
+            label: "transition",
+            kind: monaco.languages.CompletionItemKind.Keyword,
+            insertText: "transition",
+            range,
+          },
+        ],
+      };
+    },
+  });
+}
+
+async function initMonaco(): Promise<MonacoContextValue> {
+  await new Promise((resolve) => setTimeout(resolve, 4000));
+
+  // Disable all workers — no worker files will be shipped or loaded.
+  (globalThis as Record<string, unknown>).MonacoEnvironment = {
+    getWorker: undefined,
+  };
+
+  const [monaco, monacoReact] = await Promise.all([
+    import("monaco-editor") as Promise<typeof Monaco>,
+    import("@monaco-editor/react"),
+  ]);
+
+  // Use local Monaco instance — no CDN fetch.
+  monacoReact.loader.config({ monaco });
+
+  disableBuiltInTypeScriptFeatures(monaco);
+  registerCompletionProvider(monaco);
+  return { monaco, Editor: monacoReact.default };
+}
+
+export const MonacoProvider: React.FC<{ children: React.ReactNode }> = ({
+  children,
+}) => {
+  // Stable promise reference — created once, never changes.
+  const monacoPromise = initMonaco();
+
+  return (
+    <MonacoContext.Provider value={monacoPromise}>
+      {children}
+    </MonacoContext.Provider>
+  );
+};
diff --git a/libs/@hashintel/petrinaut/src/petrinaut.tsx b/libs/@hashintel/petrinaut/src/petrinaut.tsx
index c56d600da35..ad1d83d5008 100644
--- a/libs/@hashintel/petrinaut/src/petrinaut.tsx
+++ b/libs/@hashintel/petrinaut/src/petrinaut.tsx
@@ -11,7 +11,7 @@ import type {
   SDCPN,
   Transition,
 } from "./core/types/sdcpn";
-import { useMonacoGlobalTypings } from "./hooks/use-monaco-global-typings";
+import { MonacoProvider } from "./monaco/provider";
 import { NotificationsProvider } from "./notifications/notifications-provider";
 import { PlaybackProvider } from "./playback/provider";
 import { SimulationProvider } from "./simulation/provider";
@@ -33,15 +33,6 @@ export type {
   Transition,
 };
 
-/**
- * Internal component to initialize Monaco global typings.
- * Must be inside SDCPNProvider to access the store.
- */
-const MonacoSetup: React.FC = () => {
-  useMonacoGlobalTypings();
-  return null;
-};
-
 export type PetrinautProps = {
   /**
    * Nets other than this one which are available for selection, e.g. to switch to or to link from a transition.
@@ -107,18 +98,19 @@ export const Petrinaut = ({
   return (
     <NotificationsProvider>
       <SDCPNProvider {...rest}>
-        <CheckerProvider>
-          <SimulationProvider>
-            <PlaybackProvider>
-              <EditorProvider>
-                <MonacoSetup />
-                <EditorView
-                  hideNetManagementControls={hideNetManagementControls}
-                />
-              </EditorProvider>
-            </PlaybackProvider>
-          </SimulationProvider>
-        </CheckerProvider>
+        <MonacoProvider>
+          <CheckerProvider>
+            <SimulationProvider>
+              <PlaybackProvider>
+                <EditorProvider>
+                  <EditorView
+                    hideNetManagementControls={hideNetManagementControls}
+                  />
+                </EditorProvider>
+              </PlaybackProvider>
+            </SimulationProvider>
+          </CheckerProvider>
+        </MonacoProvider>
       </SDCPNProvider>
     </NotificationsProvider>
   );
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx
index d2c5db0bcfb..330dbc1e7c4 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx
@@ -5,7 +5,6 @@ import { useState } from "react";
 import { TbDotsVertical, TbSparkles } from "react-icons/tb";
 
 import { Button } from "../../../../components/button";
-import { CodeEditor } from "../../../../components/code-editor";
 import { Input } from "../../../../components/input";
 import { Menu } from "../../../../components/menu";
 import { Tooltip } from "../../../../components/tooltip";
@@ -19,6 +18,7 @@ import type {
   DifferentialEquation,
   Place,
 } from "../../../../core/types/sdcpn";
+import { CodeEditor } from "../../../../monaco/code-editor";
 import { useIsReadOnly } from "../../../../state/use-is-read-only";
 
 const containerStyle = css({
@@ -476,6 +476,7 @@ export const DifferentialEquationProperties: React.FC<
           )}
         </div>
         <CodeEditor
+          path={`inmemory://sdcpn/differential-equations/${differentialEquation.id}.ts`}
           language="typescript"
           value={differentialEquation.code}
           height="100%"
@@ -487,7 +488,6 @@ export const DifferentialEquationProperties: React.FC<
               },
             );
           }}
-          path={`inmemory://sdcpn/differential-equations/${differentialEquation.id}.ts`}
           options={{ readOnly: isReadOnly }}
           tooltip={isReadOnly ? UI_MESSAGES.READ_ONLY_MODE : undefined}
         />
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/place-properties.tsx b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/place-properties.tsx
index c80ae9f7119..7758dde9c67 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/place-properties.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/place-properties.tsx
@@ -1,6 +1,5 @@
 /* eslint-disable id-length */
 import { css } from "@hashintel/ds-helpers/css";
-import MonacoEditor from "@monaco-editor/react";
 import { use, useEffect, useMemo, useRef, useState } from "react";
 import {
   TbArrowRight,
@@ -28,6 +27,7 @@ import type {
   DifferentialEquation,
   Place,
 } from "../../../../core/types/sdcpn";
+import { CodeEditor } from "../../../../monaco/code-editor";
 import { PlaybackContext } from "../../../../playback/context";
 import { EditorContext } from "../../../../state/editor-context";
 import { SDCPNContext } from "../../../../state/sdcpn-context";
@@ -142,12 +142,6 @@ const codeHeaderLabelStyle = css({
   fontSize: "[12px]",
 });
 
-const editorBorderStyle = css({
-  border: "[1px solid rgba(0, 0, 0, 0.1)]",
-  borderRadius: "[4px]",
-  overflow: "hidden",
-});
-
 const aiMenuItemStyle = css({
   display: "flex",
   alignItems: "center",
@@ -549,33 +543,17 @@ export const PlaceProperties: React.FC<PlacePropertiesProps> = ({
                 ]}
               />
             </div>
-            <div className={editorBorderStyle}>
-              <MonacoEditor
-                key={`visualizer-${place.colorId ?? "no-type"}`}
-                language="typescript"
-                path={`inmemory://sdcpn/places/${place.id}/visualizer.tsx`}
-                height={400}
-                value={place.visualizerCode}
-                onChange={(value) => {
-                  updatePlace(place.id, (existingPlace) => {
-                    existingPlace.visualizerCode = value ?? "";
-                  });
-                }}
-                theme="vs-light"
-                options={{
-                  minimap: { enabled: false },
-                  scrollBeyondLastLine: false,
-                  fontSize: 12,
-                  lineNumbers: "off",
-                  folding: true,
-                  glyphMargin: false,
-                  lineDecorationsWidth: 0,
-                  lineNumbersMinChars: 3,
-                  padding: { top: 8, bottom: 8 },
-                  fixedOverflowWidgets: true,
-                }}
-              />
-            </div>
+            <CodeEditor
+              path={`inmemory://sdcpn/places/${place.id}/visualizer.tsx`}
+              language="typescript"
+              height={400}
+              value={place.visualizerCode}
+              onChange={(value) => {
+                updatePlace(place.id, (existingPlace) => {
+                  existingPlace.visualizerCode = value ?? "";
+                });
+              }}
+            />
           </div>
         )}
       </div>
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
index 22201b754ee..f3bd01ef93e 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
@@ -19,7 +19,6 @@ import { css } from "@hashintel/ds-helpers/css";
 import { use } from "react";
 import { TbDotsVertical, TbSparkles, TbTrash } from "react-icons/tb";
 
-import { CodeEditor } from "../../../../components/code-editor";
 import { IconButton } from "../../../../components/icon-button";
 import { Input } from "../../../../components/input";
 import { Menu } from "../../../../components/menu";
@@ -31,6 +30,7 @@ import {
   generateDefaultTransitionKernelCode,
 } from "../../../../core/default-codes";
 import type { Color, Place, Transition } from "../../../../core/types/sdcpn";
+import { CodeEditor } from "../../../../monaco/code-editor";
 import { EditorContext } from "../../../../state/editor-context";
 import { SDCPNContext } from "../../../../state/sdcpn-context";
 import { useIsReadOnly } from "../../../../state/use-is-read-only";
@@ -480,12 +480,12 @@ export const TransitionProperties: React.FC<TransitionPropertiesProps> = ({
           )}
         </div>
         <CodeEditor
+          path={`inmemory://sdcpn/transitions/${transition.id}/lambda.ts`}
           key={`lambda-${transition.lambdaType}-${transition.inputArcs
             .map((a) => `${a.placeId}:${a.weight}`)
             .join("-")}`}
           language="typescript"
           value={transition.lambdaCode || ""}
-          path={`inmemory://sdcpn/transitions/${transition.id}/lambda.ts`}
           height={340}
           onChange={(value) => {
             updateTransition(transition.id, (existingTransition) => {
@@ -583,14 +583,9 @@ export const TransitionProperties: React.FC<TransitionPropertiesProps> = ({
             )}
           </div>
           <CodeEditor
-            key={`kernel-${transition.inputArcs
-              .map((a) => `${a.placeId}:${a.weight}`)
-              .join("-")}-${transition.outputArcs
-              .map((a) => `${a.placeId}:${a.weight}`)
-              .join("-")}`}
+            path={`inmemory://sdcpn/transitions/${transition.id}/transition-kernel.ts`}
             language="typescript"
             value={transition.transitionKernelCode || ""}
-            path={`inmemory://sdcpn/transitions/${transition.id}/transition-kernel.ts`}
             height={400}
             onChange={(value) => {
               updateTransition(transition.id, (existingTransition) => {

From 16ab5396ffd7f4c640badaa7878174ef858b7ca1 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 19 Feb 2026 14:21:33 +0100
Subject: [PATCH 02/22] H-5839: Consolidate checker into self-contained module

Move core/checker/ to checker/lib/ and state/checker-context +
checker-provider into checker/context and checker/provider, mirroring
the monaco/ module structure. Add rollup-plugin-visualizer and
externalize @monaco-editor/react from the bundle.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 libs/@hashintel/petrinaut/package.json        |  1 +
 .../checker-context.ts => checker/context.ts} |  2 +-
 .../checker => checker/lib}/checker.test.ts   |  0
 .../{core/checker => checker/lib}/checker.ts  |  0
 .../lib}/create-language-service-host.ts      |  0
 .../lib}/create-sdcpn-language-service.ts     |  0
 .../checker => checker/lib}/file-paths.ts     |  0
 .../lib}/helper/create-sdcpn.ts               |  0
 .../provider.tsx}                             |  6 ++---
 libs/@hashintel/petrinaut/src/petrinaut.tsx   | 10 +++----
 .../components/BottomBar/bottom-bar.tsx       |  2 +-
 .../BottomBar/diagnostics-indicator.tsx       |  2 +-
 .../src/views/Editor/subviews/diagnostics.tsx |  2 +-
 yarn.lock                                     | 27 +++++++++++++++++--
 14 files changed, 38 insertions(+), 14 deletions(-)
 rename libs/@hashintel/petrinaut/src/{state/checker-context.ts => checker/context.ts} (89%)
 rename libs/@hashintel/petrinaut/src/{core/checker => checker/lib}/checker.test.ts (100%)
 rename libs/@hashintel/petrinaut/src/{core/checker => checker/lib}/checker.ts (100%)
 rename libs/@hashintel/petrinaut/src/{core/checker => checker/lib}/create-language-service-host.ts (100%)
 rename libs/@hashintel/petrinaut/src/{core/checker => checker/lib}/create-sdcpn-language-service.ts (100%)
 rename libs/@hashintel/petrinaut/src/{core/checker => checker/lib}/file-paths.ts (100%)
 rename libs/@hashintel/petrinaut/src/{core/checker => checker/lib}/helper/create-sdcpn.ts (100%)
 rename libs/@hashintel/petrinaut/src/{state/checker-provider.tsx => checker/provider.tsx} (77%)

diff --git a/libs/@hashintel/petrinaut/package.json b/libs/@hashintel/petrinaut/package.json
index c07cdbc5008..78fa44ebf01 100644
--- a/libs/@hashintel/petrinaut/package.json
+++ b/libs/@hashintel/petrinaut/package.json
@@ -77,6 +77,7 @@
     "jsdom": "24.1.3",
     "react": "19.2.3",
     "react-dom": "19.2.3",
+    "rollup-plugin-visualizer": "6.0.5",
     "vite": "8.0.0-beta.14",
     "vite-plugin-dts": "4.5.4",
     "vitest": "4.0.18"
diff --git a/libs/@hashintel/petrinaut/src/state/checker-context.ts b/libs/@hashintel/petrinaut/src/checker/context.ts
similarity index 89%
rename from libs/@hashintel/petrinaut/src/state/checker-context.ts
rename to libs/@hashintel/petrinaut/src/checker/context.ts
index 1ebf40f6f67..e7aa5988812 100644
--- a/libs/@hashintel/petrinaut/src/state/checker-context.ts
+++ b/libs/@hashintel/petrinaut/src/checker/context.ts
@@ -1,6 +1,6 @@
 import { createContext } from "react";
 
-import type { SDCPNCheckResult } from "../core/checker/checker";
+import type { SDCPNCheckResult } from "./lib/checker";
 
 export type CheckResult = SDCPNCheckResult;
 
diff --git a/libs/@hashintel/petrinaut/src/core/checker/checker.test.ts b/libs/@hashintel/petrinaut/src/checker/lib/checker.test.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/core/checker/checker.test.ts
rename to libs/@hashintel/petrinaut/src/checker/lib/checker.test.ts
diff --git a/libs/@hashintel/petrinaut/src/core/checker/checker.ts b/libs/@hashintel/petrinaut/src/checker/lib/checker.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/core/checker/checker.ts
rename to libs/@hashintel/petrinaut/src/checker/lib/checker.ts
diff --git a/libs/@hashintel/petrinaut/src/core/checker/create-language-service-host.ts b/libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/core/checker/create-language-service-host.ts
rename to libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts
diff --git a/libs/@hashintel/petrinaut/src/core/checker/create-sdcpn-language-service.ts b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/core/checker/create-sdcpn-language-service.ts
rename to libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
diff --git a/libs/@hashintel/petrinaut/src/core/checker/file-paths.ts b/libs/@hashintel/petrinaut/src/checker/lib/file-paths.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/core/checker/file-paths.ts
rename to libs/@hashintel/petrinaut/src/checker/lib/file-paths.ts
diff --git a/libs/@hashintel/petrinaut/src/core/checker/helper/create-sdcpn.ts b/libs/@hashintel/petrinaut/src/checker/lib/helper/create-sdcpn.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/core/checker/helper/create-sdcpn.ts
rename to libs/@hashintel/petrinaut/src/checker/lib/helper/create-sdcpn.ts
diff --git a/libs/@hashintel/petrinaut/src/state/checker-provider.tsx b/libs/@hashintel/petrinaut/src/checker/provider.tsx
similarity index 77%
rename from libs/@hashintel/petrinaut/src/state/checker-provider.tsx
rename to libs/@hashintel/petrinaut/src/checker/provider.tsx
index e4ba7b2c337..97b5a01ab11 100644
--- a/libs/@hashintel/petrinaut/src/state/checker-provider.tsx
+++ b/libs/@hashintel/petrinaut/src/checker/provider.tsx
@@ -1,8 +1,8 @@
 import { use } from "react";
 
-import { checkSDCPN } from "../core/checker/checker";
-import { CheckerContext } from "./checker-context";
-import { SDCPNContext } from "./sdcpn-context";
+import { SDCPNContext } from "../state/sdcpn-context";
+import { checkSDCPN } from "./lib/checker";
+import { CheckerContext } from "./context";
 
 export const CheckerProvider: React.FC<{ children: React.ReactNode }> = ({
   children,
diff --git a/libs/@hashintel/petrinaut/src/petrinaut.tsx b/libs/@hashintel/petrinaut/src/petrinaut.tsx
index ad1d83d5008..022f4f91816 100644
--- a/libs/@hashintel/petrinaut/src/petrinaut.tsx
+++ b/libs/@hashintel/petrinaut/src/petrinaut.tsx
@@ -15,7 +15,7 @@ import { MonacoProvider } from "./monaco/provider";
 import { NotificationsProvider } from "./notifications/notifications-provider";
 import { PlaybackProvider } from "./playback/provider";
 import { SimulationProvider } from "./simulation/provider";
-import { CheckerProvider } from "./state/checker-provider";
+import { CheckerProvider } from "./checker/provider";
 import { EditorProvider } from "./state/editor-provider";
 import { SDCPNProvider } from "./state/sdcpn-provider";
 import { EditorView } from "./views/Editor/editor-view";
@@ -98,8 +98,8 @@ export const Petrinaut = ({
   return (
     <NotificationsProvider>
       <SDCPNProvider {...rest}>
-        <MonacoProvider>
-          <CheckerProvider>
+        <CheckerProvider>
+          <MonacoProvider>
             <SimulationProvider>
               <PlaybackProvider>
                 <EditorProvider>
@@ -109,8 +109,8 @@ export const Petrinaut = ({
                 </EditorProvider>
               </PlaybackProvider>
             </SimulationProvider>
-          </CheckerProvider>
-        </MonacoProvider>
+          </MonacoProvider>
+        </CheckerProvider>
       </SDCPNProvider>
     </NotificationsProvider>
   );
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx
index 0fca9f54bce..e82175b8e04 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx
@@ -3,7 +3,7 @@ import { refractive } from "@hashintel/refractive";
 import { use, useCallback, useEffect } from "react";
 import { FaChevronDown, FaChevronUp } from "react-icons/fa6";
 
-import { CheckerContext } from "../../../../state/checker-context";
+import { CheckerContext } from "../../../../checker/context";
 import {
   EditorContext,
   type EditorState,
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx
index 145fb52b49e..648a2b0ffa4 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx
@@ -2,7 +2,7 @@ import { css, cva } from "@hashintel/ds-helpers/css";
 import { use } from "react";
 import { FaCheck, FaXmark } from "react-icons/fa6";
 
-import { CheckerContext } from "../../../../state/checker-context";
+import { CheckerContext } from "../../../../checker/context";
 import { ToolbarButton } from "./toolbar-button";
 
 const iconContainerStyle = cva({
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
index b966ef21413..dafd2c85cfe 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
@@ -4,7 +4,7 @@ import { FaChevronDown, FaChevronRight } from "react-icons/fa6";
 import ts from "typescript";
 
 import type { SubView } from "../../../components/sub-view/types";
-import { CheckerContext } from "../../../state/checker-context";
+import { CheckerContext } from "../../../checker/context";
 import { EditorContext } from "../../../state/editor-context";
 import { SDCPNContext } from "../../../state/sdcpn-context";
 
diff --git a/yarn.lock b/yarn.lock
index 408032f7a54..36d17909c59 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -7824,6 +7824,7 @@ __metadata:
     react-dom: "npm:19.2.3"
     react-icons: "npm:5.5.0"
     reactflow: "npm:11.11.4"
+    rollup-plugin-visualizer: "npm:6.0.5"
     typescript: "npm:5.9.3"
     uuid: "npm:13.0.0"
     vite: "npm:8.0.0-beta.14"
@@ -36724,7 +36725,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"open@npm:^8.0.4, open@npm:^8.4.2":
+"open@npm:^8.0.0, open@npm:^8.0.4, open@npm:^8.4.2":
   version: 8.4.2
   resolution: "open@npm:8.4.2"
   dependencies:
@@ -40631,6 +40632,28 @@ __metadata:
   languageName: node
   linkType: hard
 
+"rollup-plugin-visualizer@npm:6.0.5":
+  version: 6.0.5
+  resolution: "rollup-plugin-visualizer@npm:6.0.5"
+  dependencies:
+    open: "npm:^8.0.0"
+    picomatch: "npm:^4.0.2"
+    source-map: "npm:^0.7.4"
+    yargs: "npm:^17.5.1"
+  peerDependencies:
+    rolldown: 1.x || ^1.0.0-beta
+    rollup: 2.x || 3.x || 4.x
+  peerDependenciesMeta:
+    rolldown:
+      optional: true
+    rollup:
+      optional: true
+  bin:
+    rollup-plugin-visualizer: dist/bin/cli.js
+  checksum: 10c0/3824626e97d5033fbb3aa1bbe93c8c17a8569bc47e33c941bde6b90404f2cae70b26fec1b623bd393c3e076338014196c91726ed2c96218edc67e1f21676f7ef
+  languageName: node
+  linkType: hard
+
 "rollup@npm:4.57.1, rollup@npm:^4.34.9, rollup@npm:^4.35.0, rollup@npm:^4.43.0":
   version: 4.57.1
   resolution: "rollup@npm:4.57.1"
@@ -46397,7 +46420,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"yargs@npm:17.7.2, yargs@npm:^17.0.0, yargs@npm:^17.0.1, yargs@npm:^17.7.1, yargs@npm:^17.7.2":
+"yargs@npm:17.7.2, yargs@npm:^17.0.0, yargs@npm:^17.0.1, yargs@npm:^17.5.1, yargs@npm:^17.7.1, yargs@npm:^17.7.2":
   version: 17.7.2
   resolution: "yargs@npm:17.7.2"
   dependencies:

From 61de26200b55681699320d831989c12ee3e2e987 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 19 Feb 2026 14:38:09 +0100
Subject: [PATCH 03/22] H-5839: Fix import paths after checker move and remove
 debug delay

Update relative imports in checker/lib/ to reach core/types/sdcpn at
the new depth. Sort imports per linter rules. Remove the 4s debug
setTimeout from Monaco init.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 libs/@hashintel/petrinaut/src/checker/lib/checker.ts            | 2 +-
 .../petrinaut/src/checker/lib/create-sdcpn-language-service.ts  | 2 +-
 .../@hashintel/petrinaut/src/checker/lib/helper/create-sdcpn.ts | 2 +-
 libs/@hashintel/petrinaut/src/checker/provider.tsx              | 2 +-
 libs/@hashintel/petrinaut/src/monaco/provider.tsx               | 2 --
 libs/@hashintel/petrinaut/src/petrinaut.tsx                     | 2 +-
 .../petrinaut/src/views/Editor/subviews/diagnostics.tsx         | 2 +-
 7 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/libs/@hashintel/petrinaut/src/checker/lib/checker.ts b/libs/@hashintel/petrinaut/src/checker/lib/checker.ts
index 33d170423ae..349b3aeddf1 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/checker.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/checker.ts
@@ -1,6 +1,6 @@
 import type ts from "typescript";
 
-import type { SDCPN } from "../types/sdcpn";
+import type { SDCPN } from "../../core/types/sdcpn";
 import { createSDCPNLanguageService } from "./create-sdcpn-language-service";
 import { getItemFilePath } from "./file-paths";
 
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
index 87d16b0274e..33a05d29a34 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
@@ -1,6 +1,6 @@
 import ts from "typescript";
 
-import type { SDCPN } from "../types/sdcpn";
+import type { SDCPN } from "../../core/types/sdcpn";
 import {
   createLanguageServiceHost,
   type VirtualFile,
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/helper/create-sdcpn.ts b/libs/@hashintel/petrinaut/src/checker/lib/helper/create-sdcpn.ts
index eec76175ef0..644c469521d 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/helper/create-sdcpn.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/helper/create-sdcpn.ts
@@ -5,7 +5,7 @@ import type {
   Place,
   SDCPN,
   Transition,
-} from "../../types/sdcpn";
+} from "../../../core/types/sdcpn";
 
 type PartialColor = Omit<Partial<Color>, "elements"> & {
   elements?: Array<Partial<Color["elements"][number]>>;
diff --git a/libs/@hashintel/petrinaut/src/checker/provider.tsx b/libs/@hashintel/petrinaut/src/checker/provider.tsx
index 97b5a01ab11..c39d4d24807 100644
--- a/libs/@hashintel/petrinaut/src/checker/provider.tsx
+++ b/libs/@hashintel/petrinaut/src/checker/provider.tsx
@@ -1,8 +1,8 @@
 import { use } from "react";
 
 import { SDCPNContext } from "../state/sdcpn-context";
-import { checkSDCPN } from "./lib/checker";
 import { CheckerContext } from "./context";
+import { checkSDCPN } from "./lib/checker";
 
 export const CheckerProvider: React.FC<{ children: React.ReactNode }> = ({
   children,
diff --git a/libs/@hashintel/petrinaut/src/monaco/provider.tsx b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
index b09b0644aea..7922ec5fce5 100644
--- a/libs/@hashintel/petrinaut/src/monaco/provider.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
@@ -74,8 +74,6 @@ function registerCompletionProvider(monaco: typeof Monaco) {
 }
 
 async function initMonaco(): Promise<MonacoContextValue> {
-  await new Promise((resolve) => setTimeout(resolve, 4000));
-
   // Disable all workers — no worker files will be shipped or loaded.
   (globalThis as Record<string, unknown>).MonacoEnvironment = {
     getWorker: undefined,
diff --git a/libs/@hashintel/petrinaut/src/petrinaut.tsx b/libs/@hashintel/petrinaut/src/petrinaut.tsx
index 022f4f91816..df237136b33 100644
--- a/libs/@hashintel/petrinaut/src/petrinaut.tsx
+++ b/libs/@hashintel/petrinaut/src/petrinaut.tsx
@@ -1,6 +1,7 @@
 import "reactflow/dist/style.css";
 import "./index.css";
 
+import { CheckerProvider } from "./checker/provider";
 import type {
   Color,
   DifferentialEquation,
@@ -15,7 +16,6 @@ import { MonacoProvider } from "./monaco/provider";
 import { NotificationsProvider } from "./notifications/notifications-provider";
 import { PlaybackProvider } from "./playback/provider";
 import { SimulationProvider } from "./simulation/provider";
-import { CheckerProvider } from "./checker/provider";
 import { EditorProvider } from "./state/editor-provider";
 import { SDCPNProvider } from "./state/sdcpn-provider";
 import { EditorView } from "./views/Editor/editor-view";
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
index dafd2c85cfe..f2e3f8bd2f1 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
@@ -3,8 +3,8 @@ import { use, useCallback, useMemo, useState } from "react";
 import { FaChevronDown, FaChevronRight } from "react-icons/fa6";
 import ts from "typescript";
 
-import type { SubView } from "../../../components/sub-view/types";
 import { CheckerContext } from "../../../checker/context";
+import type { SubView } from "../../../components/sub-view/types";
 import { EditorContext } from "../../../state/editor-context";
 import { SDCPNContext } from "../../../state/sdcpn-context";
 

From 27509b8e83ea14f538fe8d7a60ea4b7a2e5e2b84 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 19 Feb 2026 15:54:29 +0100
Subject: [PATCH 04/22] H-5839: Move checker to WebWorker with JSON-RPC
 protocol

Run TypeScript validation off the main thread to keep the UI responsive.
The checker worker communicates via JSON-RPC 2.0 over postMessage, with
diagnostics serialized (messageText flattened, ts.SourceFile stripped)
before crossing the worker boundary.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../petrinaut/src/checker/context.ts          | 10 +--
 .../petrinaut/src/checker/provider.tsx        | 27 +++++-
 .../src/checker/worker/checker.worker.ts      | 62 +++++++++++++
 .../petrinaut/src/checker/worker/protocol.ts  | 63 ++++++++++++++
 .../src/checker/worker/use-checker-worker.ts  | 86 +++++++++++++++++++
 .../src/views/Editor/subviews/diagnostics.tsx | 22 +----
 6 files changed, 242 insertions(+), 28 deletions(-)
 create mode 100644 libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
 create mode 100644 libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
 create mode 100644 libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts

diff --git a/libs/@hashintel/petrinaut/src/checker/context.ts b/libs/@hashintel/petrinaut/src/checker/context.ts
index e7aa5988812..c312b234f96 100644
--- a/libs/@hashintel/petrinaut/src/checker/context.ts
+++ b/libs/@hashintel/petrinaut/src/checker/context.ts
@@ -1,13 +1,11 @@
 import { createContext } from "react";
 
-import type { SDCPNCheckResult } from "./lib/checker";
-
-export type CheckResult = SDCPNCheckResult;
+import type { CheckerResult } from "./worker/protocol";
 
 export interface CheckerContextValue {
-  /** The result of the last SDCPN check */
-  checkResult: SDCPNCheckResult;
-  /** Total count of all diagnostics across all items */
+  /** Result of the last SDCPN validation run. */
+  checkResult: CheckerResult;
+  /** Total number of diagnostics across all items. */
   totalDiagnosticsCount: number;
 }
 
diff --git a/libs/@hashintel/petrinaut/src/checker/provider.tsx b/libs/@hashintel/petrinaut/src/checker/provider.tsx
index c39d4d24807..4e5388cb950 100644
--- a/libs/@hashintel/petrinaut/src/checker/provider.tsx
+++ b/libs/@hashintel/petrinaut/src/checker/provider.tsx
@@ -1,15 +1,36 @@
-import { use } from "react";
+import { use, useEffect, useState } from "react";
 
 import { SDCPNContext } from "../state/sdcpn-context";
 import { CheckerContext } from "./context";
-import { checkSDCPN } from "./lib/checker";
+import type { CheckerResult } from "./worker/protocol";
+import { useCheckerWorker } from "./worker/use-checker-worker";
+
+const EMPTY_RESULT: CheckerResult = {
+  isValid: true,
+  itemDiagnostics: [],
+};
 
 export const CheckerProvider: React.FC<{ children: React.ReactNode }> = ({
   children,
 }) => {
   const { petriNetDefinition } = use(SDCPNContext);
+  const { checkSDCPN } = useCheckerWorker();
+
+  const [checkResult, setCheckerResult] = useState<CheckerResult>(EMPTY_RESULT);
+
+  useEffect(() => {
+    let cancelled = false;
+
+    void checkSDCPN(petriNetDefinition).then((result) => {
+      if (!cancelled) {
+        setCheckerResult(result);
+      }
+    });
 
-  const checkResult = checkSDCPN(petriNetDefinition);
+    return () => {
+      cancelled = true;
+    };
+  }, [petriNetDefinition, checkSDCPN]);
 
   const totalDiagnosticsCount = checkResult.itemDiagnostics.reduce(
     (sum, item) => sum + item.diagnostics.length,
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
new file mode 100644
index 00000000000..bbc0f96ad6c
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
@@ -0,0 +1,62 @@
+/* eslint-disable no-restricted-globals */
+/**
+ * Checker WebWorker — runs TypeScript validation off the main thread.
+ *
+ * Receives JSON-RPC requests via `postMessage`, delegates to `checkSDCPN`,
+ * serializes the diagnostics (flatten messageText, strip non-cloneable fields),
+ * and posts the result back.
+ */
+import ts from "typescript";
+
+import { checkSDCPN } from "../lib/checker";
+import type {
+  CheckerDiagnostic,
+  CheckerItemDiagnostics,
+  CheckerResult,
+  JsonRpcRequest,
+  JsonRpcResponse,
+} from "./protocol";
+
+/** Strip `ts.SourceFile` and flatten `DiagnosticMessageChain` for structured clone. */
+function serializeDiagnostic(diag: ts.Diagnostic): CheckerDiagnostic {
+  return {
+    category: diag.category,
+    code: diag.code,
+    messageText: ts.flattenDiagnosticMessageText(diag.messageText, "\n"),
+    start: diag.start,
+    length: diag.length,
+  };
+}
+
+self.onmessage = ({ data: { id, params } }: MessageEvent<JsonRpcRequest>) => {
+  try {
+    const raw = checkSDCPN(params.sdcpn);
+
+    const result: CheckerResult = {
+      isValid: raw.isValid,
+      itemDiagnostics: raw.itemDiagnostics.map(
+        (item): CheckerItemDiagnostics => ({
+          itemId: item.itemId,
+          itemType: item.itemType,
+          filePath: item.filePath,
+          diagnostics: item.diagnostics.map(serializeDiagnostic),
+        }),
+      ),
+    };
+
+    self.postMessage({
+      jsonrpc: "2.0",
+      id,
+      result,
+    } satisfies JsonRpcResponse<CheckerResult>);
+  } catch (err) {
+    self.postMessage({
+      jsonrpc: "2.0",
+      id,
+      error: {
+        code: -32603,
+        message: err instanceof Error ? err.message : String(err),
+      },
+    } satisfies JsonRpcResponse);
+  }
+};
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts b/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
new file mode 100644
index 00000000000..fc63218f0a3
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
@@ -0,0 +1,63 @@
+/**
+ * JSON-RPC 2.0 protocol types for the checker WebWorker.
+ *
+ * These types define the contract between the main thread and the worker.
+ * Diagnostic types are serializable (no `ts.SourceFile` references) so they
+ * can cross the postMessage boundary via structured clone.
+ */
+import type { SDCPN } from "../../core/types/sdcpn";
+
+// ---------------------------------------------------------------------------
+// Diagnostics — serializable variants of ts.Diagnostic
+// ---------------------------------------------------------------------------
+
+/** A single TypeScript diagnostic, safe for structured clone. */
+export type CheckerDiagnostic = {
+  /** @see ts.DiagnosticCategory */
+  category: number;
+  /** TypeScript error code (e.g. 2322 for type mismatch). */
+  code: number;
+  /** Human-readable message, pre-flattened from `ts.DiagnosticMessageChain`. */
+  messageText: string;
+  /** Character offset in user code where the error starts. */
+  start: number | undefined;
+  /** Length of the error span in characters. */
+  length: number | undefined;
+};
+
+/** Diagnostics grouped by SDCPN item (one transition function or differential equation). */
+export type CheckerItemDiagnostics = {
+  /** ID of the transition or differential equation. */
+  itemId: string;
+  /** Which piece of code was checked. */
+  itemType: "transition-lambda" | "transition-kernel" | "differential-equation";
+  /** Path in the virtual file system used by the TS LanguageService. */
+  filePath: string;
+  /** All diagnostics found in this item's code. */
+  diagnostics: CheckerDiagnostic[];
+};
+
+/** Result of validating an entire SDCPN model. */
+export type CheckerResult = {
+  /** `true` when every item compiles without errors. */
+  isValid: boolean;
+  /** Per-item diagnostics (empty when valid). */
+  itemDiagnostics: CheckerItemDiagnostics[];
+};
+
+// ---------------------------------------------------------------------------
+// JSON-RPC 2.0
+// ---------------------------------------------------------------------------
+
+/** A JSON-RPC request sent from the main thread to the worker. */
+export type JsonRpcRequest = {
+  jsonrpc: "2.0";
+  id: number;
+  method: "checkSDCPN";
+  params: { sdcpn: SDCPN };
+};
+
+/** A JSON-RPC response sent from the worker back to the main thread. */
+export type JsonRpcResponse<Result = unknown> =
+  | { jsonrpc: "2.0"; id: number; result: Result }
+  | { jsonrpc: "2.0"; id: number; error: { code: number; message: string } };
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts b/libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts
new file mode 100644
index 00000000000..e5627c34cd6
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts
@@ -0,0 +1,86 @@
+import { useEffect, useRef } from "react";
+
+import type { SDCPN } from "../../core/types/sdcpn";
+import type {
+  CheckerResult,
+  JsonRpcRequest,
+  JsonRpcResponse,
+} from "./protocol";
+
+type Pending = {
+  resolve: (result: CheckerResult) => void;
+  reject: (error: Error) => void;
+};
+
+/** Methods exposed by the checker WebWorker. */
+export type CheckerWorkerApi = {
+  /** Validate all user code in an SDCPN model. Runs off the main thread. */
+  checkSDCPN: (sdcpn: SDCPN) => Promise<CheckerResult>;
+};
+
+/**
+ * Spawn a checker WebWorker and return a Promise-based API to interact with it.
+ * The worker is created on mount and terminated on unmount.
+ */
+export function useCheckerWorker(): CheckerWorkerApi {
+  const workerRef = useRef<Worker | null>(null);
+  const pendingRef = useRef(new Map<number, Pending>());
+  const nextId = useRef(0);
+
+  useEffect(() => {
+    const worker = new Worker(new URL("./checker.worker.ts", import.meta.url), {
+      type: "module",
+    });
+
+    worker.onmessage = (
+      event: MessageEvent<JsonRpcResponse<CheckerResult>>,
+    ) => {
+      const response = event.data;
+      const pending = pendingRef.current.get(response.id);
+      if (!pending) {
+        return;
+      }
+      pendingRef.current.delete(response.id);
+
+      if ("error" in response) {
+        pending.reject(new Error(response.error.message));
+      } else {
+        pending.resolve(response.result);
+      }
+    };
+
+    workerRef.current = worker;
+    const pending = pendingRef.current;
+
+    return () => {
+      worker.terminate();
+      workerRef.current = null;
+      for (const entry of pending.values()) {
+        entry.reject(new Error("Worker terminated"));
+      }
+      pending.clear();
+    };
+  }, []);
+
+  const checkSDCPN = (sdcpn: SDCPN): Promise<CheckerResult> => {
+    const worker = workerRef.current;
+    if (!worker) {
+      return Promise.reject(new Error("Worker not initialized"));
+    }
+
+    const id = nextId.current++;
+    const request: JsonRpcRequest = {
+      jsonrpc: "2.0",
+      id,
+      method: "checkSDCPN",
+      params: { sdcpn },
+    };
+
+    return new Promise<CheckerResult>((resolve, reject) => {
+      pendingRef.current.set(id, { resolve, reject });
+      worker.postMessage(request);
+    });
+  };
+
+  return { checkSDCPN };
+}
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
index f2e3f8bd2f1..0eac7e15e5c 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
@@ -1,9 +1,9 @@
 import { css } from "@hashintel/ds-helpers/css";
 import { use, useCallback, useMemo, useState } from "react";
 import { FaChevronDown, FaChevronRight } from "react-icons/fa6";
-import ts from "typescript";
 
 import { CheckerContext } from "../../../checker/context";
+import type { CheckerDiagnostic } from "../../../checker/worker/protocol";
 import type { SubView } from "../../../components/sub-view/types";
 import { EditorContext } from "../../../state/editor-context";
 import { SDCPNContext } from "../../../state/sdcpn-context";
@@ -94,22 +94,6 @@ const positionStyle = css({
   marginLeft: "[8px]",
 });
 
-// --- Helpers ---
-
-/**
- * Formats a TypeScript diagnostic message to a readable string
- */
-function formatDiagnosticMessage(
-  messageText: string | ts.DiagnosticMessageChain,
-): string {
-  if (typeof messageText === "string") {
-    return messageText;
-  }
-  return ts.flattenDiagnosticMessageText(messageText, "\n");
-}
-
-// --- Types ---
-
 type EntityType = "transition" | "differential-equation";
 
 interface GroupedDiagnostics {
@@ -119,7 +103,7 @@ interface GroupedDiagnostics {
   errorCount: number;
   items: Array<{
     subType: "lambda" | "kernel" | null;
-    diagnostics: ts.Diagnostic[];
+    diagnostics: CheckerDiagnostic[];
   }>;
 }
 
@@ -269,7 +253,7 @@ const DiagnosticsContent: React.FC = () => {
                             className={diagnosticButtonStyle}
                           >
                             <span className={bulletStyle}>•</span>
-                            {formatDiagnosticMessage(diagnostic.messageText)}
+                            {diagnostic.messageText}
                             {diagnostic.start !== undefined && (
                               <span className={positionStyle}>
                                 (pos: {diagnostic.start})

From 3eb9d44f1194009428ec05ac2859e866a4508256 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 19 Feb 2026 17:10:00 +0100
Subject: [PATCH 05/22] H-5839: Bundle TypeScript into worker and remove
 CDN/polyfill workarounds

TypeScript now runs exclusively in the checker WebWorker, so:
- Move `typescript` from main bundle externals into worker bundle
- Add process/isNodeLikeSystem replacements to eliminate Node.js code paths
- Remove Node.js polyfill fallbacks from next.config.js (fs, path, os, etc.)
- Remove CDN CSP exceptions since Monaco is loaded locally

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 apps/hash-frontend/next.config.js        | 14 --------------
 apps/hash-frontend/src/lib/csp.ts        | 13 +------------
 libs/@hashintel/petrinaut/vite.config.ts | 12 +++++++++++-
 3 files changed, 12 insertions(+), 27 deletions(-)

diff --git a/apps/hash-frontend/next.config.js b/apps/hash-frontend/next.config.js
index 606f4cd6a5f..d6d6bef9f1d 100644
--- a/apps/hash-frontend/next.config.js
+++ b/apps/hash-frontend/next.config.js
@@ -219,20 +219,6 @@ export default withSentryConfig(
         // eslint-disable-next-line no-param-reassign
         webpackConfig.resolve.alias.canvas = false;
 
-        if (!isServer) {
-          // Stub Node.js built-ins for browser — needed by `typescript` (used by
-          // @hashintel/petrinaut's in-browser language service)
-          // eslint-disable-next-line no-param-reassign
-          webpackConfig.resolve.fallback = {
-            ...webpackConfig.resolve.fallback,
-            module: false,
-            fs: false,
-            path: false,
-            os: false,
-            perf_hooks: false,
-          };
-        }
-
         webpackConfig.plugins.push(
           new DefinePlugin({
             __SENTRY_DEBUG__: false,
diff --git a/apps/hash-frontend/src/lib/csp.ts b/apps/hash-frontend/src/lib/csp.ts
index 171adfb0a5b..4b327029a49 100644
--- a/apps/hash-frontend/src/lib/csp.ts
+++ b/apps/hash-frontend/src/lib/csp.ts
@@ -24,9 +24,6 @@ export const buildCspHeader = (nonce: string): string => {
       "https://apis.google.com",
       // Vercel toolbar / live preview widget
       "https://vercel.live",
-      // @todo FE-488 will make this unnecessary
-      // Monaco Editor loaded from CDN by @monaco-editor/react (used by petrinaut)
-      "https://cdn.jsdelivr.net",
     ],
 
     "style-src": [
@@ -34,9 +31,6 @@ export const buildCspHeader = (nonce: string): string => {
       // Required for Emotion/MUI CSS-in-JS inline style injection.
       // @todo Use nonce-based approach via Emotion's cache `nonce` option.
       "'unsafe-inline'",
-      // @todo FE-488 will make this unnecessary
-      // Monaco Editor stylesheet loaded from CDN by @monaco-editor/react (used by petrinaut)
-      "https://cdn.jsdelivr.net",
     ],
 
     "img-src": [
@@ -51,12 +45,7 @@ export const buildCspHeader = (nonce: string): string => {
       ...(process.env.NODE_ENV === "development" ? ["http:"] : []),
     ],
 
-    "font-src": [
-      "'self'",
-      // @todo FE-488 will make this unnecessary
-      // Monaco Editor CSS embeds the Codicon icon font as an inline base64 data URI
-      "data:",
-    ],
+    "font-src": ["'self'"],
 
     "connect-src": [
       "'self'",
diff --git a/libs/@hashintel/petrinaut/vite.config.ts b/libs/@hashintel/petrinaut/vite.config.ts
index 744ed2004b9..d5b64395b11 100644
--- a/libs/@hashintel/petrinaut/vite.config.ts
+++ b/libs/@hashintel/petrinaut/vite.config.ts
@@ -21,7 +21,6 @@ export default defineConfig({
         "react",
         "react-dom",
         "reactflow",
-        "typescript",
         "monaco-editor",
         "@babel/standalone",
       ],
@@ -48,7 +47,17 @@ export default defineConfig({
         // This causes crashes in Web Workers, since `window` is not defined there.
         // To prevent this, we do this resolution on our side.
         "typeof window": '"undefined"',
+        // TypeScript's internals reference process, process.versions.pnp, etc.
+        "typeof process": "'undefined'",
+        "typeof process.versions.pnp": "'undefined'",
       }),
+      // Separate replacePlugin for call-expression replacements:
+      // 1. Empty end delimiter because \b can't match after `)` (non-word → non-word).
+      // 2. Negative lookbehind skips the function definition (`function isNodeLikeSystem`).
+      replacePlugin(
+        { "isNodeLikeSystem()": "false" },
+        { delimiters: ["(?<!function )\\b", ""] },
+      ),
     ],
   },
 
@@ -59,6 +68,7 @@ export default defineConfig({
         "react/compiler-runtime",
         "react/jsx-runtime",
         "react/jsx-dev-runtime",
+        "typescript",
       ],
     }),
 

From be47f6cec086b210a1b4dfb9bff8ac75e75c9ef2 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 19 Feb 2026 17:32:52 +0100
Subject: [PATCH 06/22] H-5839: Remove @dnd-kit and feature flags

Arc reordering was behind a permanently-off feature flag, so all dnd-kit
usage was dead code. Strip the DndContext/SortableContext wrappers, simplify
SortableArcItem to a plain ArcItem, remove @dnd-kit dependencies, and
delete the now-empty feature-flags module.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../@hashintel/petrinaut/src/feature-flags.ts |   3 -
 .../PropertiesPanel/sortable-arc-item.tsx     |  64 +-----
 .../PropertiesPanel/transition-properties.tsx | 186 +++++-------------
 3 files changed, 51 insertions(+), 202 deletions(-)
 delete mode 100644 libs/@hashintel/petrinaut/src/feature-flags.ts

diff --git a/libs/@hashintel/petrinaut/src/feature-flags.ts b/libs/@hashintel/petrinaut/src/feature-flags.ts
deleted file mode 100644
index 5d4bcdf431d..00000000000
--- a/libs/@hashintel/petrinaut/src/feature-flags.ts
+++ /dev/null
@@ -1,3 +0,0 @@
-export const FEATURE_FLAGS = {
-  REORDER_TRANSITION_ARCS: false,
-};
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/sortable-arc-item.tsx b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/sortable-arc-item.tsx
index d68cb37ac3c..bbfa37c91ce 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/sortable-arc-item.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/sortable-arc-item.tsx
@@ -1,12 +1,8 @@
-import { useSortable } from "@dnd-kit/sortable";
-import { CSS } from "@dnd-kit/utilities";
-import { css, cva } from "@hashintel/ds-helpers/css";
-import { MdDragIndicator } from "react-icons/md";
+import { css } from "@hashintel/ds-helpers/css";
 import { TbTrash } from "react-icons/tb";
 
 import { IconButton } from "../../../../components/icon-button";
 import { NumberInput } from "../../../../components/number-input";
-import { FEATURE_FLAGS } from "../../../../feature-flags";
 
 const containerStyle = css({
   display: "flex",
@@ -17,28 +13,6 @@ const containerStyle = css({
   borderBottom: "[1px solid rgba(0, 0, 0, 0.06)]",
 });
 
-const dragHandleStyle = cva({
-  base: {
-    display: "flex",
-    alignItems: "center",
-    flexShrink: 0,
-  },
-  variants: {
-    isDisabled: {
-      true: {
-        cursor: "default",
-        color: "[#ccc]",
-        pointerEvents: "none",
-      },
-      false: {
-        cursor: "grab",
-        color: "[#999]",
-        pointerEvents: "auto",
-      },
-    },
-  },
-});
-
 const placeNameStyle = css({
   flex: "[1]",
   fontSize: "[14px]",
@@ -68,22 +42,16 @@ const weightInputStyle = css({
   padding: "[4px 8px]",
 });
 
-/**
- * SortableArcItem - A draggable arc item that displays place name and weight
- */
-interface SortableArcItemProps {
-  id: string;
+interface ArcItemProps {
   placeName: string;
   weight: number;
   disabled?: boolean;
-  /** Tooltip to show when disabled (e.g., for read-only mode) */
   tooltip?: string;
   onWeightChange: (weight: number) => void;
   onDelete?: () => void;
 }
 
-export const SortableArcItem: React.FC<SortableArcItemProps> = ({
-  id,
+export const ArcItem: React.FC<ArcItemProps> = ({
   placeName,
   weight,
   disabled = false,
@@ -91,32 +59,8 @@ export const SortableArcItem: React.FC<SortableArcItemProps> = ({
   onWeightChange,
   onDelete,
 }) => {
-  const {
-    attributes,
-    listeners,
-    setNodeRef,
-    transform,
-    transition,
-    isDragging,
-  } = useSortable({ id, disabled });
-
-  const transformStyle = {
-    transform: CSS.Transform.toString(transform),
-    transition,
-    opacity: isDragging ? 0.5 : 1,
-  };
-
   return (
-    <div ref={setNodeRef} style={transformStyle} className={containerStyle}>
-      {FEATURE_FLAGS.REORDER_TRANSITION_ARCS && (
-        <div
-          {...attributes}
-          {...listeners}
-          className={dragHandleStyle({ isDisabled: disabled })}
-        >
-          <MdDragIndicator size={16} />
-        </div>
-      )}
+    <div className={containerStyle}>
       <div className={placeNameStyle}>{placeName}</div>
       <div className={weightContainerStyle}>
         <span className={weightLabelStyle}>weight</span>
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
index f3bd01ef93e..73a4b2a1e3e 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
@@ -1,20 +1,5 @@
 /* eslint-disable id-length */
 /* eslint-disable curly */
-import type { DragEndEvent } from "@dnd-kit/core";
-import {
-  closestCenter,
-  DndContext,
-  KeyboardSensor,
-  PointerSensor,
-  useSensor,
-  useSensors,
-} from "@dnd-kit/core";
-import {
-  arrayMove,
-  SortableContext,
-  sortableKeyboardCoordinates,
-  verticalListSortingStrategy,
-} from "@dnd-kit/sortable";
 import { css } from "@hashintel/ds-helpers/css";
 import { use } from "react";
 import { TbDotsVertical, TbSparkles, TbTrash } from "react-icons/tb";
@@ -34,7 +19,7 @@ import { CodeEditor } from "../../../../monaco/code-editor";
 import { EditorContext } from "../../../../state/editor-context";
 import { SDCPNContext } from "../../../../state/sdcpn-context";
 import { useIsReadOnly } from "../../../../state/use-is-read-only";
-import { SortableArcItem } from "./sortable-arc-item";
+import { ArcItem } from "./sortable-arc-item";
 
 const containerStyle = css({
   display: "flex",
@@ -182,55 +167,6 @@ export const TransitionProperties: React.FC<TransitionPropertiesProps> = ({
   const isReadOnly = useIsReadOnly();
   const { globalMode } = use(EditorContext);
 
-  const sensors = useSensors(
-    useSensor(PointerSensor),
-    useSensor(KeyboardSensor, {
-      coordinateGetter: sortableKeyboardCoordinates,
-    }),
-  );
-
-  const handleInputArcDragEnd = (event: DragEndEvent) => {
-    const { active, over } = event;
-
-    if (over && active.id !== over.id) {
-      const oldIndex = transition.inputArcs.findIndex(
-        (arc) => arc.placeId === active.id,
-      );
-      const newIndex = transition.inputArcs.findIndex(
-        (arc) => arc.placeId === over.id,
-      );
-
-      updateTransition(transition.id, (existingTransition) => {
-        existingTransition.inputArcs = arrayMove(
-          existingTransition.inputArcs,
-          oldIndex,
-          newIndex,
-        );
-      });
-    }
-  };
-
-  const handleOutputArcDragEnd = (event: DragEndEvent) => {
-    const { active, over } = event;
-
-    if (over && active.id !== over.id) {
-      const oldIndex = transition.outputArcs.findIndex(
-        (arc) => arc.placeId === active.id,
-      );
-      const newIndex = transition.outputArcs.findIndex(
-        (arc) => arc.placeId === over.id,
-      );
-
-      updateTransition(transition.id, (existingTransition) => {
-        existingTransition.outputArcs = arrayMove(
-          existingTransition.outputArcs,
-          oldIndex,
-          newIndex,
-        );
-      });
-    }
-  };
-
   const handleDeleteInputArc = (placeId: string) => {
     updateTransition(transition.id, (existingTransition) => {
       const index = existingTransition.inputArcs.findIndex(
@@ -308,43 +244,29 @@ export const TransitionProperties: React.FC<TransitionPropertiesProps> = ({
           </div>
         ) : (
           <div className={arcListContainerStyle}>
-            <DndContext
-              sensors={isReadOnly ? [] : sensors}
-              collisionDetection={closestCenter}
-              onDragEnd={handleInputArcDragEnd}
-            >
-              <SortableContext
-                items={transition.inputArcs.map((arc) => arc.placeId)}
-                strategy={verticalListSortingStrategy}
-              >
-                {transition.inputArcs.map((arc) => {
-                  const place = places.find(
-                    (placeItem) => placeItem.id === arc.placeId,
-                  );
-                  return (
-                    <SortableArcItem
-                      key={arc.placeId}
-                      id={arc.placeId}
-                      placeName={place?.name ?? arc.placeId}
-                      weight={arc.weight}
-                      disabled={isReadOnly}
-                      tooltip={
-                        isReadOnly ? UI_MESSAGES.READ_ONLY_MODE : undefined
-                      }
-                      onWeightChange={(weight) => {
-                        onArcWeightUpdate(
-                          transition.id,
-                          "input",
-                          arc.placeId,
-                          weight,
-                        );
-                      }}
-                      onDelete={() => handleDeleteInputArc(arc.placeId)}
-                    />
-                  );
-                })}
-              </SortableContext>
-            </DndContext>
+            {transition.inputArcs.map((arc) => {
+              const place = places.find(
+                (placeItem) => placeItem.id === arc.placeId,
+              );
+              return (
+                <ArcItem
+                  key={arc.placeId}
+                  placeName={place?.name ?? arc.placeId}
+                  weight={arc.weight}
+                  disabled={isReadOnly}
+                  tooltip={isReadOnly ? UI_MESSAGES.READ_ONLY_MODE : undefined}
+                  onWeightChange={(weight) => {
+                    onArcWeightUpdate(
+                      transition.id,
+                      "input",
+                      arc.placeId,
+                      weight,
+                    );
+                  }}
+                  onDelete={() => handleDeleteInputArc(arc.placeId)}
+                />
+              );
+            })}
           </div>
         )}
       </div>
@@ -357,43 +279,29 @@ export const TransitionProperties: React.FC<TransitionPropertiesProps> = ({
           </div>
         ) : (
           <div className={arcListContainerStyle}>
-            <DndContext
-              sensors={isReadOnly ? [] : sensors}
-              collisionDetection={closestCenter}
-              onDragEnd={handleOutputArcDragEnd}
-            >
-              <SortableContext
-                items={transition.outputArcs.map((arc) => arc.placeId)}
-                strategy={verticalListSortingStrategy}
-              >
-                {transition.outputArcs.map((arc) => {
-                  const place = places.find(
-                    (placeItem) => placeItem.id === arc.placeId,
-                  );
-                  return (
-                    <SortableArcItem
-                      key={arc.placeId}
-                      id={arc.placeId}
-                      placeName={place?.name ?? arc.placeId}
-                      weight={arc.weight}
-                      disabled={isReadOnly}
-                      tooltip={
-                        isReadOnly ? UI_MESSAGES.READ_ONLY_MODE : undefined
-                      }
-                      onWeightChange={(weight) => {
-                        onArcWeightUpdate(
-                          transition.id,
-                          "output",
-                          arc.placeId,
-                          weight,
-                        );
-                      }}
-                      onDelete={() => handleDeleteOutputArc(arc.placeId)}
-                    />
-                  );
-                })}
-              </SortableContext>
-            </DndContext>
+            {transition.outputArcs.map((arc) => {
+              const place = places.find(
+                (placeItem) => placeItem.id === arc.placeId,
+              );
+              return (
+                <ArcItem
+                  key={arc.placeId}
+                  placeName={place?.name ?? arc.placeId}
+                  weight={arc.weight}
+                  disabled={isReadOnly}
+                  tooltip={isReadOnly ? UI_MESSAGES.READ_ONLY_MODE : undefined}
+                  onWeightChange={(weight) => {
+                    onArcWeightUpdate(
+                      transition.id,
+                      "output",
+                      arc.placeId,
+                      weight,
+                    );
+                  }}
+                  onDelete={() => handleDeleteOutputArc(arc.placeId)}
+                />
+              );
+            })}
           </div>
         )}
       </div>

From 806b1c72dd339fd1052e649c9d0917e68a88958d Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 19 Feb 2026 17:43:19 +0100
Subject: [PATCH 07/22] H-5839: Remove rollup-plugin-visualizer dependency

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 libs/@hashintel/petrinaut/package.json |  1 -
 yarn.lock                              | 27 ++------------------------
 2 files changed, 2 insertions(+), 26 deletions(-)

diff --git a/libs/@hashintel/petrinaut/package.json b/libs/@hashintel/petrinaut/package.json
index 78fa44ebf01..c07cdbc5008 100644
--- a/libs/@hashintel/petrinaut/package.json
+++ b/libs/@hashintel/petrinaut/package.json
@@ -77,7 +77,6 @@
     "jsdom": "24.1.3",
     "react": "19.2.3",
     "react-dom": "19.2.3",
-    "rollup-plugin-visualizer": "6.0.5",
     "vite": "8.0.0-beta.14",
     "vite-plugin-dts": "4.5.4",
     "vitest": "4.0.18"
diff --git a/yarn.lock b/yarn.lock
index 36d17909c59..408032f7a54 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -7824,7 +7824,6 @@ __metadata:
     react-dom: "npm:19.2.3"
     react-icons: "npm:5.5.0"
     reactflow: "npm:11.11.4"
-    rollup-plugin-visualizer: "npm:6.0.5"
     typescript: "npm:5.9.3"
     uuid: "npm:13.0.0"
     vite: "npm:8.0.0-beta.14"
@@ -36725,7 +36724,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"open@npm:^8.0.0, open@npm:^8.0.4, open@npm:^8.4.2":
+"open@npm:^8.0.4, open@npm:^8.4.2":
   version: 8.4.2
   resolution: "open@npm:8.4.2"
   dependencies:
@@ -40632,28 +40631,6 @@ __metadata:
   languageName: node
   linkType: hard
 
-"rollup-plugin-visualizer@npm:6.0.5":
-  version: 6.0.5
-  resolution: "rollup-plugin-visualizer@npm:6.0.5"
-  dependencies:
-    open: "npm:^8.0.0"
-    picomatch: "npm:^4.0.2"
-    source-map: "npm:^0.7.4"
-    yargs: "npm:^17.5.1"
-  peerDependencies:
-    rolldown: 1.x || ^1.0.0-beta
-    rollup: 2.x || 3.x || 4.x
-  peerDependenciesMeta:
-    rolldown:
-      optional: true
-    rollup:
-      optional: true
-  bin:
-    rollup-plugin-visualizer: dist/bin/cli.js
-  checksum: 10c0/3824626e97d5033fbb3aa1bbe93c8c17a8569bc47e33c941bde6b90404f2cae70b26fec1b623bd393c3e076338014196c91726ed2c96218edc67e1f21676f7ef
-  languageName: node
-  linkType: hard
-
 "rollup@npm:4.57.1, rollup@npm:^4.34.9, rollup@npm:^4.35.0, rollup@npm:^4.43.0":
   version: 4.57.1
   resolution: "rollup@npm:4.57.1"
@@ -46420,7 +46397,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"yargs@npm:17.7.2, yargs@npm:^17.0.0, yargs@npm:^17.0.1, yargs@npm:^17.5.1, yargs@npm:^17.7.1, yargs@npm:^17.7.2":
+"yargs@npm:17.7.2, yargs@npm:^17.0.0, yargs@npm:^17.0.1, yargs@npm:^17.7.1, yargs@npm:^17.7.2":
   version: 17.7.2
   resolution: "yargs@npm:17.7.2"
   dependencies:

From 56eba32298c248fb3c543f71d113967df5ed2122 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 19 Feb 2026 18:49:09 +0100
Subject: [PATCH 08/22] H-5839: Wire checker diagnostics into Monaco editors as
 inline markers

Bridge CheckerContext diagnostics to Monaco model markers so CodeEditor
instances show squiggly underlines for type errors. Centralizes editor
model paths in a single getEditorPath function used by both CodeEditor
path props and the new DiagnosticsSync component.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../petrinaut/src/monaco/diagnostics-sync.tsx | 108 ++++++++++++++++++
 .../petrinaut/src/monaco/editor-paths.ts      |  16 +++
 .../petrinaut/src/monaco/provider.tsx         |   2 +
 .../differential-equation-properties.tsx      |   3 +-
 .../PropertiesPanel/transition-properties.tsx |   5 +-
 5 files changed, 131 insertions(+), 3 deletions(-)
 create mode 100644 libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
 create mode 100644 libs/@hashintel/petrinaut/src/monaco/editor-paths.ts

diff --git a/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
new file mode 100644
index 00000000000..9bd907d02d5
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
@@ -0,0 +1,108 @@
+import type * as Monaco from "monaco-editor";
+import { Suspense, use, useEffect, useRef } from "react";
+
+import { CheckerContext } from "../checker/context";
+import type { CheckerDiagnostic } from "../checker/worker/protocol";
+import { MonacoContext } from "./context";
+import { getEditorPath } from "./editor-paths";
+
+const OWNER = "checker";
+
+/** Convert ts.DiagnosticCategory number to Monaco MarkerSeverity. */
+function toMarkerSeverity(
+  category: number,
+  monaco: typeof Monaco,
+): Monaco.MarkerSeverity {
+  switch (category) {
+    case 0:
+      return monaco.MarkerSeverity.Warning;
+    case 1:
+      return monaco.MarkerSeverity.Error;
+    case 2:
+      return monaco.MarkerSeverity.Hint;
+    case 3:
+      return monaco.MarkerSeverity.Info;
+    default:
+      return monaco.MarkerSeverity.Error;
+  }
+}
+
+/** Convert CheckerDiagnostic[] to IMarkerData[] using the model for offset→position. */
+function diagnosticsToMarkers(
+  model: Monaco.editor.ITextModel,
+  diagnostics: CheckerDiagnostic[],
+  monaco: typeof Monaco,
+): Monaco.editor.IMarkerData[] {
+  return diagnostics.map((diag) => {
+    const start = model.getPositionAt(diag.start ?? 0);
+    const end = model.getPositionAt((diag.start ?? 0) + (diag.length ?? 0));
+    return {
+      severity: toMarkerSeverity(diag.category, monaco),
+      message: diag.messageText,
+      startLineNumber: start.lineNumber,
+      startColumn: start.column,
+      endLineNumber: end.lineNumber,
+      endColumn: end.column,
+      code: String(diag.code),
+    };
+  });
+}
+
+const DiagnosticsSyncInner = () => {
+  const { monaco } = use(use(MonacoContext));
+  const { checkResult } = use(CheckerContext);
+  const prevPathsRef = useRef<Set<string>>(new Set());
+
+  useEffect(() => {
+    const currentPaths = new Set<string>();
+
+    for (const item of checkResult.itemDiagnostics) {
+      const path = getEditorPath(item.itemType, item.itemId);
+      const uri = monaco.Uri.parse(path);
+      const model = monaco.editor.getModel(uri);
+      if (model) {
+        const markers = diagnosticsToMarkers(model, item.diagnostics, monaco);
+        monaco.editor.setModelMarkers(model, OWNER, markers);
+      }
+      currentPaths.add(path);
+    }
+
+    // Clear markers from models that no longer have diagnostics
+    for (const path of prevPathsRef.current) {
+      if (!currentPaths.has(path)) {
+        const uri = monaco.Uri.parse(path);
+        const model = monaco.editor.getModel(uri);
+        if (model) {
+          monaco.editor.setModelMarkers(model, OWNER, []);
+        }
+      }
+    }
+
+    prevPathsRef.current = currentPaths;
+
+    // Handle models created after diagnostics arrived
+    const disposable = monaco.editor.onDidCreateModel((model) => {
+      const modelUri = model.uri.toString();
+      const item = checkResult.itemDiagnostics.find(
+        (i) =>
+          monaco.Uri.parse(getEditorPath(i.itemType, i.itemId)).toString() ===
+          modelUri,
+      );
+      if (item) {
+        const markers = diagnosticsToMarkers(model, item.diagnostics, monaco);
+        monaco.editor.setModelMarkers(model, OWNER, markers);
+      }
+    });
+
+    return () => disposable.dispose();
+  }, [checkResult, monaco]);
+
+  return null;
+};
+
+/** Renders nothing visible — syncs CheckerContext diagnostics to Monaco model markers. */
+export const DiagnosticsSync: React.FC = () => (
+  <Suspense fallback={null}>
+    <DiagnosticsSyncInner />
+  </Suspense>
+);
diff --git a/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts b/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
new file mode 100644
index 00000000000..cef45bd237d
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
@@ -0,0 +1,16 @@
+import type { CheckerItemDiagnostics } from "../checker/worker/protocol";
+
+/** Generates the Monaco model path for a given SDCPN item. */
+export function getEditorPath(
+  itemType: CheckerItemDiagnostics["itemType"],
+  itemId: string,
+): string {
+  switch (itemType) {
+    case "transition-lambda":
+      return `inmemory://sdcpn/transitions/${itemId}/lambda.ts`;
+    case "transition-kernel":
+      return `inmemory://sdcpn/transitions/${itemId}/kernel.ts`;
+    case "differential-equation":
+      return `inmemory://sdcpn/differential-equations/${itemId}.ts`;
+  }
+}
diff --git a/libs/@hashintel/petrinaut/src/monaco/provider.tsx b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
index 7922ec5fce5..ce18102f437 100644
--- a/libs/@hashintel/petrinaut/src/monaco/provider.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
@@ -2,6 +2,7 @@ import type * as Monaco from "monaco-editor";
 
 import type { MonacoContextValue } from "./context";
 import { MonacoContext } from "./context";
+import { DiagnosticsSync } from "./diagnostics-sync";
 
 interface LanguageDefaults {
   setModeConfiguration(config: Record<string, boolean>): void;
@@ -100,6 +101,7 @@ export const MonacoProvider: React.FC<{ children: React.ReactNode }> = ({
 
   return (
     <MonacoContext.Provider value={monacoPromise}>
+      <DiagnosticsSync />
       {children}
     </MonacoContext.Provider>
   );
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx
index 330dbc1e7c4..6556c3f2712 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx
@@ -19,6 +19,7 @@ import type {
   Place,
 } from "../../../../core/types/sdcpn";
 import { CodeEditor } from "../../../../monaco/code-editor";
+import { getEditorPath } from "../../../../monaco/editor-paths";
 import { useIsReadOnly } from "../../../../state/use-is-read-only";
 
 const containerStyle = css({
@@ -476,7 +477,7 @@ export const DifferentialEquationProperties: React.FC<
           )}
         </div>
         <CodeEditor
-          path={`inmemory://sdcpn/differential-equations/${differentialEquation.id}.ts`}
+          path={getEditorPath("differential-equation", differentialEquation.id)}
           language="typescript"
           value={differentialEquation.code}
           height="100%"
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
index 73a4b2a1e3e..b803b51c5fd 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
@@ -16,6 +16,7 @@ import {
 } from "../../../../core/default-codes";
 import type { Color, Place, Transition } from "../../../../core/types/sdcpn";
 import { CodeEditor } from "../../../../monaco/code-editor";
+import { getEditorPath } from "../../../../monaco/editor-paths";
 import { EditorContext } from "../../../../state/editor-context";
 import { SDCPNContext } from "../../../../state/sdcpn-context";
 import { useIsReadOnly } from "../../../../state/use-is-read-only";
@@ -388,7 +389,7 @@ export const TransitionProperties: React.FC<TransitionPropertiesProps> = ({
           )}
         </div>
         <CodeEditor
-          path={`inmemory://sdcpn/transitions/${transition.id}/lambda.ts`}
+          path={getEditorPath("transition-lambda", transition.id)}
           key={`lambda-${transition.lambdaType}-${transition.inputArcs
             .map((a) => `${a.placeId}:${a.weight}`)
             .join("-")}`}
@@ -491,7 +492,7 @@ export const TransitionProperties: React.FC<TransitionPropertiesProps> = ({
             )}
           </div>
           <CodeEditor
-            path={`inmemory://sdcpn/transitions/${transition.id}/transition-kernel.ts`}
+            path={getEditorPath("transition-kernel", transition.id)}
             language="typescript"
             value={transition.transitionKernelCode || ""}
             height={400}

From 0ef8aa70d9add87f1da74cade46ff4247991fb8f Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Fri, 20 Feb 2026 02:43:49 +0100
Subject: [PATCH 09/22] H-5839: Add completions, hover, and signature help
 providers to Monaco editors

Wire TypeScript LanguageService features through the checker WebWorker
to Monaco editors via JSON-RPC. The LanguageServiceHost is now mutable
(per-file version tracking) so completions reflect current editor state.

- Completions: registerCompletionItemProvider with ScriptElementKind mapping
- Hover: registerHoverProvider backed by getQuickInfoAtPosition
- Signature help: registerSignatureHelpProvider backed by getSignatureHelpItems
- All three adjust offsets to account for injected declaration prefixes
- Add tests for completions and updateFileContent in the language service

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../petrinaut/src/checker/context.ts          |  29 +-
 .../petrinaut/src/checker/lib/checker.ts      |  13 +-
 .../lib/create-language-service-host.ts       |  23 +-
 .../lib/create-sdcpn-language-service.test.ts | 311 ++++++++++++++++++
 .../lib/create-sdcpn-language-service.ts      |  37 ++-
 .../petrinaut/src/checker/provider.tsx        |  10 +-
 .../src/checker/worker/checker.worker.ts      | 214 ++++++++++--
 .../petrinaut/src/checker/worker/protocol.ts  | 101 +++++-
 .../src/checker/worker/use-checker-worker.ts  | 121 +++++--
 .../petrinaut/src/monaco/completion-sync.tsx  | 127 +++++++
 .../petrinaut/src/monaco/editor-paths.ts      |  30 ++
 .../petrinaut/src/monaco/hover-sync.tsx       |  58 ++++
 .../petrinaut/src/monaco/provider.tsx         |  39 +--
 .../src/monaco/signature-help-sync.tsx        |  73 ++++
 14 files changed, 1093 insertions(+), 93 deletions(-)
 create mode 100644 libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.test.ts
 create mode 100644 libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
 create mode 100644 libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
 create mode 100644 libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx

diff --git a/libs/@hashintel/petrinaut/src/checker/context.ts b/libs/@hashintel/petrinaut/src/checker/context.ts
index c312b234f96..d95dea43201 100644
--- a/libs/@hashintel/petrinaut/src/checker/context.ts
+++ b/libs/@hashintel/petrinaut/src/checker/context.ts
@@ -1,12 +1,36 @@
 import { createContext } from "react";
 
-import type { CheckerResult } from "./worker/protocol";
+import type {
+  CheckerCompletionResult,
+  CheckerItemDiagnostics,
+  CheckerQuickInfoResult,
+  CheckerResult,
+  CheckerSignatureHelpResult,
+} from "./worker/protocol";
 
 export interface CheckerContextValue {
   /** Result of the last SDCPN validation run. */
   checkResult: CheckerResult;
   /** Total number of diagnostics across all items. */
   totalDiagnosticsCount: number;
+  /** Request completions at a position within an SDCPN item. */
+  getCompletions: (
+    itemType: CheckerItemDiagnostics["itemType"],
+    itemId: string,
+    offset: number,
+  ) => Promise<CheckerCompletionResult>;
+  /** Request quick info (hover) at a position within an SDCPN item. */
+  getQuickInfo: (
+    itemType: CheckerItemDiagnostics["itemType"],
+    itemId: string,
+    offset: number,
+  ) => Promise<CheckerQuickInfoResult>;
+  /** Request signature help at a position within an SDCPN item. */
+  getSignatureHelp: (
+    itemType: CheckerItemDiagnostics["itemType"],
+    itemId: string,
+    offset: number,
+  ) => Promise<CheckerSignatureHelpResult>;
 }
 
 const DEFAULT_CONTEXT_VALUE: CheckerContextValue = {
@@ -15,6 +39,9 @@ const DEFAULT_CONTEXT_VALUE: CheckerContextValue = {
     itemDiagnostics: [],
   },
   totalDiagnosticsCount: 0,
+  getCompletions: () => Promise.resolve({ items: [] }),
+  getQuickInfo: () => Promise.resolve(null),
+  getSignatureHelp: () => Promise.resolve(null),
 };
 
 export const CheckerContext = createContext<CheckerContextValue>(
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/checker.ts b/libs/@hashintel/petrinaut/src/checker/lib/checker.ts
index 349b3aeddf1..61a5d27799a 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/checker.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/checker.ts
@@ -1,7 +1,10 @@
 import type ts from "typescript";
 
 import type { SDCPN } from "../../core/types/sdcpn";
-import { createSDCPNLanguageService } from "./create-sdcpn-language-service";
+import {
+  createSDCPNLanguageService,
+  type SDCPNLanguageService,
+} from "./create-sdcpn-language-service";
 import { getItemFilePath } from "./file-paths";
 
 export type SDCPNDiagnostic = {
@@ -29,8 +32,12 @@ export type SDCPNCheckResult = {
  * @param sdcpn - The SDCPN to check
  * @returns A result object indicating validity and any diagnostics
  */
-export function checkSDCPN(sdcpn: SDCPN): SDCPNCheckResult {
-  const languageService = createSDCPNLanguageService(sdcpn);
+export function checkSDCPN(
+  sdcpn: SDCPN,
+  existingLanguageService?: SDCPNLanguageService,
+): SDCPNCheckResult {
+  const languageService =
+    existingLanguageService ?? createSDCPNLanguageService(sdcpn);
   const itemDiagnostics: SDCPNDiagnostic[] = [];
 
   // Check all differential equations
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts b/libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts
index 3e85556c015..4e22135e8ce 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts
@@ -42,9 +42,12 @@ export type VirtualFile = {
  *
  * Creates a TypeScript LanguageServiceHost for virtual SDCPN files
  */
-export function createLanguageServiceHost(
-  files: Map<string, VirtualFile>,
-): ts.LanguageServiceHost {
+export function createLanguageServiceHost(files: Map<string, VirtualFile>): {
+  host: ts.LanguageServiceHost;
+  updateFileContent: (fileName: string, content: string) => void;
+} {
+  const versions = new Map<string, number>();
+
   const getFileContent = (fileName: string): string | undefined => {
     const entry = files.get(fileName);
     if (entry) {
@@ -62,10 +65,18 @@ export function createLanguageServiceHost(
     return undefined;
   };
 
-  return {
+  const updateFileContent = (fileName: string, content: string) => {
+    const entry = files.get(fileName);
+    if (entry) {
+      entry.content = content;
+      versions.set(fileName, (versions.get(fileName) ?? 0) + 1);
+    }
+  };
+
+  const host: ts.LanguageServiceHost = {
     getScriptFileNames: () => [...files.keys()],
     getCompilationSettings: () => COMPILER_OPTIONS,
-    getScriptVersion: () => "0",
+    getScriptVersion: (fileName) => String(versions.get(fileName) ?? 0),
     getCurrentDirectory: () => "/",
     getDefaultLibFileName: () => "/lib.es2015.core.d.ts",
 
@@ -82,4 +93,6 @@ export function createLanguageServiceHost(
       return getFileContent(path);
     },
   };
+
+  return { host, updateFileContent };
 }
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.test.ts b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.test.ts
new file mode 100644
index 00000000000..791b66f2c23
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.test.ts
@@ -0,0 +1,311 @@
+import { describe, expect, it } from "vitest";
+
+import { createSDCPNLanguageService } from "./create-sdcpn-language-service";
+import { getItemFilePath } from "./file-paths";
+import { createSDCPN } from "./helper/create-sdcpn";
+
+/** Cursor marker used in test code strings to indicate the completion position. */
+const CURSOR = "∫";
+
+/**
+ * Find the offset of a cursor marker in user code.
+ * Returns the offset (without the marker) and the clean code.
+ */
+function parseCursor(codeWithCursor: string): {
+  offset: number;
+  code: string;
+} {
+  const offset = codeWithCursor.indexOf(CURSOR);
+  if (offset === -1) {
+    throw new Error(`No cursor marker \`${CURSOR}\` found in code`);
+  }
+  const code =
+    codeWithCursor.slice(0, offset) + codeWithCursor.slice(offset + 1);
+  return { offset, code };
+}
+
+/** Extract just the completion names from a position. */
+function getCompletionNames(
+  sdcpnOptions: Parameters<typeof createSDCPN>[0],
+  codeWithCursor: string,
+  target:
+    | { type: "transition-lambda"; transitionId?: string }
+    | { type: "transition-kernel"; transitionId?: string }
+    | { type: "differential-equation"; deId?: string },
+): string[] {
+  const { offset, code } = parseCursor(codeWithCursor);
+
+  // Patch the SDCPN to inject the clean code at the right item
+  const patched = { ...sdcpnOptions };
+  if (
+    target.type === "transition-lambda" ||
+    target.type === "transition-kernel"
+  ) {
+    const transitionId = target.transitionId ?? "t1";
+    patched.transitions = (patched.transitions ?? []).map((tr) =>
+      (tr.id ?? "t1") === transitionId
+        ? {
+            ...tr,
+            ...(target.type === "transition-lambda"
+              ? { lambdaCode: code }
+              : { transitionKernelCode: code }),
+          }
+        : tr,
+    );
+  } else {
+    const deId = target.deId ?? "de_1";
+    patched.differentialEquations = (patched.differentialEquations ?? []).map(
+      (de, index) =>
+        (de.id ?? `de_${index + 1}`) === deId ? { ...de, code } : de,
+    );
+  }
+
+  const sdcpn = createSDCPN(patched);
+  const ls = createSDCPNLanguageService(sdcpn);
+
+  let filePath: string;
+  if (target.type === "transition-lambda") {
+    filePath = getItemFilePath("transition-lambda-code", {
+      transitionId: target.transitionId ?? "t1",
+    });
+  } else if (target.type === "transition-kernel") {
+    filePath = getItemFilePath("transition-kernel-code", {
+      transitionId: target.transitionId ?? "t1",
+    });
+  } else {
+    filePath = getItemFilePath("differential-equation-code", {
+      id: target.deId ?? "de_1",
+    });
+  }
+
+  const completions = ls.getCompletionsAtPosition(filePath, offset, undefined);
+  return (completions?.entries ?? []).map((entry) => entry.name);
+}
+
+describe("createSDCPNLanguageService completions", () => {
+  const baseSdcpn = {
+    types: [{ id: "color1", elements: [{ name: "x", type: "real" as const }] }],
+    places: [
+      { id: "place1", name: "Source", colorId: "color1" },
+      { id: "place2", name: "Target", colorId: "color1" },
+    ],
+    parameters: [
+      { id: "p1", variableName: "alpha", type: "real" as const },
+      { id: "p2", variableName: "enabled", type: "boolean" as const },
+    ],
+    transitions: [
+      {
+        id: "t1",
+        lambdaType: "predicate" as const,
+        inputArcs: [{ placeId: "place1", weight: 1 }],
+        outputArcs: [{ placeId: "place2", weight: 1 }],
+        lambdaCode: "",
+        transitionKernelCode: "",
+      },
+    ],
+  };
+
+  describe("member access completions (dot completions)", () => {
+    it("returns Number methods after `a.` where a is a number", () => {
+      const names = getCompletionNames(
+        baseSdcpn,
+        `const a = 42;\na.${CURSOR}`,
+        {
+          type: "transition-lambda",
+        },
+      );
+
+      expect(names).toContain("toFixed");
+      expect(names).toContain("toString");
+      expect(names).toContain("valueOf");
+      // Should NOT contain global scope items
+      expect(names).not.toContain("Array");
+      expect(names).not.toContain("Lambda");
+    });
+
+    it("returns parameter names after `parameters.`", () => {
+      const names = getCompletionNames(
+        baseSdcpn,
+        `export default Lambda((input, parameters) => {\n  return parameters.${CURSOR};\n});`,
+        { type: "transition-lambda" },
+      );
+
+      expect(names).toContain("alpha");
+      expect(names).toContain("enabled");
+      // Should NOT contain globals
+      expect(names).not.toContain("Array");
+    });
+
+    it("returns place names after `input.`", () => {
+      const names = getCompletionNames(
+        baseSdcpn,
+        `export default Lambda((input, parameters) => {\n  return input.${CURSOR};\n});`,
+        { type: "transition-lambda" },
+      );
+
+      expect(names).toContain("Source");
+      // Should NOT contain unrelated globals
+      expect(names).not.toContain("Array");
+    });
+
+    it("returns token properties after `input.Source[0].`", () => {
+      const names = getCompletionNames(
+        baseSdcpn,
+        `export default Lambda((input, parameters) => {\n  const token = input.Source[0];\n  return token.${CURSOR};\n});`,
+        { type: "transition-lambda" },
+      );
+
+      expect(names).toContain("x");
+    });
+
+    it("returns String methods after string expression", () => {
+      const names = getCompletionNames(
+        baseSdcpn,
+        `const s = "hello";\ns.${CURSOR}`,
+        {
+          type: "transition-lambda",
+        },
+      );
+
+      expect(names).toContain("charAt");
+      expect(names).toContain("indexOf");
+      expect(names).not.toContain("Array");
+    });
+  });
+
+  describe("top-level completions (no dot)", () => {
+    it("returns globals and declared identifiers at top level", () => {
+      const names = getCompletionNames(baseSdcpn, `const a = 42;\n${CURSOR}`, {
+        type: "transition-lambda",
+      });
+
+      // Should include user-defined and prefix-declared identifiers
+      expect(names).toContain("a");
+      expect(names).toContain("Lambda");
+      // Should include globals
+      expect(names).toContain("Array");
+    });
+  });
+
+  describe("updateFileContent", () => {
+    it("returns completions for updated code, not original code", () => {
+      // Start with number code
+      const sdcpn = createSDCPN({
+        ...baseSdcpn,
+        transitions: [
+          {
+            ...baseSdcpn.transitions[0]!,
+            lambdaCode: "const a = 42;\na.",
+          },
+        ],
+      });
+
+      const ls = createSDCPNLanguageService(sdcpn);
+      const filePath = getItemFilePath("transition-lambda-code", {
+        transitionId: "t1",
+      });
+
+      // Update to string code
+      const { offset, code } = parseCursor(`const s = "hello";\ns.${CURSOR}`);
+      ls.updateFileContent(filePath, code);
+
+      const completions = ls.getCompletionsAtPosition(
+        filePath,
+        offset,
+        undefined,
+      );
+      const names = (completions?.entries ?? []).map((entry) => entry.name);
+
+      // Should have String methods
+      expect(names).toContain("charAt");
+      expect(names).toContain("indexOf");
+      // Should NOT have Number methods
+      expect(names).not.toContain("toFixed");
+    });
+
+    it("reflects new content after multiple updates", () => {
+      const sdcpn = createSDCPN({
+        ...baseSdcpn,
+        transitions: [
+          {
+            ...baseSdcpn.transitions[0]!,
+            lambdaCode: "",
+          },
+        ],
+      });
+
+      const ls = createSDCPNLanguageService(sdcpn);
+      const filePath = getItemFilePath("transition-lambda-code", {
+        transitionId: "t1",
+      });
+
+      // First update: number
+      const first = parseCursor(`const a = 42;\na.${CURSOR}`);
+      ls.updateFileContent(filePath, first.code);
+      const firstCompletions = ls.getCompletionsAtPosition(
+        filePath,
+        first.offset,
+        undefined,
+      );
+      const firstNames = (firstCompletions?.entries ?? []).map(
+        (entry) => entry.name,
+      );
+      expect(firstNames).toContain("toFixed");
+
+      // Second update: string
+      const second = parseCursor(`const s = "hello";\ns.${CURSOR}`);
+      ls.updateFileContent(filePath, second.code);
+      const secondCompletions = ls.getCompletionsAtPosition(
+        filePath,
+        second.offset,
+        undefined,
+      );
+      const secondNames = (secondCompletions?.entries ?? []).map(
+        (entry) => entry.name,
+      );
+      expect(secondNames).toContain("charAt");
+      expect(secondNames).not.toContain("toFixed");
+    });
+  });
+
+  describe("differential equation completions", () => {
+    const deSdcpn = {
+      types: [
+        {
+          id: "color1",
+          elements: [{ name: "velocity", type: "real" as const }],
+        },
+      ],
+      parameters: [
+        { id: "p1", variableName: "gravity", type: "real" as const },
+      ],
+      differentialEquations: [
+        {
+          id: "de1",
+          colorId: "color1",
+          code: "",
+        },
+      ],
+    };
+
+    it("returns token properties after `tokens[0].`", () => {
+      const names = getCompletionNames(
+        deSdcpn,
+        `export default Dynamics((tokens, parameters) => {\n  const t = tokens[0];\n  return t.${CURSOR};\n});`,
+        { type: "differential-equation", deId: "de1" },
+      );
+
+      expect(names).toContain("velocity");
+    });
+
+    it("returns parameter names after `parameters.`", () => {
+      const names = getCompletionNames(
+        deSdcpn,
+        `export default Dynamics((tokens, parameters) => {\n  return parameters.${CURSOR};\n});`,
+        { type: "differential-equation", deId: "de1" },
+      );
+
+      expect(names).toContain("gravity");
+    });
+  });
+});
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
index 33a05d29a34..02064505ca5 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
@@ -7,7 +7,9 @@ import {
 } from "./create-language-service-host";
 import { getItemFilePath } from "./file-paths";
 
-export type SDCPNLanguageService = ts.LanguageService;
+export type SDCPNLanguageService = ts.LanguageService & {
+  updateFileContent: (fileName: string, content: string) => void;
+};
 
 /**
  * Sanitizes a color ID to be a valid TypeScript identifier.
@@ -250,13 +252,15 @@ function adjustDiagnostics<T extends ts.Diagnostic>(
  */
 export function createSDCPNLanguageService(sdcpn: SDCPN): SDCPNLanguageService {
   const files = generateVirtualFiles(sdcpn);
-  const host = createLanguageServiceHost(files);
+  const { host, updateFileContent } = createLanguageServiceHost(files);
   const baseService = ts.createLanguageService(host);
 
   // Proxy service to adjust positions for injected prefixes
   return {
     ...baseService,
 
+    updateFileContent,
+
     getSemanticDiagnostics(fileName) {
       const entry = files.get(fileName);
       const prefixLength = entry?.prefix?.length ?? 0;
@@ -280,5 +284,34 @@ export function createSDCPNLanguageService(sdcpn: SDCPN): SDCPNLanguageService {
         options,
       );
     },
+
+    getQuickInfoAtPosition(fileName, position) {
+      const entry = files.get(fileName);
+      const prefixLength = entry?.prefix?.length ?? 0;
+      const info = baseService.getQuickInfoAtPosition(
+        fileName,
+        position + prefixLength,
+      );
+      if (!info) {
+        return undefined;
+      }
+      return {
+        ...info,
+        textSpan: {
+          start: info.textSpan.start - prefixLength,
+          length: info.textSpan.length,
+        },
+      };
+    },
+
+    getSignatureHelpItems(fileName, position, options) {
+      const entry = files.get(fileName);
+      const prefixLength = entry?.prefix?.length ?? 0;
+      return baseService.getSignatureHelpItems(
+        fileName,
+        position + prefixLength,
+        options,
+      );
+    },
   };
 }
diff --git a/libs/@hashintel/petrinaut/src/checker/provider.tsx b/libs/@hashintel/petrinaut/src/checker/provider.tsx
index 4e5388cb950..04ef268eef0 100644
--- a/libs/@hashintel/petrinaut/src/checker/provider.tsx
+++ b/libs/@hashintel/petrinaut/src/checker/provider.tsx
@@ -14,14 +14,15 @@ export const CheckerProvider: React.FC<{ children: React.ReactNode }> = ({
   children,
 }) => {
   const { petriNetDefinition } = use(SDCPNContext);
-  const { checkSDCPN } = useCheckerWorker();
+  const { setSDCPN, getCompletions, getQuickInfo, getSignatureHelp } =
+    useCheckerWorker();
 
   const [checkResult, setCheckerResult] = useState<CheckerResult>(EMPTY_RESULT);
 
   useEffect(() => {
     let cancelled = false;
 
-    void checkSDCPN(petriNetDefinition).then((result) => {
+    void setSDCPN(petriNetDefinition).then((result) => {
       if (!cancelled) {
         setCheckerResult(result);
       }
@@ -30,7 +31,7 @@ export const CheckerProvider: React.FC<{ children: React.ReactNode }> = ({
     return () => {
       cancelled = true;
     };
-  }, [petriNetDefinition, checkSDCPN]);
+  }, [petriNetDefinition, setSDCPN]);
 
   const totalDiagnosticsCount = checkResult.itemDiagnostics.reduce(
     (sum, item) => sum + item.diagnostics.length,
@@ -42,6 +43,9 @@ export const CheckerProvider: React.FC<{ children: React.ReactNode }> = ({
       value={{
         checkResult,
         totalDiagnosticsCount,
+        getCompletions,
+        getQuickInfo,
+        getSignatureHelp,
       }}
     >
       {children}
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
index bbc0f96ad6c..caa4b826ca7 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
+++ b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
@@ -5,18 +5,34 @@
  * Receives JSON-RPC requests via `postMessage`, delegates to `checkSDCPN`,
  * serializes the diagnostics (flatten messageText, strip non-cloneable fields),
  * and posts the result back.
+ *
+ * The LanguageService is persisted between calls so that `getCompletions`
+ * can reuse it without re-sending the full SDCPN model.
  */
 import ts from "typescript";
 
 import { checkSDCPN } from "../lib/checker";
+import {
+  createSDCPNLanguageService,
+  type SDCPNLanguageService,
+} from "../lib/create-sdcpn-language-service";
+import { getItemFilePath } from "../lib/file-paths";
 import type {
+  CheckerCompletionItem,
+  CheckerCompletionResult,
   CheckerDiagnostic,
   CheckerItemDiagnostics,
+  CheckerQuickInfoResult,
   CheckerResult,
+  CheckerSignatureHelpResult,
+  CheckerSignatureInfo,
   JsonRpcRequest,
   JsonRpcResponse,
 } from "./protocol";
 
+/** Persisted LanguageService — created on `setSDCPN`, reused by `getCompletions`. */
+let languageService: SDCPNLanguageService | null = null;
+
 /** Strip `ts.SourceFile` and flatten `DiagnosticMessageChain` for structured clone. */
 function serializeDiagnostic(diag: ts.Diagnostic): CheckerDiagnostic {
   return {
@@ -28,27 +44,187 @@ function serializeDiagnostic(diag: ts.Diagnostic): CheckerDiagnostic {
   };
 }
 
-self.onmessage = ({ data: { id, params } }: MessageEvent<JsonRpcRequest>) => {
+/** Map (itemType, itemId) → checker virtual file path. */
+function getCheckerFilePath(
+  itemType: CheckerItemDiagnostics["itemType"],
+  itemId: string,
+): string {
+  switch (itemType) {
+    case "transition-lambda":
+      return getItemFilePath("transition-lambda-code", {
+        transitionId: itemId,
+      });
+    case "transition-kernel":
+      return getItemFilePath("transition-kernel-code", {
+        transitionId: itemId,
+      });
+    case "differential-equation":
+      return getItemFilePath("differential-equation-code", { id: itemId });
+  }
+}
+
+self.onmessage = async ({ data }: MessageEvent<JsonRpcRequest>) => {
+  const { id, method } = data;
+
   try {
-    const raw = checkSDCPN(params.sdcpn);
-
-    const result: CheckerResult = {
-      isValid: raw.isValid,
-      itemDiagnostics: raw.itemDiagnostics.map(
-        (item): CheckerItemDiagnostics => ({
-          itemId: item.itemId,
-          itemType: item.itemType,
-          filePath: item.filePath,
-          diagnostics: item.diagnostics.map(serializeDiagnostic),
-        }),
-      ),
-    };
+    switch (method) {
+      case "setSDCPN": {
+        const { sdcpn } = data.params;
 
-    self.postMessage({
-      jsonrpc: "2.0",
-      id,
-      result,
-    } satisfies JsonRpcResponse<CheckerResult>);
+        languageService = createSDCPNLanguageService(sdcpn);
+        const raw = checkSDCPN(sdcpn, languageService);
+
+        const result: CheckerResult = {
+          isValid: raw.isValid,
+          itemDiagnostics: raw.itemDiagnostics.map(
+            (item): CheckerItemDiagnostics => ({
+              itemId: item.itemId,
+              itemType: item.itemType,
+              filePath: item.filePath,
+              diagnostics: item.diagnostics.map(serializeDiagnostic),
+            }),
+          ),
+        };
+
+        self.postMessage({
+          jsonrpc: "2.0",
+          id,
+          result,
+        } satisfies JsonRpcResponse<CheckerResult>);
+        break;
+      }
+
+      case "getCompletions": {
+        const { itemType, itemId, offset } = data.params;
+
+        // Wait before requesting completions, to be sure the file content is updated
+        await new Promise<void>((resolve) => {
+          setTimeout(resolve, 50);
+        });
+
+        if (!languageService) {
+          self.postMessage({
+            jsonrpc: "2.0",
+            id,
+            result: { items: [] },
+          } satisfies JsonRpcResponse<CheckerCompletionResult>);
+          break;
+        }
+
+        const filePath = getCheckerFilePath(itemType, itemId);
+        const completions = languageService.getCompletionsAtPosition(
+          filePath,
+          offset,
+          undefined,
+        );
+
+        const items: CheckerCompletionItem[] = (completions?.entries ?? []).map(
+          (entry) => ({
+            name: entry.name,
+            kind: entry.kind,
+            sortText: entry.sortText,
+            insertText: entry.insertText,
+          }),
+        );
+
+        self.postMessage({
+          jsonrpc: "2.0",
+          id,
+          result: { items },
+        } satisfies JsonRpcResponse<CheckerCompletionResult>);
+        break;
+      }
+
+      case "getQuickInfo": {
+        const { itemType, itemId, offset } = data.params;
+
+        if (!languageService) {
+          self.postMessage({
+            jsonrpc: "2.0",
+            id,
+            result: null,
+          } satisfies JsonRpcResponse<CheckerQuickInfoResult>);
+          break;
+        }
+
+        const filePath = getCheckerFilePath(itemType, itemId);
+        const info = languageService.getQuickInfoAtPosition(filePath, offset);
+
+        const result: CheckerQuickInfoResult = info
+          ? {
+              displayParts: ts.displayPartsToString(info.displayParts),
+              documentation: ts.displayPartsToString(info.documentation),
+              start: info.textSpan.start,
+              length: info.textSpan.length,
+            }
+          : null;
+
+        self.postMessage({
+          jsonrpc: "2.0",
+          id,
+          result,
+        } satisfies JsonRpcResponse<CheckerQuickInfoResult>);
+        break;
+      }
+
+      case "getSignatureHelp": {
+        const { itemType, itemId, offset } = data.params;
+
+        // Wait a bit before requesting completions, to be sure the file content is updated
+        await new Promise<void>((resolve) => {
+          setTimeout(resolve, 50);
+        });
+
+        if (!languageService) {
+          self.postMessage({
+            jsonrpc: "2.0",
+            id,
+            result: null,
+          } satisfies JsonRpcResponse<CheckerSignatureHelpResult>);
+          break;
+        }
+
+        const filePath = getCheckerFilePath(itemType, itemId);
+        const help = languageService.getSignatureHelpItems(
+          filePath,
+          offset,
+          undefined,
+        );
+
+        const result: CheckerSignatureHelpResult = help
+          ? {
+              activeSignature: help.selectedItemIndex,
+              activeParameter: help.argumentIndex,
+              signatures: help.items.map(
+                (item): CheckerSignatureInfo => ({
+                  label: [
+                    ...item.prefixDisplayParts,
+                    ...item.parameters.flatMap((param, idx) => [
+                      ...(idx > 0 ? item.separatorDisplayParts : []),
+                      ...param.displayParts,
+                    ]),
+                    ...item.suffixDisplayParts,
+                  ]
+                    .map((part) => part.text)
+                    .join(""),
+                  documentation: ts.displayPartsToString(item.documentation),
+                  parameters: item.parameters.map((param) => ({
+                    label: ts.displayPartsToString(param.displayParts),
+                    documentation: ts.displayPartsToString(param.documentation),
+                  })),
+                }),
+              ),
+            }
+          : null;
+
+        self.postMessage({
+          jsonrpc: "2.0",
+          id,
+          result,
+        } satisfies JsonRpcResponse<CheckerSignatureHelpResult>);
+        break;
+      }
+    }
   } catch (err) {
     self.postMessage({
       jsonrpc: "2.0",
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts b/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
index fc63218f0a3..7e52045904c 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
+++ b/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
@@ -45,17 +45,106 @@ export type CheckerResult = {
   itemDiagnostics: CheckerItemDiagnostics[];
 };
 
+// ---------------------------------------------------------------------------
+// Completions — serializable variants of ts.CompletionEntry
+// ---------------------------------------------------------------------------
+
+/** A single completion suggestion, safe for structured clone. */
+export type CheckerCompletionItem = {
+  name: string;
+  /** @see ts.ScriptElementKind */
+  kind: string;
+  sortText: string;
+  insertText?: string;
+};
+
+/** Result of requesting completions at a position. */
+export type CheckerCompletionResult = {
+  items: CheckerCompletionItem[];
+};
+
+// ---------------------------------------------------------------------------
+// Quick Info (hover) — serializable variant of ts.QuickInfo
+// ---------------------------------------------------------------------------
+
+/** Result of requesting quick info (hover) at a position. */
+export type CheckerQuickInfoResult = {
+  /** Type/signature display string. */
+  displayParts: string;
+  /** JSDoc documentation string. */
+  documentation: string;
+  /** Offset in user code where the hovered symbol starts. */
+  start: number;
+  /** Length of the hovered symbol span. */
+  length: number;
+} | null;
+
+// ---------------------------------------------------------------------------
+// Signature Help — serializable variant of ts.SignatureHelpItems
+// ---------------------------------------------------------------------------
+
+/** A single parameter in a signature. */
+export type CheckerSignatureParameter = {
+  label: string;
+  documentation: string;
+};
+
+/** A single signature (overload). */
+export type CheckerSignatureInfo = {
+  label: string;
+  documentation: string;
+  parameters: CheckerSignatureParameter[];
+};
+
+/** Result of requesting signature help at a position. */
+export type CheckerSignatureHelpResult = {
+  signatures: CheckerSignatureInfo[];
+  activeSignature: number;
+  activeParameter: number;
+} | null;
+
 // ---------------------------------------------------------------------------
 // JSON-RPC 2.0
 // ---------------------------------------------------------------------------
 
 /** A JSON-RPC request sent from the main thread to the worker. */
-export type JsonRpcRequest = {
-  jsonrpc: "2.0";
-  id: number;
-  method: "checkSDCPN";
-  params: { sdcpn: SDCPN };
-};
+export type JsonRpcRequest =
+  | {
+      jsonrpc: "2.0";
+      id: number;
+      method: "setSDCPN";
+      params: { sdcpn: SDCPN };
+    }
+  | {
+      jsonrpc: "2.0";
+      id: number;
+      method: "getCompletions";
+      params: {
+        itemType: CheckerItemDiagnostics["itemType"];
+        itemId: string;
+        offset: number;
+      };
+    }
+  | {
+      jsonrpc: "2.0";
+      id: number;
+      method: "getQuickInfo";
+      params: {
+        itemType: CheckerItemDiagnostics["itemType"];
+        itemId: string;
+        offset: number;
+      };
+    }
+  | {
+      jsonrpc: "2.0";
+      id: number;
+      method: "getSignatureHelp";
+      params: {
+        itemType: CheckerItemDiagnostics["itemType"];
+        itemId: string;
+        offset: number;
+      };
+    };
 
 /** A JSON-RPC response sent from the worker back to the main thread. */
 export type JsonRpcResponse<Result = unknown> =
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts b/libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts
index e5627c34cd6..82d63cbca62 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts
+++ b/libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts
@@ -1,21 +1,43 @@
-import { useEffect, useRef } from "react";
+import { useCallback, useEffect, useRef } from "react";
 
 import type { SDCPN } from "../../core/types/sdcpn";
 import type {
+  CheckerCompletionResult,
+  CheckerItemDiagnostics,
+  CheckerQuickInfoResult,
   CheckerResult,
+  CheckerSignatureHelpResult,
   JsonRpcRequest,
   JsonRpcResponse,
 } from "./protocol";
 
 type Pending = {
-  resolve: (result: CheckerResult) => void;
+  resolve: (result: never) => void;
   reject: (error: Error) => void;
 };
 
 /** Methods exposed by the checker WebWorker. */
 export type CheckerWorkerApi = {
-  /** Validate all user code in an SDCPN model. Runs off the main thread. */
-  checkSDCPN: (sdcpn: SDCPN) => Promise<CheckerResult>;
+  /** Send an SDCPN model to the worker. Persists the LanguageService and returns diagnostics. */
+  setSDCPN: (sdcpn: SDCPN) => Promise<CheckerResult>;
+  /** Request completions at a position within an SDCPN item. */
+  getCompletions: (
+    itemType: CheckerItemDiagnostics["itemType"],
+    itemId: string,
+    offset: number,
+  ) => Promise<CheckerCompletionResult>;
+  /** Request quick info (hover) at a position within an SDCPN item. */
+  getQuickInfo: (
+    itemType: CheckerItemDiagnostics["itemType"],
+    itemId: string,
+    offset: number,
+  ) => Promise<CheckerQuickInfoResult>;
+  /** Request signature help at a position within an SDCPN item. */
+  getSignatureHelp: (
+    itemType: CheckerItemDiagnostics["itemType"],
+    itemId: string,
+    offset: number,
+  ) => Promise<CheckerSignatureHelpResult>;
 };
 
 /**
@@ -32,9 +54,7 @@ export function useCheckerWorker(): CheckerWorkerApi {
       type: "module",
     });
 
-    worker.onmessage = (
-      event: MessageEvent<JsonRpcResponse<CheckerResult>>,
-    ) => {
+    worker.onmessage = (event: MessageEvent<JsonRpcResponse>) => {
       const response = event.data;
       const pending = pendingRef.current.get(response.id);
       if (!pending) {
@@ -45,7 +65,7 @@ export function useCheckerWorker(): CheckerWorkerApi {
       if ("error" in response) {
         pending.reject(new Error(response.error.message));
       } else {
-        pending.resolve(response.result);
+        pending.resolve(response.result as never);
       }
     };
 
@@ -62,25 +82,84 @@ export function useCheckerWorker(): CheckerWorkerApi {
     };
   }, []);
 
-  const checkSDCPN = (sdcpn: SDCPN): Promise<CheckerResult> => {
+  const sendRequest = useCallback(<T>(request: JsonRpcRequest): Promise<T> => {
     const worker = workerRef.current;
     if (!worker) {
       return Promise.reject(new Error("Worker not initialized"));
     }
 
-    const id = nextId.current++;
-    const request: JsonRpcRequest = {
-      jsonrpc: "2.0",
-      id,
-      method: "checkSDCPN",
-      params: { sdcpn },
-    };
-
-    return new Promise<CheckerResult>((resolve, reject) => {
-      pendingRef.current.set(id, { resolve, reject });
+    return new Promise<T>((resolve, reject) => {
+      pendingRef.current.set(request.id, {
+        resolve: resolve as (result: never) => void,
+        reject,
+      });
       worker.postMessage(request);
     });
-  };
+  }, []);
+
+  const setSDCPN = useCallback(
+    (sdcpn: SDCPN): Promise<CheckerResult> => {
+      const id = nextId.current++;
+      return sendRequest<CheckerResult>({
+        jsonrpc: "2.0",
+        id,
+        method: "setSDCPN",
+        params: { sdcpn },
+      });
+    },
+    [sendRequest],
+  );
+
+  const getCompletions = useCallback(
+    (
+      itemType: CheckerItemDiagnostics["itemType"],
+      itemId: string,
+      offset: number,
+    ): Promise<CheckerCompletionResult> => {
+      const id = nextId.current++;
+      return sendRequest<CheckerCompletionResult>({
+        jsonrpc: "2.0",
+        id,
+        method: "getCompletions",
+        params: { itemType, itemId, offset },
+      });
+    },
+    [sendRequest],
+  );
+
+  const getQuickInfo = useCallback(
+    (
+      itemType: CheckerItemDiagnostics["itemType"],
+      itemId: string,
+      offset: number,
+    ): Promise<CheckerQuickInfoResult> => {
+      const id = nextId.current++;
+      return sendRequest<CheckerQuickInfoResult>({
+        jsonrpc: "2.0",
+        id,
+        method: "getQuickInfo",
+        params: { itemType, itemId, offset },
+      });
+    },
+    [sendRequest],
+  );
+
+  const getSignatureHelp = useCallback(
+    (
+      itemType: CheckerItemDiagnostics["itemType"],
+      itemId: string,
+      offset: number,
+    ): Promise<CheckerSignatureHelpResult> => {
+      const id = nextId.current++;
+      return sendRequest<CheckerSignatureHelpResult>({
+        jsonrpc: "2.0",
+        id,
+        method: "getSignatureHelp",
+        params: { itemType, itemId, offset },
+      });
+    },
+    [sendRequest],
+  );
 
-  return { checkSDCPN };
+  return { setSDCPN, getCompletions, getQuickInfo, getSignatureHelp };
 }
diff --git a/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
new file mode 100644
index 00000000000..9a9cbbe0868
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
@@ -0,0 +1,127 @@
+import type * as Monaco from "monaco-editor";
+import { Suspense, use, useEffect } from "react";
+
+import { CheckerContext } from "../checker/context";
+import type { CheckerCompletionItem } from "../checker/worker/protocol";
+import { MonacoContext } from "./context";
+import { parseEditorPath } from "./editor-paths";
+
+/**
+ * Map TypeScript `ScriptElementKind` strings to Monaco `CompletionItemKind`.
+ * @see https://github.com/microsoft/TypeScript/blob/main/src/services/types.ts
+ */
+function toCompletionItemKind(
+  kind: string,
+  monaco: typeof Monaco,
+): Monaco.languages.CompletionItemKind {
+  switch (kind) {
+    case "method":
+    case "construct":
+      return monaco.languages.CompletionItemKind.Method;
+    case "function":
+    case "local function":
+      return monaco.languages.CompletionItemKind.Function;
+    case "constructor":
+      return monaco.languages.CompletionItemKind.Constructor;
+    case "property":
+    case "getter":
+    case "setter":
+      return monaco.languages.CompletionItemKind.Property;
+    case "parameter":
+    case "var":
+    case "local var":
+    case "let":
+      return monaco.languages.CompletionItemKind.Variable;
+    case "const":
+      return monaco.languages.CompletionItemKind.Variable;
+    case "class":
+      return monaco.languages.CompletionItemKind.Class;
+    case "interface":
+      return monaco.languages.CompletionItemKind.Interface;
+    case "type":
+    case "type parameter":
+    case "primitive type":
+    case "alias":
+      return monaco.languages.CompletionItemKind.TypeParameter;
+    case "enum":
+      return monaco.languages.CompletionItemKind.Enum;
+    case "enum member":
+      return monaco.languages.CompletionItemKind.EnumMember;
+    case "module":
+    case "external module name":
+      return monaco.languages.CompletionItemKind.Module;
+    case "keyword":
+      return monaco.languages.CompletionItemKind.Keyword;
+    case "string":
+      return monaco.languages.CompletionItemKind.Value;
+    default:
+      return monaco.languages.CompletionItemKind.Text;
+  }
+}
+
+function toMonacoCompletion(
+  entry: CheckerCompletionItem,
+  range: Monaco.IRange,
+  monaco: typeof Monaco,
+): Monaco.languages.CompletionItem {
+  return {
+    label: entry.name,
+    kind: toCompletionItemKind(entry.kind, monaco),
+    insertText: entry.insertText ?? entry.name,
+    sortText: entry.sortText,
+    range,
+  };
+}
+
+const CompletionSyncInner = () => {
+  const { monaco } = use(use(MonacoContext));
+  const { getCompletions } = use(CheckerContext);
+
+  useEffect(() => {
+    const disposable = monaco.languages.registerCompletionItemProvider(
+      "typescript",
+      {
+        triggerCharacters: ["."],
+
+        async provideCompletionItems(model, position) {
+          const parsed = parseEditorPath(model.uri.toString());
+          if (!parsed) {
+            return { suggestions: [] };
+          }
+
+          const offset = model.getOffsetAt(position);
+          const result = await getCompletions(
+            parsed.itemType,
+            parsed.itemId,
+            offset,
+          );
+
+          const word = model.getWordUntilPosition(position);
+          const range: Monaco.IRange = {
+            startLineNumber: position.lineNumber,
+            endLineNumber: position.lineNumber,
+            startColumn: word.startColumn,
+            endColumn: word.endColumn,
+          };
+
+          return {
+            suggestions: result.items.map((item) =>
+              toMonacoCompletion(item, range, monaco),
+            ),
+          };
+        },
+      },
+    );
+
+    return () => disposable.dispose();
+  }, [monaco, getCompletions]);
+
+  return null;
+};
+
+/** Renders nothing visible — registers a Monaco CompletionItemProvider backed by the checker worker. */
+export const CompletionSync: React.FC = () => (
+  <Suspense fallback={null}>
+    <CompletionSyncInner />
+  </Suspense>
+);
diff --git a/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts b/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
index cef45bd237d..f6f167e5ef6 100644
--- a/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
+++ b/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
@@ -14,3 +14,33 @@ export function getEditorPath(
       return `inmemory://sdcpn/differential-equations/${itemId}.ts`;
   }
 }
+
+const TRANSITION_LAMBDA_RE =
+  /^inmemory:\/\/sdcpn\/transitions\/([^/]+)\/lambda\.ts$/;
+const TRANSITION_KERNEL_RE =
+  /^inmemory:\/\/sdcpn\/transitions\/([^/]+)\/kernel\.ts$/;
+const DIFFERENTIAL_EQUATION_RE =
+  /^inmemory:\/\/sdcpn\/differential-equations\/([^/]+)\.ts$/;
+
+/** Extract `(itemType, itemId)` from a Monaco model URI string. */
+export function parseEditorPath(uri: string): {
+  itemType: CheckerItemDiagnostics["itemType"];
+  itemId: string;
+} | null {
+  let match = TRANSITION_LAMBDA_RE.exec(uri);
+  if (match) {
+    return { itemType: "transition-lambda", itemId: match[1]! };
+  }
+
+  match = TRANSITION_KERNEL_RE.exec(uri);
+  if (match) {
+    return { itemType: "transition-kernel", itemId: match[1]! };
+  }
+
+  match = DIFFERENTIAL_EQUATION_RE.exec(uri);
+  if (match) {
+    return { itemType: "differential-equation", itemId: match[1]! };
+  }
+
+  return null;
+}
diff --git a/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
new file mode 100644
index 00000000000..06b3bd14d87
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
@@ -0,0 +1,58 @@
+import type * as Monaco from "monaco-editor";
+import { Suspense, use, useEffect } from "react";
+
+import { CheckerContext } from "../checker/context";
+import { MonacoContext } from "./context";
+import { parseEditorPath } from "./editor-paths";
+
+const HoverSyncInner = () => {
+  const { monaco } = use(use(MonacoContext));
+  const { getQuickInfo } = use(CheckerContext);
+
+  useEffect(() => {
+    const disposable = monaco.languages.registerHoverProvider("typescript", {
+      async provideHover(model, position) {
+        const parsed = parseEditorPath(model.uri.toString());
+        if (!parsed) {
+          return null;
+        }
+
+        const offset = model.getOffsetAt(position);
+        const info = await getQuickInfo(parsed.itemType, parsed.itemId, offset);
+
+        if (!info) {
+          return null;
+        }
+
+        const startPos = model.getPositionAt(info.start);
+        const endPos = model.getPositionAt(info.start + info.length);
+        const range: Monaco.IRange = {
+          startLineNumber: startPos.lineNumber,
+          startColumn: startPos.column,
+          endLineNumber: endPos.lineNumber,
+          endColumn: endPos.column,
+        };
+
+        const contents: Monaco.IMarkdownString[] = [
+          { value: `\`\`\`typescript\n${info.displayParts}\n\`\`\`` },
+        ];
+        if (info.documentation) {
+          contents.push({ value: info.documentation });
+        }
+
+        return { range, contents };
+      },
+    });
+
+    return () => disposable.dispose();
+  }, [monaco, getQuickInfo]);
+
+  return null;
+};
+
+/** Renders nothing visible — registers a Monaco HoverProvider backed by the checker worker. */
+export const HoverSync: React.FC = () => (
+  <Suspense fallback={null}>
+    <HoverSyncInner />
+  </Suspense>
+);
diff --git a/libs/@hashintel/petrinaut/src/monaco/provider.tsx b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
index ce18102f437..6488438e526 100644
--- a/libs/@hashintel/petrinaut/src/monaco/provider.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
@@ -1,8 +1,11 @@
 import type * as Monaco from "monaco-editor";
 
+import { CompletionSync } from "./completion-sync";
 import type { MonacoContextValue } from "./context";
 import { MonacoContext } from "./context";
 import { DiagnosticsSync } from "./diagnostics-sync";
+import { HoverSync } from "./hover-sync";
+import { SignatureHelpSync } from "./signature-help-sync";
 
 interface LanguageDefaults {
   setModeConfiguration(config: Record<string, boolean>): void;
@@ -42,38 +45,6 @@ function disableBuiltInTypeScriptFeatures(monaco: typeof Monaco) {
   ts.javascriptDefaults.setModeConfiguration(modeConfiguration);
 }
 
-function registerCompletionProvider(monaco: typeof Monaco) {
-  monaco.languages.registerCompletionItemProvider("typescript", {
-    provideCompletionItems(model, position) {
-      const word = model.getWordUntilPosition(position);
-      const range = {
-        startLineNumber: position.lineNumber,
-        endLineNumber: position.lineNumber,
-        startColumn: word.startColumn,
-        endColumn: word.endColumn,
-      };
-
-      // eslint-disable-next-line no-console
-      console.log("Completion requested", {
-        position: { line: position.lineNumber, column: position.column },
-        word: word.word,
-        range,
-      });
-
-      return {
-        suggestions: [
-          {
-            label: "transition",
-            kind: monaco.languages.CompletionItemKind.Keyword,
-            insertText: "transition",
-            range,
-          },
-        ],
-      };
-    },
-  });
-}
-
 async function initMonaco(): Promise<MonacoContextValue> {
   // Disable all workers — no worker files will be shipped or loaded.
   (globalThis as Record<string, unknown>).MonacoEnvironment = {
@@ -89,7 +60,6 @@ async function initMonaco(): Promise<MonacoContextValue> {
   monacoReact.loader.config({ monaco });
 
   disableBuiltInTypeScriptFeatures(monaco);
-  registerCompletionProvider(monaco);
   return { monaco, Editor: monacoReact.default };
 }
 
@@ -102,6 +72,9 @@ export const MonacoProvider: React.FC<{ children: React.ReactNode }> = ({
   return (
     <MonacoContext.Provider value={monacoPromise}>
       <DiagnosticsSync />
+      <CompletionSync />
+      <HoverSync />
+      <SignatureHelpSync />
       {children}
     </MonacoContext.Provider>
   );
diff --git a/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
new file mode 100644
index 00000000000..54f71f59f8b
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
@@ -0,0 +1,73 @@
+import type * as Monaco from "monaco-editor";
+import { Suspense, use, useEffect } from "react";
+
+import { CheckerContext } from "../checker/context";
+import type { CheckerSignatureHelpResult } from "../checker/worker/protocol";
+import { MonacoContext } from "./context";
+import { parseEditorPath } from "./editor-paths";
+
+function toMonacoSignatureHelp(
+  result: NonNullable<CheckerSignatureHelpResult>,
+): Monaco.languages.SignatureHelp {
+  return {
+    activeSignature: result.activeSignature,
+    activeParameter: result.activeParameter,
+    signatures: result.signatures.map((sig) => ({
+      label: sig.label,
+      documentation: sig.documentation || undefined,
+      parameters: sig.parameters.map((param) => ({
+        label: param.label,
+        documentation: param.documentation || undefined,
+      })),
+    })),
+  };
+}
+
+const SignatureHelpSyncInner = () => {
+  const { monaco } = use(use(MonacoContext));
+  const { getSignatureHelp } = use(CheckerContext);
+
+  useEffect(() => {
+    const disposable = monaco.languages.registerSignatureHelpProvider(
+      "typescript",
+      {
+        signatureHelpTriggerCharacters: ["(", ","],
+        signatureHelpRetriggerCharacters: [","],
+
+        async provideSignatureHelp(model, position) {
+          const parsed = parseEditorPath(model.uri.toString());
+          if (!parsed) {
+            return null;
+          }
+
+          const offset = model.getOffsetAt(position);
+          const result = await getSignatureHelp(
+            parsed.itemType,
+            parsed.itemId,
+            offset,
+          );
+
+          if (!result) {
+            return null;
+          }
+
+          return {
+            value: toMonacoSignatureHelp(result),
+            dispose() {},
+          };
+        },
+      },
+    );
+
+    return () => disposable.dispose();
+  }, [monaco, getSignatureHelp]);
+
+  return null;
+};
+
+/** Renders nothing visible — registers a Monaco SignatureHelpProvider backed by the checker worker. */
+export const SignatureHelpSync: React.FC = () => (
+  <Suspense fallback={null}>
+    <SignatureHelpSyncInner />
+  </Suspense>
+);

From bc532615e72ffa3c21137ac2ff55fa3111b66c80 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 13:13:30 +0100
Subject: [PATCH 10/22] H-5839: Adopt LSP-like protocol and reuse TS
 LanguageService across SDCPN changes

Convert the checker communication layer from ad-hoc method names and types to an
LSP-inspired protocol (initialize, sdcpn/didChange, textDocument/didChange,
textDocument/completion, textDocument/hover, textDocument/signatureHelp), with
push-based diagnostics via textDocument/publishDiagnostics notifications.

Replace the factory function with a persistent SDCPNLanguageServer class that
creates ts.createLanguageService() once and incrementally syncs virtual files
(add/remove/update) when the SDCPN structure changes, instead of recreating
from scratch on every mutation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../petrinaut/src/checker/context.ts          |  62 ++-
 .../petrinaut/src/checker/lib/checker.test.ts |  49 ++-
 .../petrinaut/src/checker/lib/checker.ts      |  33 +-
 .../lib/create-language-service-host.ts       |  65 +++-
 .../lib/create-sdcpn-language-service.test.ts | 111 +++++-
 .../lib/create-sdcpn-language-service.ts      | 187 +++++----
 .../petrinaut/src/checker/provider.tsx        |  97 +++--
 .../src/checker/worker/checker.worker.ts      | 366 +++++++++++-------
 .../petrinaut/src/checker/worker/protocol.ts  | 165 ++++----
 .../src/checker/worker/use-checker-worker.ts  | 165 --------
 .../src/checker/worker/use-language-client.ts | 211 ++++++++++
 .../petrinaut/src/monaco/completion-sync.tsx  |  29 +-
 .../petrinaut/src/monaco/diagnostics-sync.tsx |  69 ++--
 .../petrinaut/src/monaco/editor-paths.ts      |  15 +-
 .../petrinaut/src/monaco/hover-sync.tsx       |  17 +-
 .../petrinaut/src/monaco/provider.tsx         |  11 +-
 .../src/monaco/signature-help-sync.tsx        |  25 +-
 libs/@hashintel/petrinaut/src/petrinaut.tsx   |   6 +-
 .../components/BottomBar/bottom-bar.tsx       |   4 +-
 .../BottomBar/diagnostics-indicator.tsx       |   4 +-
 .../differential-equation-properties.tsx      |   7 +-
 .../PropertiesPanel/transition-properties.tsx |   6 +-
 .../src/views/Editor/subviews/diagnostics.tsx |  32 +-
 23 files changed, 1044 insertions(+), 692 deletions(-)
 delete mode 100644 libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts
 create mode 100644 libs/@hashintel/petrinaut/src/checker/worker/use-language-client.ts

diff --git a/libs/@hashintel/petrinaut/src/checker/context.ts b/libs/@hashintel/petrinaut/src/checker/context.ts
index d95dea43201..eeacedf8c1f 100644
--- a/libs/@hashintel/petrinaut/src/checker/context.ts
+++ b/libs/@hashintel/petrinaut/src/checker/context.ts
@@ -1,49 +1,43 @@
 import { createContext } from "react";
 
 import type {
-  CheckerCompletionResult,
-  CheckerItemDiagnostics,
-  CheckerQuickInfoResult,
-  CheckerResult,
-  CheckerSignatureHelpResult,
+  CompletionList,
+  Diagnostic,
+  DocumentUri,
+  Hover,
+  SignatureHelp,
 } from "./worker/protocol";
 
-export interface CheckerContextValue {
-  /** Result of the last SDCPN validation run. */
-  checkResult: CheckerResult;
-  /** Total number of diagnostics across all items. */
+export interface LanguageClientContextValue {
+  /** Per-URI diagnostics pushed from the language server. */
+  diagnosticsByUri: Map<DocumentUri, Diagnostic[]>;
+  /** Total number of diagnostics across all documents. */
   totalDiagnosticsCount: number;
-  /** Request completions at a position within an SDCPN item. */
-  getCompletions: (
-    itemType: CheckerItemDiagnostics["itemType"],
-    itemId: string,
+  /** Notify the server that a document's content changed. */
+  notifyDocumentChanged: (uri: DocumentUri, text: string) => void;
+  /** Request completions at a position within a document. */
+  requestCompletion: (
+    uri: DocumentUri,
     offset: number,
-  ) => Promise<CheckerCompletionResult>;
-  /** Request quick info (hover) at a position within an SDCPN item. */
-  getQuickInfo: (
-    itemType: CheckerItemDiagnostics["itemType"],
-    itemId: string,
+  ) => Promise<CompletionList>;
+  /** Request hover info at a position within a document. */
+  requestHover: (uri: DocumentUri, offset: number) => Promise<Hover>;
+  /** Request signature help at a position within a document. */
+  requestSignatureHelp: (
+    uri: DocumentUri,
     offset: number,
-  ) => Promise<CheckerQuickInfoResult>;
-  /** Request signature help at a position within an SDCPN item. */
-  getSignatureHelp: (
-    itemType: CheckerItemDiagnostics["itemType"],
-    itemId: string,
-    offset: number,
-  ) => Promise<CheckerSignatureHelpResult>;
+  ) => Promise<SignatureHelp>;
 }
 
-const DEFAULT_CONTEXT_VALUE: CheckerContextValue = {
-  checkResult: {
-    isValid: true,
-    itemDiagnostics: [],
-  },
+const DEFAULT_CONTEXT_VALUE: LanguageClientContextValue = {
+  diagnosticsByUri: new Map(),
   totalDiagnosticsCount: 0,
-  getCompletions: () => Promise.resolve({ items: [] }),
-  getQuickInfo: () => Promise.resolve(null),
-  getSignatureHelp: () => Promise.resolve(null),
+  notifyDocumentChanged: () => {},
+  requestCompletion: () => Promise.resolve({ items: [] }),
+  requestHover: () => Promise.resolve(null),
+  requestSignatureHelp: () => Promise.resolve(null),
 };
 
-export const CheckerContext = createContext<CheckerContextValue>(
+export const LanguageClientContext = createContext<LanguageClientContextValue>(
   DEFAULT_CONTEXT_VALUE,
 );
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/checker.test.ts b/libs/@hashintel/petrinaut/src/checker/lib/checker.test.ts
index 8140ea9f151..4fdb53700e5 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/checker.test.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/checker.test.ts
@@ -1,8 +1,17 @@
 import { describe, expect, it } from "vitest";
 
+import type { SDCPN } from "../../core/types/sdcpn";
 import { checkSDCPN } from "./checker";
+import { SDCPNLanguageServer } from "./create-sdcpn-language-service";
 import { createSDCPN } from "./helper/create-sdcpn";
 
+/** Create a server, sync the SDCPN, and run diagnostics. */
+function check(sdcpn: SDCPN) {
+  const server = new SDCPNLanguageServer();
+  server.syncFiles(sdcpn);
+  return checkSDCPN(sdcpn, server);
+}
+
 describe("checkSDCPN", () => {
   describe("Color IDs with special characters", () => {
     it("handles UUID-style color IDs with dashes", () => {
@@ -43,7 +52,7 @@ describe("checkSDCPN", () => {
       });
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN - should be valid
       expect(result.isValid).toBe(true);
@@ -68,7 +77,7 @@ describe("checkSDCPN", () => {
       });
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(true);
@@ -101,7 +110,7 @@ describe("checkSDCPN", () => {
       });
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(true);
@@ -130,7 +139,7 @@ describe("checkSDCPN", () => {
       const de = sdcpn.differentialEquations[0]!;
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(false);
@@ -166,7 +175,7 @@ describe("checkSDCPN", () => {
       const de = sdcpn.differentialEquations[0]!;
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(false);
@@ -193,7 +202,7 @@ describe("checkSDCPN", () => {
       const de = sdcpn.differentialEquations[0]!;
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(false);
@@ -225,7 +234,7 @@ describe("checkSDCPN", () => {
       });
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(true);
@@ -253,7 +262,7 @@ describe("checkSDCPN", () => {
       const transition = sdcpn.transitions[0]!;
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(false);
@@ -285,7 +294,7 @@ describe("checkSDCPN", () => {
       const transition = sdcpn.transitions[0]!;
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(false);
@@ -313,7 +322,7 @@ describe("checkSDCPN", () => {
       });
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(true);
@@ -343,7 +352,7 @@ describe("checkSDCPN", () => {
       });
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(true);
@@ -372,7 +381,7 @@ describe("checkSDCPN", () => {
       const transition = sdcpn.transitions[0]!;
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(false);
@@ -403,7 +412,7 @@ describe("checkSDCPN", () => {
       const transition = sdcpn.transitions[0]!;
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(false);
@@ -437,7 +446,7 @@ describe("checkSDCPN", () => {
       });
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(true);
@@ -472,7 +481,7 @@ describe("checkSDCPN", () => {
       const transition = sdcpn.transitions[0]!;
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN - should error because Untyped is not in the input type
       expect(result.isValid).toBe(false);
@@ -511,7 +520,7 @@ describe("checkSDCPN", () => {
       const transition = sdcpn.transitions[0]!;
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN - should error because Target is missing from the output type
       expect(result.isValid).toBe(false);
@@ -544,7 +553,7 @@ describe("checkSDCPN", () => {
       const transition = sdcpn.transitions[0]!;
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN - should error because nonExistentProperty doesn't exist on the token type
       expect(result.isValid).toBe(false);
@@ -586,7 +595,7 @@ describe("checkSDCPN", () => {
       });
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN
       expect(result.isValid).toBe(false);
@@ -643,7 +652,7 @@ describe("checkSDCPN", () => {
       });
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN - Should be valid because TransitionKernel is not checked when no output places
       expect(result.isValid).toBe(true);
@@ -678,7 +687,7 @@ describe("checkSDCPN", () => {
       });
 
       // WHEN
-      const result = checkSDCPN(sdcpn);
+      const result = check(sdcpn);
 
       // THEN - Should be valid because TransitionKernel is not checked when no coloured output places
       expect(result.isValid).toBe(true);
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/checker.ts b/libs/@hashintel/petrinaut/src/checker/lib/checker.ts
index 61a5d27799a..3c5029c5fe3 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/checker.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/checker.ts
@@ -1,17 +1,19 @@
 import type ts from "typescript";
 
 import type { SDCPN } from "../../core/types/sdcpn";
-import {
-  createSDCPNLanguageService,
-  type SDCPNLanguageService,
-} from "./create-sdcpn-language-service";
+import type { SDCPNLanguageServer } from "./create-sdcpn-language-service";
 import { getItemFilePath } from "./file-paths";
 
+export type ItemType =
+  | "transition-lambda"
+  | "transition-kernel"
+  | "differential-equation";
+
 export type SDCPNDiagnostic = {
   /** The ID of the SDCPN item (transition or differential equation) */
   itemId: string;
   /** The type of the item */
-  itemType: "transition-lambda" | "transition-kernel" | "differential-equation";
+  itemType: ItemType;
   /** The file path in the virtual file system */
   filePath: string;
   /** TypeScript diagnostics for this file */
@@ -28,16 +30,11 @@ export type SDCPNCheckResult = {
 /**
  * Checks the validity of an SDCPN by running TypeScript validation
  * on all user-provided code (transitions and differential equations).
- *
- * @param sdcpn - The SDCPN to check
- * @returns A result object indicating validity and any diagnostics
  */
 export function checkSDCPN(
   sdcpn: SDCPN,
-  existingLanguageService?: SDCPNLanguageService,
+  server: SDCPNLanguageServer,
 ): SDCPNCheckResult {
-  const languageService =
-    existingLanguageService ?? createSDCPNLanguageService(sdcpn);
   const itemDiagnostics: SDCPNDiagnostic[] = [];
 
   // Check all differential equations
@@ -45,10 +42,8 @@ export function checkSDCPN(
     const filePath = getItemFilePath("differential-equation-code", {
       id: de.id,
     });
-    const semanticDiagnostics =
-      languageService.getSemanticDiagnostics(filePath);
-    const syntacticDiagnostics =
-      languageService.getSyntacticDiagnostics(filePath);
+    const semanticDiagnostics = server.getSemanticDiagnostics(filePath);
+    const syntacticDiagnostics = server.getSyntacticDiagnostics(filePath);
     const allDiagnostics = [...syntacticDiagnostics, ...semanticDiagnostics];
 
     if (allDiagnostics.length > 0) {
@@ -68,9 +63,9 @@ export function checkSDCPN(
       transitionId: transition.id,
     });
     const lambdaSemanticDiagnostics =
-      languageService.getSemanticDiagnostics(lambdaFilePath);
+      server.getSemanticDiagnostics(lambdaFilePath);
     const lambdaSyntacticDiagnostics =
-      languageService.getSyntacticDiagnostics(lambdaFilePath);
+      server.getSyntacticDiagnostics(lambdaFilePath);
     const lambdaDiagnostics = [
       ...lambdaSyntacticDiagnostics,
       ...lambdaSemanticDiagnostics,
@@ -97,9 +92,9 @@ export function checkSDCPN(
         transitionId: transition.id,
       });
       const kernelSemanticDiagnostics =
-        languageService.getSemanticDiagnostics(kernelFilePath);
+        server.getSemanticDiagnostics(kernelFilePath);
       const kernelSyntacticDiagnostics =
-        languageService.getSyntacticDiagnostics(kernelFilePath);
+        server.getSyntacticDiagnostics(kernelFilePath);
       const kernelDiagnostics = [
         ...kernelSyntacticDiagnostics,
         ...kernelSemanticDiagnostics,
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts b/libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts
index 4e22135e8ce..8a5df10640e 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts
@@ -37,15 +37,34 @@ export type VirtualFile = {
   content: string;
 };
 
+/** Controller for the virtual file system backing the LanguageServiceHost. */
+export type LanguageServiceHostController = {
+  host: ts.LanguageServiceHost;
+  /** Add a new file to the virtual file system. */
+  addFile: (fileName: string, file: VirtualFile) => void;
+  /** Remove a file from the virtual file system. */
+  removeFile: (fileName: string) => void;
+  /** Replace an entire file entry (prefix + content) and bump its version. */
+  updateFile: (fileName: string, file: VirtualFile) => void;
+  /** Update only the user content of an existing file (preserves prefix). */
+  updateContent: (fileName: string, content: string) => void;
+  /** Check whether a file exists in the virtual file system. */
+  hasFile: (fileName: string) => boolean;
+  /** Return all file names currently in the virtual file system. */
+  getFileNames: () => string[];
+  /** Get the VirtualFile entry for a given file name. */
+  getFile: (fileName: string) => VirtualFile | undefined;
+};
+
 /**
- * @private Used by `createSDCPNLanguageService`.
+ * Creates a TypeScript LanguageServiceHost backed by a virtual file system.
  *
- * Creates a TypeScript LanguageServiceHost for virtual SDCPN files
+ * The returned controller allows incremental mutations (add/remove/update)
+ * without recreating the host or the LanguageService that consumes it.
  */
-export function createLanguageServiceHost(files: Map<string, VirtualFile>): {
-  host: ts.LanguageServiceHost;
-  updateFileContent: (fileName: string, content: string) => void;
-} {
+export function createLanguageServiceHost(
+  files: Map<string, VirtualFile>,
+): LanguageServiceHostController {
   const versions = new Map<string, number>();
 
   const getFileContent = (fileName: string): string | undefined => {
@@ -65,11 +84,30 @@ export function createLanguageServiceHost(files: Map<string, VirtualFile>): {
     return undefined;
   };
 
-  const updateFileContent = (fileName: string, content: string) => {
+  const bumpVersion = (fileName: string) => {
+    versions.set(fileName, (versions.get(fileName) ?? 0) + 1);
+  };
+
+  const addFile = (fileName: string, file: VirtualFile) => {
+    files.set(fileName, file);
+    versions.set(fileName, 0);
+  };
+
+  const removeFile = (fileName: string) => {
+    files.delete(fileName);
+    versions.delete(fileName);
+  };
+
+  const updateFile = (fileName: string, file: VirtualFile) => {
+    files.set(fileName, file);
+    bumpVersion(fileName);
+  };
+
+  const updateContent = (fileName: string, content: string) => {
     const entry = files.get(fileName);
     if (entry) {
       entry.content = content;
-      versions.set(fileName, (versions.get(fileName) ?? 0) + 1);
+      bumpVersion(fileName);
     }
   };
 
@@ -94,5 +132,14 @@ export function createLanguageServiceHost(files: Map<string, VirtualFile>): {
     },
   };
 
-  return { host, updateFileContent };
+  return {
+    host,
+    addFile,
+    removeFile,
+    updateFile,
+    updateContent,
+    hasFile: (fileName) => files.has(fileName),
+    getFileNames: () => [...files.keys()],
+    getFile: (fileName) => files.get(fileName),
+  };
 }
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.test.ts b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.test.ts
index 791b66f2c23..68d0a6e7ccf 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.test.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 
-import { createSDCPNLanguageService } from "./create-sdcpn-language-service";
+import { SDCPNLanguageServer } from "./create-sdcpn-language-service";
 import { getItemFilePath } from "./file-paths";
 import { createSDCPN } from "./helper/create-sdcpn";
 
@@ -24,6 +24,16 @@ function parseCursor(codeWithCursor: string): {
   return { offset, code };
 }
 
+/** Create a server initialized with the given SDCPN and return it. */
+function createServer(
+  sdcpnOptions: Parameters<typeof createSDCPN>[0],
+): SDCPNLanguageServer {
+  const sdcpn = createSDCPN(sdcpnOptions);
+  const server = new SDCPNLanguageServer();
+  server.syncFiles(sdcpn);
+  return server;
+}
+
 /** Extract just the completion names from a position. */
 function getCompletionNames(
   sdcpnOptions: Parameters<typeof createSDCPN>[0],
@@ -60,8 +70,7 @@ function getCompletionNames(
     );
   }
 
-  const sdcpn = createSDCPN(patched);
-  const ls = createSDCPNLanguageService(sdcpn);
+  const server = createServer(patched);
 
   let filePath: string;
   if (target.type === "transition-lambda") {
@@ -78,11 +87,15 @@ function getCompletionNames(
     });
   }
 
-  const completions = ls.getCompletionsAtPosition(filePath, offset, undefined);
+  const completions = server.getCompletionsAtPosition(
+    filePath,
+    offset,
+    undefined,
+  );
   return (completions?.entries ?? []).map((entry) => entry.name);
 }
 
-describe("createSDCPNLanguageService completions", () => {
+describe("SDCPNLanguageServer completions", () => {
   const baseSdcpn = {
     types: [{ id: "color1", elements: [{ name: "x", type: "real" as const }] }],
     places: [
@@ -187,10 +200,10 @@ describe("createSDCPNLanguageService completions", () => {
     });
   });
 
-  describe("updateFileContent", () => {
+  describe("updateDocumentContent", () => {
     it("returns completions for updated code, not original code", () => {
       // Start with number code
-      const sdcpn = createSDCPN({
+      const server = createServer({
         ...baseSdcpn,
         transitions: [
           {
@@ -199,17 +212,15 @@ describe("createSDCPNLanguageService completions", () => {
           },
         ],
       });
-
-      const ls = createSDCPNLanguageService(sdcpn);
       const filePath = getItemFilePath("transition-lambda-code", {
         transitionId: "t1",
       });
 
       // Update to string code
       const { offset, code } = parseCursor(`const s = "hello";\ns.${CURSOR}`);
-      ls.updateFileContent(filePath, code);
+      server.updateDocumentContent(filePath, code);
 
-      const completions = ls.getCompletionsAtPosition(
+      const completions = server.getCompletionsAtPosition(
         filePath,
         offset,
         undefined,
@@ -224,7 +235,7 @@ describe("createSDCPNLanguageService completions", () => {
     });
 
     it("reflects new content after multiple updates", () => {
-      const sdcpn = createSDCPN({
+      const server = createServer({
         ...baseSdcpn,
         transitions: [
           {
@@ -233,16 +244,14 @@ describe("createSDCPNLanguageService completions", () => {
           },
         ],
       });
-
-      const ls = createSDCPNLanguageService(sdcpn);
       const filePath = getItemFilePath("transition-lambda-code", {
         transitionId: "t1",
       });
 
       // First update: number
       const first = parseCursor(`const a = 42;\na.${CURSOR}`);
-      ls.updateFileContent(filePath, first.code);
-      const firstCompletions = ls.getCompletionsAtPosition(
+      server.updateDocumentContent(filePath, first.code);
+      const firstCompletions = server.getCompletionsAtPosition(
         filePath,
         first.offset,
         undefined,
@@ -254,8 +263,8 @@ describe("createSDCPNLanguageService completions", () => {
 
       // Second update: string
       const second = parseCursor(`const s = "hello";\ns.${CURSOR}`);
-      ls.updateFileContent(filePath, second.code);
-      const secondCompletions = ls.getCompletionsAtPosition(
+      server.updateDocumentContent(filePath, second.code);
+      const secondCompletions = server.getCompletionsAtPosition(
         filePath,
         second.offset,
         undefined,
@@ -268,6 +277,72 @@ describe("createSDCPNLanguageService completions", () => {
     });
   });
 
+  describe("syncFiles", () => {
+    it("updates types when SDCPN changes structurally", () => {
+      // Start with one color element
+      const server = new SDCPNLanguageServer();
+      const sdcpn1 = createSDCPN({
+        types: [
+          { id: "color1", elements: [{ name: "x", type: "real" as const }] },
+        ],
+        places: [{ id: "place1", name: "Source", colorId: "color1" }],
+        transitions: [
+          {
+            id: "t1",
+            lambdaType: "predicate" as const,
+            inputArcs: [{ placeId: "place1", weight: 1 }],
+            outputArcs: [],
+            lambdaCode: "",
+          },
+        ],
+      });
+      server.syncFiles(sdcpn1);
+
+      // Add a new element to the color
+      const sdcpn2 = createSDCPN({
+        types: [
+          {
+            id: "color1",
+            elements: [
+              { name: "x", type: "real" as const },
+              { name: "y", type: "real" as const },
+            ],
+          },
+        ],
+        places: [{ id: "place1", name: "Source", colorId: "color1" }],
+        transitions: [
+          {
+            id: "t1",
+            lambdaType: "predicate" as const,
+            inputArcs: [{ placeId: "place1", weight: 1 }],
+            outputArcs: [],
+            lambdaCode: `export default Lambda((input) => {\n  const t = input.Source[0];\n  return t.`,
+          },
+        ],
+      });
+      server.syncFiles(sdcpn2);
+
+      const filePath = getItemFilePath("transition-lambda-code", {
+        transitionId: "t1",
+      });
+      const { offset, code } = parseCursor(
+        `export default Lambda((input) => {\n  const t = input.Source[0];\n  return t.${CURSOR}`,
+      );
+      server.updateDocumentContent(filePath, code);
+
+      const completions = server.getCompletionsAtPosition(
+        filePath,
+        offset,
+        undefined,
+      );
+      const names = (completions?.entries ?? []).map((entry) => entry.name);
+
+      // Should now include both x and y
+      expect(names).toContain("x");
+      expect(names).toContain("y");
+    });
+  });
+
   describe("differential equation completions", () => {
     const deSdcpn = {
       types: [
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
index 02064505ca5..e790d0effc5 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
@@ -3,14 +3,11 @@ import ts from "typescript";
 import type { SDCPN } from "../../core/types/sdcpn";
 import {
   createLanguageServiceHost,
+  type LanguageServiceHostController,
   type VirtualFile,
 } from "./create-language-service-host";
 import { getItemFilePath } from "./file-paths";
 
-export type SDCPNLanguageService = ts.LanguageService & {
-  updateFileContent: (fileName: string, content: string) => void;
-};
-
 /**
  * Sanitizes a color ID to be a valid TypeScript identifier.
  * Removes all characters that are not valid suffixes for TypeScript identifiers
@@ -245,73 +242,123 @@ function adjustDiagnostics<T extends ts.Diagnostic>(
 }
 
 /**
- * Creates a TypeScript language service for SDCPN code validation.
+ * Persistent TypeScript language server for SDCPN code validation.
  *
- * @param sdcpn - The SDCPN model to create the service for
- * @returns A TypeScript LanguageService instance
+ * Creates the `ts.LanguageService` once and reuses it across SDCPN changes
+ * by diffing virtual files (add/remove/update) rather than recreating everything.
  */
-export function createSDCPNLanguageService(sdcpn: SDCPN): SDCPNLanguageService {
-  const files = generateVirtualFiles(sdcpn);
-  const { host, updateFileContent } = createLanguageServiceHost(files);
-  const baseService = ts.createLanguageService(host);
-
-  // Proxy service to adjust positions for injected prefixes
-  return {
-    ...baseService,
-
-    updateFileContent,
-
-    getSemanticDiagnostics(fileName) {
-      const entry = files.get(fileName);
-      const prefixLength = entry?.prefix?.length ?? 0;
-      const diagnostics = baseService.getSemanticDiagnostics(fileName);
-      return adjustDiagnostics(diagnostics, prefixLength);
-    },
-
-    getSyntacticDiagnostics(fileName) {
-      const entry = files.get(fileName);
-      const prefixLength = entry?.prefix?.length ?? 0;
-      const diagnostics = baseService.getSyntacticDiagnostics(fileName);
-      return adjustDiagnostics(diagnostics, prefixLength);
-    },
-
-    getCompletionsAtPosition(fileName, position, options) {
-      const entry = files.get(fileName);
-      const prefixLength = entry?.prefix?.length ?? 0;
-      return baseService.getCompletionsAtPosition(
-        fileName,
-        position + prefixLength,
-        options,
-      );
-    },
-
-    getQuickInfoAtPosition(fileName, position) {
-      const entry = files.get(fileName);
-      const prefixLength = entry?.prefix?.length ?? 0;
-      const info = baseService.getQuickInfoAtPosition(
-        fileName,
-        position + prefixLength,
-      );
-      if (!info) {
-        return undefined;
+export class SDCPNLanguageServer {
+  private files: Map<string, VirtualFile>;
+  private controller: LanguageServiceHostController;
+  private service: ts.LanguageService;
+
+  constructor() {
+    this.files = new Map();
+    this.controller = createLanguageServiceHost(this.files);
+    this.service = ts.createLanguageService(this.controller.host);
+  }
+
+  /**
+   * Sync virtual files to match the given SDCPN model.
+   * Diffs against the current state: adds new files, updates changed files,
+   * removes files that no longer exist.
+   */
+  syncFiles(sdcpn: SDCPN): void {
+    const newFiles = generateVirtualFiles(sdcpn);
+
+    // Remove files that no longer exist
+    for (const existingName of this.controller.getFileNames()) {
+      if (!newFiles.has(existingName)) {
+        this.controller.removeFile(existingName);
+      }
+    }
+
+    // Add or update files
+    for (const [name, newFile] of newFiles) {
+      if (!this.controller.hasFile(name)) {
+        this.controller.addFile(name, newFile);
+      } else {
+        const existing = this.controller.getFile(name);
+        if (
+          existing?.content !== newFile.content ||
+          existing?.prefix !== newFile.prefix
+        ) {
+          this.controller.updateFile(name, newFile);
+        }
       }
-      return {
-        ...info,
-        textSpan: {
-          start: info.textSpan.start - prefixLength,
-          length: info.textSpan.length,
-        },
-      };
-    },
-
-    getSignatureHelpItems(fileName, position, options) {
-      const entry = files.get(fileName);
-      const prefixLength = entry?.prefix?.length ?? 0;
-      return baseService.getSignatureHelpItems(
-        fileName,
-        position + prefixLength,
-        options,
-      );
-    },
-  };
+    }
+  }
+
+  /** Update only the user content of a single file (e.g., when the user types in an editor). */
+  updateDocumentContent(fileName: string, content: string): void {
+    this.controller.updateContent(fileName, content);
+  }
+
+  getSemanticDiagnostics(fileName: string): ts.Diagnostic[] {
+    const entry = this.controller.getFile(fileName);
+    const prefixLength = entry?.prefix?.length ?? 0;
+    const diagnostics = this.service.getSemanticDiagnostics(fileName);
+    return adjustDiagnostics(diagnostics, prefixLength);
+  }
+
+  getSyntacticDiagnostics(fileName: string): ts.Diagnostic[] {
+    const entry = this.controller.getFile(fileName);
+    const prefixLength = entry?.prefix?.length ?? 0;
+    const diagnostics = this.service.getSyntacticDiagnostics(fileName);
+    return adjustDiagnostics(diagnostics, prefixLength);
+  }
+
+  getCompletionsAtPosition(
+    fileName: string,
+    position: number,
+    options: ts.GetCompletionsAtPositionOptions | undefined,
+  ): ts.CompletionInfo | undefined {
+    const entry = this.controller.getFile(fileName);
+    const prefixLength = entry?.prefix?.length ?? 0;
+    return this.service.getCompletionsAtPosition(
+      fileName,
+      position + prefixLength,
+      options,
+    );
+  }
+
+  getQuickInfoAtPosition(
+    fileName: string,
+    position: number,
+  ): ts.QuickInfo | undefined {
+    const entry = this.controller.getFile(fileName);
+    const prefixLength = entry?.prefix?.length ?? 0;
+    const info = this.service.getQuickInfoAtPosition(
+      fileName,
+      position + prefixLength,
+    );
+    if (!info) {
+      return undefined;
+    }
+    return {
+      ...info,
+      textSpan: {
+        start: info.textSpan.start - prefixLength,
+        length: info.textSpan.length,
+      },
+    };
+  }
+
+  getSignatureHelpItems(
+    fileName: string,
+    position: number,
+    options: ts.SignatureHelpItemsOptions | undefined,
+  ): ts.SignatureHelpItems | undefined {
+    const entry = this.controller.getFile(fileName);
+    const prefixLength = entry?.prefix?.length ?? 0;
+    return this.service.getSignatureHelpItems(
+      fileName,
+      position + prefixLength,
+      options,
+    );
+  }
+
+  dispose(): void {
+    this.service.dispose();
+  }
 }
diff --git a/libs/@hashintel/petrinaut/src/checker/provider.tsx b/libs/@hashintel/petrinaut/src/checker/provider.tsx
index 04ef268eef0..a2c95cb3137 100644
--- a/libs/@hashintel/petrinaut/src/checker/provider.tsx
+++ b/libs/@hashintel/petrinaut/src/checker/provider.tsx
@@ -1,54 +1,75 @@
-import { use, useEffect, useState } from "react";
+import { use, useCallback, useEffect, useMemo, useRef, useState } from "react";
 
 import { SDCPNContext } from "../state/sdcpn-context";
-import { CheckerContext } from "./context";
-import type { CheckerResult } from "./worker/protocol";
-import { useCheckerWorker } from "./worker/use-checker-worker";
+import { LanguageClientContext } from "./context";
+import type {
+  Diagnostic,
+  DocumentUri,
+  PublishDiagnosticsParams,
+} from "./worker/protocol";
+import { useLanguageClient } from "./worker/use-language-client";
 
-const EMPTY_RESULT: CheckerResult = {
-  isValid: true,
-  itemDiagnostics: [],
-};
-
-export const CheckerProvider: React.FC<{ children: React.ReactNode }> = ({
-  children,
-}) => {
+export const LanguageClientProvider: React.FC<{
+  children: React.ReactNode;
+}> = ({ children }) => {
   const { petriNetDefinition } = use(SDCPNContext);
-  const { setSDCPN, getCompletions, getQuickInfo, getSignatureHelp } =
-    useCheckerWorker();
+  const client = useLanguageClient();
 
-  const [checkResult, setCheckerResult] = useState<CheckerResult>(EMPTY_RESULT);
+  const [diagnosticsByUri, setDiagnosticsByUri] = useState<
+    Map<DocumentUri, Diagnostic[]>
+  >(new Map());
 
-  useEffect(() => {
-    let cancelled = false;
-
-    void setSDCPN(petriNetDefinition).then((result) => {
-      if (!cancelled) {
-        setCheckerResult(result);
-      }
-    });
-
-    return () => {
-      cancelled = true;
-    };
-  }, [petriNetDefinition, setSDCPN]);
-
-  const totalDiagnosticsCount = checkResult.itemDiagnostics.reduce(
-    (sum, item) => sum + item.diagnostics.length,
-    0,
+  // Subscribe to diagnostics pushed from the server
+  const handleDiagnostics = useCallback(
+    (allParams: PublishDiagnosticsParams[]) => {
+      setDiagnosticsByUri(() => {
+        const next = new Map<DocumentUri, Diagnostic[]>();
+        for (const param of allParams) {
+          if (param.diagnostics.length > 0) {
+            next.set(param.uri, param.diagnostics);
+          }
+        }
+        return next;
+      });
+    },
+    [],
   );
 
+  useEffect(() => {
+    client.onDiagnostics(handleDiagnostics);
+  }, [client, handleDiagnostics]);
+
+  // Initialize on first mount, then send incremental updates
+  const initializedRef = useRef(false);
+  useEffect(() => {
+    if (!initializedRef.current) {
+      client.initialize(petriNetDefinition);
+      initializedRef.current = true;
+    } else {
+      client.notifySDCPNChanged(petriNetDefinition);
+    }
+  }, [petriNetDefinition, client]);
+
+  const totalDiagnosticsCount = useMemo(() => {
+    let count = 0;
+    for (const diagnostics of diagnosticsByUri.values()) {
+      count += diagnostics.length;
+    }
+    return count;
+  }, [diagnosticsByUri]);
+
   return (
-    <CheckerContext.Provider
+    <LanguageClientContext.Provider
       value={{
-        checkResult,
+        diagnosticsByUri,
         totalDiagnosticsCount,
-        getCompletions,
-        getQuickInfo,
-        getSignatureHelp,
+        notifyDocumentChanged: client.notifyDocumentChanged,
+        requestCompletion: client.requestCompletion,
+        requestHover: client.requestHover,
+        requestSignatureHelp: client.requestSignatureHelp,
       }}
     >
       {children}
-    </CheckerContext.Provider>
+    </LanguageClientContext.Provider>
   );
 };
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
index caa4b826ca7..df0f1ef56f5 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
+++ b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
@@ -2,155 +2,270 @@
 /**
  * Checker WebWorker — runs TypeScript validation off the main thread.
  *
- * Receives JSON-RPC requests via `postMessage`, delegates to `checkSDCPN`,
- * serializes the diagnostics (flatten messageText, strip non-cloneable fields),
- * and posts the result back.
+ * Implements an LSP-inspired protocol over JSON-RPC 2.0:
+ * - Notifications: `initialize`, `sdcpn/didChange`, `textDocument/didChange`
+ * - Requests: `textDocument/completion`, `textDocument/hover`, `textDocument/signatureHelp`
+ * - Server push: `textDocument/publishDiagnostics`
  *
- * The LanguageService is persisted between calls so that `getCompletions`
- * can reuse it without re-sending the full SDCPN model.
+ * The LanguageService is created once and reused across SDCPN changes.
  */
 import ts from "typescript";
 
+import type { SDCPN } from "../../core/types/sdcpn";
 import { checkSDCPN } from "../lib/checker";
-import {
-  createSDCPNLanguageService,
-  type SDCPNLanguageService,
-} from "../lib/create-sdcpn-language-service";
+import { SDCPNLanguageServer } from "../lib/create-sdcpn-language-service";
 import { getItemFilePath } from "../lib/file-paths";
 import type {
-  CheckerCompletionItem,
-  CheckerCompletionResult,
-  CheckerDiagnostic,
-  CheckerItemDiagnostics,
-  CheckerQuickInfoResult,
-  CheckerResult,
-  CheckerSignatureHelpResult,
-  CheckerSignatureInfo,
-  JsonRpcRequest,
-  JsonRpcResponse,
+  ClientMessage,
+  CompletionItem,
+  CompletionList,
+  Diagnostic,
+  DocumentUri,
+  Hover,
+  PublishDiagnosticsParams,
+  ServerMessage,
+  SignatureHelp,
+  SignatureInformation,
 } from "./protocol";
 
-/** Persisted LanguageService — created on `setSDCPN`, reused by `getCompletions`. */
-let languageService: SDCPNLanguageService | null = null;
+// ---------------------------------------------------------------------------
+// URI ↔ internal file path mapping
+// ---------------------------------------------------------------------------
 
-/** Strip `ts.SourceFile` and flatten `DiagnosticMessageChain` for structured clone. */
-function serializeDiagnostic(diag: ts.Diagnostic): CheckerDiagnostic {
+const TRANSITION_LAMBDA_URI_RE =
+  /^inmemory:\/\/sdcpn\/transitions\/([^/]+)\/lambda\.ts$/;
+const TRANSITION_KERNEL_URI_RE =
+  /^inmemory:\/\/sdcpn\/transitions\/([^/]+)\/kernel\.ts$/;
+const DE_URI_RE = /^inmemory:\/\/sdcpn\/differential-equations\/([^/]+)\.ts$/;
+
+/** Convert a document URI to the internal virtual file path used by the TS LanguageService. */
+function uriToFilePath(uri: DocumentUri): string | null {
+  let match = TRANSITION_LAMBDA_URI_RE.exec(uri);
+  if (match) {
+    return getItemFilePath("transition-lambda-code", {
+      transitionId: match[1]!,
+    });
+  }
+
+  match = TRANSITION_KERNEL_URI_RE.exec(uri);
+  if (match) {
+    return getItemFilePath("transition-kernel-code", {
+      transitionId: match[1]!,
+    });
+  }
+
+  match = DE_URI_RE.exec(uri);
+  if (match) {
+    return getItemFilePath("differential-equation-code", { id: match[1]! });
+  }
+
+  return null;
+}
+
+const TRANSITION_LAMBDA_PATH_RE = /^\/transitions\/([^/]+)\/lambda\/code\.ts$/;
+const TRANSITION_KERNEL_PATH_RE = /^\/transitions\/([^/]+)\/kernel\/code\.ts$/;
+const DE_PATH_RE = /^\/differential_equations\/([^/]+)\/code\.ts$/;
+
+/** Convert an internal file path to the document URI used by Monaco. */
+function filePathToUri(filePath: string): DocumentUri | null {
+  let match = TRANSITION_LAMBDA_PATH_RE.exec(filePath);
+  if (match) {
+    return `inmemory://sdcpn/transitions/${match[1]!}/lambda.ts`;
+  }
+
+  match = TRANSITION_KERNEL_PATH_RE.exec(filePath);
+  if (match) {
+    return `inmemory://sdcpn/transitions/${match[1]!}/kernel.ts`;
+  }
+
+  match = DE_PATH_RE.exec(filePath);
+  if (match) {
+    return `inmemory://sdcpn/differential-equations/${match[1]!}.ts`;
+  }
+
+  return null;
+}
+
+// ---------------------------------------------------------------------------
+// Serialization helpers
+// ---------------------------------------------------------------------------
+
+/**
+ * Map `ts.DiagnosticCategory` to LSP-like severity.
+ * TS: 0=Warning, 1=Error, 2=Suggestion, 3=Message
+ * LSP: 1=Error, 2=Warning, 3=Information, 4=Hint
+ */
+function toLspSeverity(category: number): number {
+  switch (category) {
+    case 0:
+      return 2; // Warning
+    case 1:
+      return 1; // Error
+    case 2:
+      return 4; // Hint (Suggestion)
+    case 3:
+      return 3; // Information (Message)
+    default:
+      return 1;
+  }
+}
+
+function serializeDiagnostic(diag: ts.Diagnostic): Diagnostic {
   return {
-    category: diag.category,
+    severity: toLspSeverity(diag.category),
     code: diag.code,
-    messageText: ts.flattenDiagnosticMessageText(diag.messageText, "\n"),
+    message: ts.flattenDiagnosticMessageText(diag.messageText, "\n"),
     start: diag.start,
     length: diag.length,
   };
 }
 
-/** Map (itemType, itemId) → checker virtual file path. */
-function getCheckerFilePath(
-  itemType: CheckerItemDiagnostics["itemType"],
-  itemId: string,
-): string {
-  switch (itemType) {
-    case "transition-lambda":
-      return getItemFilePath("transition-lambda-code", {
-        transitionId: itemId,
-      });
-    case "transition-kernel":
-      return getItemFilePath("transition-kernel-code", {
-        transitionId: itemId,
-      });
-    case "differential-equation":
-      return getItemFilePath("differential-equation-code", { id: itemId });
+// ---------------------------------------------------------------------------
+// Server state
+// ---------------------------------------------------------------------------
+
+let server: SDCPNLanguageServer | null = null;
+
+function respond(id: number, result: unknown): void {
+  self.postMessage({
+    jsonrpc: "2.0",
+    id,
+    result,
+  } satisfies ServerMessage);
+}
+
+function respondError(id: number, message: string): void {
+  self.postMessage({
+    jsonrpc: "2.0",
+    id,
+    error: { code: -32603, message },
+  } satisfies ServerMessage);
+}
+
+/** Run diagnostics on all SDCPN code files and push results to the main thread. */
+function publishAllDiagnostics(sdcpn: SDCPN): void {
+  if (!server) {
+    return;
   }
+
+  const result = checkSDCPN(sdcpn, server);
+  const params: PublishDiagnosticsParams[] = result.itemDiagnostics.map(
+    (item) => {
+      const uri = filePathToUri(item.filePath);
+      return {
+        uri: uri ?? item.filePath,
+        diagnostics: item.diagnostics.map(serializeDiagnostic),
+      };
+    },
+  );
+
+  self.postMessage({
+    jsonrpc: "2.0",
+    method: "textDocument/publishDiagnostics",
+    params,
+  } satisfies ServerMessage);
 }
 
-self.onmessage = async ({ data }: MessageEvent<JsonRpcRequest>) => {
-  const { id, method } = data;
+// ---------------------------------------------------------------------------
+// Message handler
+// ---------------------------------------------------------------------------
+
+/** Cache the last SDCPN for re-running diagnostics after single-file changes. */
+let lastSDCPN: SDCPN | null = null;
 
+self.onmessage = ({ data }: MessageEvent<ClientMessage>) => {
   try {
-    switch (method) {
-      case "setSDCPN": {
+    switch (data.method) {
+      // --- Notifications (no response) ---
+
+      case "initialize": {
         const { sdcpn } = data.params;
+        lastSDCPN = sdcpn;
+        server = new SDCPNLanguageServer();
+        server.syncFiles(sdcpn);
+        publishAllDiagnostics(sdcpn);
+        break;
+      }
 
-        languageService = createSDCPNLanguageService(sdcpn);
-        const raw = checkSDCPN(sdcpn, languageService);
-
-        const result: CheckerResult = {
-          isValid: raw.isValid,
-          itemDiagnostics: raw.itemDiagnostics.map(
-            (item): CheckerItemDiagnostics => ({
-              itemId: item.itemId,
-              itemType: item.itemType,
-              filePath: item.filePath,
-              diagnostics: item.diagnostics.map(serializeDiagnostic),
-            }),
-          ),
-        };
-
-        self.postMessage({
-          jsonrpc: "2.0",
-          id,
-          result,
-        } satisfies JsonRpcResponse<CheckerResult>);
+      case "sdcpn/didChange": {
+        const { sdcpn } = data.params;
+        lastSDCPN = sdcpn;
+        if (!server) {
+          server = new SDCPNLanguageServer();
+        }
+        server.syncFiles(sdcpn);
+        publishAllDiagnostics(sdcpn);
         break;
       }
 
-      case "getCompletions": {
-        const { itemType, itemId, offset } = data.params;
+      case "textDocument/didChange": {
+        if (!server) {
+          break;
+        }
+        const filePath = uriToFilePath(data.params.textDocument.uri);
+        if (filePath) {
+          server.updateDocumentContent(filePath, data.params.text);
+          // Re-run full diagnostics since type changes can cascade
+          if (lastSDCPN) {
+            publishAllDiagnostics(lastSDCPN);
+          }
+        }
+        break;
+      }
 
-        // Wait before requesting completions, to be sure the file content is updated
-        await new Promise<void>((resolve) => {
-          setTimeout(resolve, 50);
-        });
+      // --- Requests (send response) ---
 
-        if (!languageService) {
-          self.postMessage({
-            jsonrpc: "2.0",
-            id,
-            result: { items: [] },
-          } satisfies JsonRpcResponse<CheckerCompletionResult>);
+      case "textDocument/completion": {
+        const { id } = data;
+        if (!server) {
+          respond(id, { items: [] } satisfies CompletionList);
           break;
         }
 
-        const filePath = getCheckerFilePath(itemType, itemId);
-        const completions = languageService.getCompletionsAtPosition(
+        const filePath = uriToFilePath(data.params.textDocument.uri);
+        if (!filePath) {
+          respond(id, { items: [] } satisfies CompletionList);
+          break;
+        }
+
+        const completions = server.getCompletionsAtPosition(
           filePath,
-          offset,
+          data.params.offset,
           undefined,
         );
 
-        const items: CheckerCompletionItem[] = (completions?.entries ?? []).map(
+        const items: CompletionItem[] = (completions?.entries ?? []).map(
           (entry) => ({
-            name: entry.name,
+            label: entry.name,
             kind: entry.kind,
             sortText: entry.sortText,
             insertText: entry.insertText,
           }),
         );
 
-        self.postMessage({
-          jsonrpc: "2.0",
-          id,
-          result: { items },
-        } satisfies JsonRpcResponse<CheckerCompletionResult>);
+        respond(id, { items } satisfies CompletionList);
         break;
       }
 
-      case "getQuickInfo": {
-        const { itemType, itemId, offset } = data.params;
+      case "textDocument/hover": {
+        const { id } = data;
+        if (!server) {
+          respond(id, null);
+          break;
+        }
 
-        if (!languageService) {
-          self.postMessage({
-            jsonrpc: "2.0",
-            id,
-            result: null,
-          } satisfies JsonRpcResponse<CheckerQuickInfoResult>);
+        const filePath = uriToFilePath(data.params.textDocument.uri);
+        if (!filePath) {
+          respond(id, null);
           break;
         }
 
-        const filePath = getCheckerFilePath(itemType, itemId);
-        const info = languageService.getQuickInfoAtPosition(filePath, offset);
+        const info = server.getQuickInfoAtPosition(
+          filePath,
+          data.params.offset,
+        );
 
-        const result: CheckerQuickInfoResult = info
+        const result: Hover = info
           ? {
               displayParts: ts.displayPartsToString(info.displayParts),
               documentation: ts.displayPartsToString(info.documentation),
@@ -159,44 +274,35 @@ self.onmessage = async ({ data }: MessageEvent<JsonRpcRequest>) => {
             }
           : null;
 
-        self.postMessage({
-          jsonrpc: "2.0",
-          id,
-          result,
-        } satisfies JsonRpcResponse<CheckerQuickInfoResult>);
+        respond(id, result);
         break;
       }
 
-      case "getSignatureHelp": {
-        const { itemType, itemId, offset } = data.params;
-
-        // Wait a bit before requesting completions, to be sure the file content is updated
-        await new Promise<void>((resolve) => {
-          setTimeout(resolve, 50);
-        });
+      case "textDocument/signatureHelp": {
+        const { id } = data;
+        if (!server) {
+          respond(id, null);
+          break;
+        }
 
-        if (!languageService) {
-          self.postMessage({
-            jsonrpc: "2.0",
-            id,
-            result: null,
-          } satisfies JsonRpcResponse<CheckerSignatureHelpResult>);
+        const filePath = uriToFilePath(data.params.textDocument.uri);
+        if (!filePath) {
+          respond(id, null);
           break;
         }
 
-        const filePath = getCheckerFilePath(itemType, itemId);
-        const help = languageService.getSignatureHelpItems(
+        const help = server.getSignatureHelpItems(
           filePath,
-          offset,
+          data.params.offset,
           undefined,
         );
 
-        const result: CheckerSignatureHelpResult = help
+        const result: SignatureHelp = help
           ? {
               activeSignature: help.selectedItemIndex,
               activeParameter: help.argumentIndex,
               signatures: help.items.map(
-                (item): CheckerSignatureInfo => ({
+                (item): SignatureInformation => ({
                   label: [
                     ...item.prefixDisplayParts,
                     ...item.parameters.flatMap((param, idx) => [
@@ -217,22 +323,14 @@ self.onmessage = async ({ data }: MessageEvent<JsonRpcRequest>) => {
             }
           : null;
 
-        self.postMessage({
-          jsonrpc: "2.0",
-          id,
-          result,
-        } satisfies JsonRpcResponse<CheckerSignatureHelpResult>);
+        respond(id, result);
         break;
       }
     }
   } catch (err) {
-    self.postMessage({
-      jsonrpc: "2.0",
-      id,
-      error: {
-        code: -32603,
-        message: err instanceof Error ? err.message : String(err),
-      },
-    } satisfies JsonRpcResponse);
+    // Only requests have an `id` that needs a response
+    if ("id" in data) {
+      respondError(data.id, err instanceof Error ? err.message : String(err));
+    }
   }
 };
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts b/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
index 7e52045904c..057336a6dae 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
+++ b/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
@@ -1,57 +1,70 @@
 /**
- * JSON-RPC 2.0 protocol types for the checker WebWorker.
+ * LSP-inspired protocol types for the checker WebWorker.
  *
- * These types define the contract between the main thread and the worker.
+ * Uses JSON-RPC 2.0 with LSP-like method names and structures.
  * Diagnostic types are serializable (no `ts.SourceFile` references) so they
  * can cross the postMessage boundary via structured clone.
+ *
+ * Deviations from LSP:
+ * - Uses character offsets instead of line/character positions (both Monaco
+ *   and the TS LanguageService work in offsets internally).
+ * - Uses `sdcpn/didChange` for structural model changes (not per-document).
  */
 import type { SDCPN } from "../../core/types/sdcpn";
 
 // ---------------------------------------------------------------------------
-// Diagnostics — serializable variants of ts.Diagnostic
+// Document identification
 // ---------------------------------------------------------------------------
 
-/** A single TypeScript diagnostic, safe for structured clone. */
-export type CheckerDiagnostic = {
-  /** @see ts.DiagnosticCategory */
-  category: number;
+/** URI identifying a text document (e.g., "inmemory://sdcpn/transitions/t1/lambda.ts"). */
+export type DocumentUri = string;
+
+export type TextDocumentIdentifier = {
+  uri: DocumentUri;
+};
+
+/**
+ * Position in a text document, specified as a character offset.
+ *
+ * Unlike standard LSP (which uses line/character), we use offsets since both
+ * Monaco and the TS LanguageService work in offsets internally.
+ */
+export type TextDocumentPositionParams = {
+  textDocument: TextDocumentIdentifier;
+  offset: number;
+};
+
+// ---------------------------------------------------------------------------
+// Diagnostics
+// ---------------------------------------------------------------------------
+
+/** A single diagnostic, safe for structured clone. */
+export type Diagnostic = {
+  /** Severity: 1=Error, 2=Warning, 3=Info, 4=Hint. */
+  severity: number;
   /** TypeScript error code (e.g. 2322 for type mismatch). */
   code: number;
   /** Human-readable message, pre-flattened from `ts.DiagnosticMessageChain`. */
-  messageText: string;
-  /** Character offset in user code where the error starts. */
+  message: string;
+  /** Character offset in user code where the diagnostic starts. */
   start: number | undefined;
-  /** Length of the error span in characters. */
+  /** Length of the diagnostic span in characters. */
   length: number | undefined;
 };
 
-/** Diagnostics grouped by SDCPN item (one transition function or differential equation). */
-export type CheckerItemDiagnostics = {
-  /** ID of the transition or differential equation. */
-  itemId: string;
-  /** Which piece of code was checked. */
-  itemType: "transition-lambda" | "transition-kernel" | "differential-equation";
-  /** Path in the virtual file system used by the TS LanguageService. */
-  filePath: string;
-  /** All diagnostics found in this item's code. */
-  diagnostics: CheckerDiagnostic[];
-};
-
-/** Result of validating an entire SDCPN model. */
-export type CheckerResult = {
-  /** `true` when every item compiles without errors. */
-  isValid: boolean;
-  /** Per-item diagnostics (empty when valid). */
-  itemDiagnostics: CheckerItemDiagnostics[];
+/** Diagnostics for a single document. */
+export type PublishDiagnosticsParams = {
+  uri: DocumentUri;
+  diagnostics: Diagnostic[];
 };
 
 // ---------------------------------------------------------------------------
-// Completions — serializable variants of ts.CompletionEntry
+// Completions
 // ---------------------------------------------------------------------------
 
 /** A single completion suggestion, safe for structured clone. */
-export type CheckerCompletionItem = {
-  name: string;
+export type CompletionItem = {
+  label: string;
   /** @see ts.ScriptElementKind */
   kind: string;
   sortText: string;
@@ -59,16 +72,16 @@ export type CheckerCompletionItem = {
 };
 
 /** Result of requesting completions at a position. */
-export type CheckerCompletionResult = {
-  items: CheckerCompletionItem[];
+export type CompletionList = {
+  items: CompletionItem[];
 };
 
 // ---------------------------------------------------------------------------
-// Quick Info (hover) — serializable variant of ts.QuickInfo
+// Hover
 // ---------------------------------------------------------------------------
 
-/** Result of requesting quick info (hover) at a position. */
-export type CheckerQuickInfoResult = {
+/** Result of requesting hover info at a position. */
+export type Hover = {
   /** Type/signature display string. */
   displayParts: string;
   /** JSDoc documentation string. */
@@ -80,73 +93,93 @@ export type CheckerQuickInfoResult = {
 } | null;
 
 // ---------------------------------------------------------------------------
-// Signature Help — serializable variant of ts.SignatureHelpItems
+// Signature Help
 // ---------------------------------------------------------------------------
 
 /** A single parameter in a signature. */
-export type CheckerSignatureParameter = {
+export type SignatureParameter = {
   label: string;
   documentation: string;
 };
 
 /** A single signature (overload). */
-export type CheckerSignatureInfo = {
+export type SignatureInformation = {
   label: string;
   documentation: string;
-  parameters: CheckerSignatureParameter[];
+  parameters: SignatureParameter[];
 };
 
 /** Result of requesting signature help at a position. */
-export type CheckerSignatureHelpResult = {
-  signatures: CheckerSignatureInfo[];
+export type SignatureHelp = {
+  signatures: SignatureInformation[];
   activeSignature: number;
   activeParameter: number;
 } | null;
 
 // ---------------------------------------------------------------------------
-// JSON-RPC 2.0
+// JSON-RPC 2.0 messages: main thread → worker
 // ---------------------------------------------------------------------------
 
-/** A JSON-RPC request sent from the main thread to the worker. */
-export type JsonRpcRequest =
+/** Notifications (fire-and-forget, no response expected). */
+type ClientNotification =
   | {
       jsonrpc: "2.0";
-      id: number;
-      method: "setSDCPN";
+      method: "initialize";
       params: { sdcpn: SDCPN };
     }
   | {
       jsonrpc: "2.0";
-      id: number;
-      method: "getCompletions";
+      method: "sdcpn/didChange";
+      params: { sdcpn: SDCPN };
+    }
+  | {
+      jsonrpc: "2.0";
+      method: "textDocument/didChange";
       params: {
-        itemType: CheckerItemDiagnostics["itemType"];
-        itemId: string;
-        offset: number;
+        textDocument: TextDocumentIdentifier;
+        text: string;
       };
+    };
+
+/** Requests (expect a response with matching `id`). */
+type ClientRequest =
+  | {
+      jsonrpc: "2.0";
+      id: number;
+      method: "textDocument/completion";
+      params: TextDocumentPositionParams;
     }
   | {
       jsonrpc: "2.0";
       id: number;
-      method: "getQuickInfo";
-      params: {
-        itemType: CheckerItemDiagnostics["itemType"];
-        itemId: string;
-        offset: number;
-      };
+      method: "textDocument/hover";
+      params: TextDocumentPositionParams;
     }
   | {
       jsonrpc: "2.0";
       id: number;
-      method: "getSignatureHelp";
-      params: {
-        itemType: CheckerItemDiagnostics["itemType"];
-        itemId: string;
-        offset: number;
-      };
+      method: "textDocument/signatureHelp";
+      params: TextDocumentPositionParams;
     };
 
-/** A JSON-RPC response sent from the worker back to the main thread. */
-export type JsonRpcResponse<Result = unknown> =
+/** Any message from the main thread to the worker. */
+export type ClientMessage = ClientNotification | ClientRequest;
+
+// ---------------------------------------------------------------------------
+// JSON-RPC 2.0 messages: worker → main thread
+// ---------------------------------------------------------------------------
+
+/** A successful response to a request. */
+type ServerResponse<Result = unknown> =
   | { jsonrpc: "2.0"; id: number; result: Result }
   | { jsonrpc: "2.0"; id: number; error: { code: number; message: string } };
+
+/** Server-initiated notifications (no `id`). */
+type ServerNotification = {
+  jsonrpc: "2.0";
+  method: "textDocument/publishDiagnostics";
+  params: PublishDiagnosticsParams[];
+};
+
+/** Any message from the worker to the main thread. */
+export type ServerMessage = ServerResponse | ServerNotification;
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts b/libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts
deleted file mode 100644
index 82d63cbca62..00000000000
--- a/libs/@hashintel/petrinaut/src/checker/worker/use-checker-worker.ts
+++ /dev/null
@@ -1,165 +0,0 @@
-import { useCallback, useEffect, useRef } from "react";
-
-import type { SDCPN } from "../../core/types/sdcpn";
-import type {
-  CheckerCompletionResult,
-  CheckerItemDiagnostics,
-  CheckerQuickInfoResult,
-  CheckerResult,
-  CheckerSignatureHelpResult,
-  JsonRpcRequest,
-  JsonRpcResponse,
-} from "./protocol";
-
-type Pending = {
-  resolve: (result: never) => void;
-  reject: (error: Error) => void;
-};
-
-/** Methods exposed by the checker WebWorker. */
-export type CheckerWorkerApi = {
-  /** Send an SDCPN model to the worker. Persists the LanguageService and returns diagnostics. */
-  setSDCPN: (sdcpn: SDCPN) => Promise<CheckerResult>;
-  /** Request completions at a position within an SDCPN item. */
-  getCompletions: (
-    itemType: CheckerItemDiagnostics["itemType"],
-    itemId: string,
-    offset: number,
-  ) => Promise<CheckerCompletionResult>;
-  /** Request quick info (hover) at a position within an SDCPN item. */
-  getQuickInfo: (
-    itemType: CheckerItemDiagnostics["itemType"],
-    itemId: string,
-    offset: number,
-  ) => Promise<CheckerQuickInfoResult>;
-  /** Request signature help at a position within an SDCPN item. */
-  getSignatureHelp: (
-    itemType: CheckerItemDiagnostics["itemType"],
-    itemId: string,
-    offset: number,
-  ) => Promise<CheckerSignatureHelpResult>;
-};
-
-/**
- * Spawn a checker WebWorker and return a Promise-based API to interact with it.
- * The worker is created on mount and terminated on unmount.
- */
-export function useCheckerWorker(): CheckerWorkerApi {
-  const workerRef = useRef<Worker | null>(null);
-  const pendingRef = useRef(new Map<number, Pending>());
-  const nextId = useRef(0);
-
-  useEffect(() => {
-    const worker = new Worker(new URL("./checker.worker.ts", import.meta.url), {
-      type: "module",
-    });
-
-    worker.onmessage = (event: MessageEvent<JsonRpcResponse>) => {
-      const response = event.data;
-      const pending = pendingRef.current.get(response.id);
-      if (!pending) {
-        return;
-      }
-      pendingRef.current.delete(response.id);
-
-      if ("error" in response) {
-        pending.reject(new Error(response.error.message));
-      } else {
-        pending.resolve(response.result as never);
-      }
-    };
-
-    workerRef.current = worker;
-    const pending = pendingRef.current;
-
-    return () => {
-      worker.terminate();
-      workerRef.current = null;
-      for (const entry of pending.values()) {
-        entry.reject(new Error("Worker terminated"));
-      }
-      pending.clear();
-    };
-  }, []);
-
-  const sendRequest = useCallback(<T>(request: JsonRpcRequest): Promise<T> => {
-    const worker = workerRef.current;
-    if (!worker) {
-      return Promise.reject(new Error("Worker not initialized"));
-    }
-
-    return new Promise<T>((resolve, reject) => {
-      pendingRef.current.set(request.id, {
-        resolve: resolve as (result: never) => void,
-        reject,
-      });
-      worker.postMessage(request);
-    });
-  }, []);
-
-  const setSDCPN = useCallback(
-    (sdcpn: SDCPN): Promise<CheckerResult> => {
-      const id = nextId.current++;
-      return sendRequest<CheckerResult>({
-        jsonrpc: "2.0",
-        id,
-        method: "setSDCPN",
-        params: { sdcpn },
-      });
-    },
-    [sendRequest],
-  );
-
-  const getCompletions = useCallback(
-    (
-      itemType: CheckerItemDiagnostics["itemType"],
-      itemId: string,
-      offset: number,
-    ): Promise<CheckerCompletionResult> => {
-      const id = nextId.current++;
-      return sendRequest<CheckerCompletionResult>({
-        jsonrpc: "2.0",
-        id,
-        method: "getCompletions",
-        params: { itemType, itemId, offset },
-      });
-    },
-    [sendRequest],
-  );
-
-  const getQuickInfo = useCallback(
-    (
-      itemType: CheckerItemDiagnostics["itemType"],
-      itemId: string,
-      offset: number,
-    ): Promise<CheckerQuickInfoResult> => {
-      const id = nextId.current++;
-      return sendRequest<CheckerQuickInfoResult>({
-        jsonrpc: "2.0",
-        id,
-        method: "getQuickInfo",
-        params: { itemType, itemId, offset },
-      });
-    },
-    [sendRequest],
-  );
-
-  const getSignatureHelp = useCallback(
-    (
-      itemType: CheckerItemDiagnostics["itemType"],
-      itemId: string,
-      offset: number,
-    ): Promise<CheckerSignatureHelpResult> => {
-      const id = nextId.current++;
-      return sendRequest<CheckerSignatureHelpResult>({
-        jsonrpc: "2.0",
-        id,
-        method: "getSignatureHelp",
-        params: { itemType, itemId, offset },
-      });
-    },
-    [sendRequest],
-  );
-
-  return { setSDCPN, getCompletions, getQuickInfo, getSignatureHelp };
-}
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/use-language-client.ts b/libs/@hashintel/petrinaut/src/checker/worker/use-language-client.ts
new file mode 100644
index 00000000000..0cc12567b95
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/checker/worker/use-language-client.ts
@@ -0,0 +1,211 @@
+import { useCallback, useEffect, useRef } from "react";
+
+import type { SDCPN } from "../../core/types/sdcpn";
+import type {
+  ClientMessage,
+  CompletionList,
+  DocumentUri,
+  Hover,
+  PublishDiagnosticsParams,
+  ServerMessage,
+  SignatureHelp,
+} from "./protocol";
+
+type Pending = {
+  resolve: (result: never) => void;
+  reject: (error: Error) => void;
+};
+
+/** Methods exposed by the language client (main-thread side of the worker). */
+export type LanguageClientApi = {
+  /** Initialize the server with the full SDCPN model (notification, no response). */
+  initialize: (sdcpn: SDCPN) => void;
+  /** Notify the server that the SDCPN model has changed structurally (notification). */
+  notifySDCPNChanged: (sdcpn: SDCPN) => void;
+  /** Notify the server that a single document's content changed (notification). */
+  notifyDocumentChanged: (uri: DocumentUri, text: string) => void;
+  /** Request completions at a position within a document. */
+  requestCompletion: (
+    uri: DocumentUri,
+    offset: number,
+  ) => Promise<CompletionList>;
+  /** Request hover info at a position within a document. */
+  requestHover: (uri: DocumentUri, offset: number) => Promise<Hover>;
+  /** Request signature help at a position within a document. */
+  requestSignatureHelp: (
+    uri: DocumentUri,
+    offset: number,
+  ) => Promise<SignatureHelp>;
+  /** Register a callback for diagnostics pushed from the server. */
+  onDiagnostics: (
+    callback: (params: PublishDiagnosticsParams[]) => void,
+  ) => void;
+};
+
+/**
+ * Spawn a checker WebWorker and return an LSP-inspired API to interact with it.
+ * The worker is created on mount and terminated on unmount.
+ */
+export function useLanguageClient(): LanguageClientApi {
+  const workerRef = useRef<Worker | null>(null);
+  const pendingRef = useRef(new Map<number, Pending>());
+  const nextId = useRef(0);
+  const diagnosticsCallbackRef = useRef<
+    ((params: PublishDiagnosticsParams[]) => void) | null
+  >(null);
+
+  useEffect(() => {
+    const worker = new Worker(new URL("./checker.worker.ts", import.meta.url), {
+      type: "module",
+    });
+
+    worker.onmessage = (event: MessageEvent<ServerMessage>) => {
+      const msg = event.data;
+
+      if ("id" in msg) {
+        // Response to a request
+        const pending = pendingRef.current.get(msg.id);
+        if (!pending) {
+          return;
+        }
+        pendingRef.current.delete(msg.id);
+
+        if ("error" in msg) {
+          pending.reject(new Error(msg.error.message));
+        } else {
+          pending.resolve(msg.result as never);
+        }
+      } else if (
+        "method" in msg &&
+        msg.method === "textDocument/publishDiagnostics"
+      ) {
+        // Server-pushed notification
+        diagnosticsCallbackRef.current?.(msg.params);
+      }
+    };
+
+    workerRef.current = worker;
+    const pending = pendingRef.current;
+
+    return () => {
+      worker.terminate();
+      workerRef.current = null;
+      for (const entry of pending.values()) {
+        entry.reject(new Error("Worker terminated"));
+      }
+      pending.clear();
+    };
+  }, []);
+
+  // --- Notifications (fire-and-forget) ---
+
+  const sendNotification = useCallback((message: Omit<ClientMessage, "id">) => {
+    workerRef.current?.postMessage(message);
+  }, []);
+
+  const initialize = useCallback(
+    (sdcpn: SDCPN) => {
+      sendNotification({
+        jsonrpc: "2.0",
+        method: "initialize",
+        params: { sdcpn },
+      });
+    },
+    [sendNotification],
+  );
+
+  const notifySDCPNChanged = useCallback(
+    (sdcpn: SDCPN) => {
+      sendNotification({
+        jsonrpc: "2.0",
+        method: "sdcpn/didChange",
+        params: { sdcpn },
+      });
+    },
+    [sendNotification],
+  );
+
+  const notifyDocumentChanged = useCallback(
+    (uri: DocumentUri, text: string) => {
+      sendNotification({
+        jsonrpc: "2.0",
+        method: "textDocument/didChange",
+        params: { textDocument: { uri }, text },
+      });
+    },
+    [sendNotification],
+  );
+
+  // --- Requests (return Promise) ---
+
+  const sendRequest = useCallback(<T>(message: ClientMessage): Promise<T> => {
+    const worker = workerRef.current;
+    if (!worker) {
+      return Promise.reject(new Error("Worker not initialized"));
+    }
+
+    return new Promise<T>((resolve, reject) => {
+      pendingRef.current.set((message as { id: number }).id, {
+        resolve: resolve as (result: never) => void,
+        reject,
+      });
+      worker.postMessage(message);
+    });
+  }, []);
+
+  const requestCompletion = useCallback(
+    (uri: DocumentUri, offset: number): Promise<CompletionList> => {
+      const id = nextId.current++;
+      return sendRequest<CompletionList>({
+        jsonrpc: "2.0",
+        id,
+        method: "textDocument/completion",
+        params: { textDocument: { uri }, offset },
+      });
+    },
+    [sendRequest],
+  );
+
+  const requestHover = useCallback(
+    (uri: DocumentUri, offset: number): Promise<Hover> => {
+      const id = nextId.current++;
+      return sendRequest<Hover>({
+        jsonrpc: "2.0",
+        id,
+        method: "textDocument/hover",
+        params: { textDocument: { uri }, offset },
+      });
+    },
+    [sendRequest],
+  );
+
+  const requestSignatureHelp = useCallback(
+    (uri: DocumentUri, offset: number): Promise<SignatureHelp> => {
+      const id = nextId.current++;
+      return sendRequest<SignatureHelp>({
+        jsonrpc: "2.0",
+        id,
+        method: "textDocument/signatureHelp",
+        params: { textDocument: { uri }, offset },
+      });
+    },
+    [sendRequest],
+  );
+
+  const onDiagnostics = useCallback(
+    (callback: (params: PublishDiagnosticsParams[]) => void) => {
+      diagnosticsCallbackRef.current = callback;
+    },
+    [],
+  );
+
+  return {
+    initialize,
+    notifySDCPNChanged,
+    notifyDocumentChanged,
+    requestCompletion,
+    requestHover,
+    requestSignatureHelp,
+    onDiagnostics,
+  };
+}
diff --git a/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
index 9a9cbbe0868..dfdb5624cca 100644
--- a/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
@@ -1,10 +1,9 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect } from "react";
 
-import { CheckerContext } from "../checker/context";
-import type { CheckerCompletionItem } from "../checker/worker/protocol";
+import { LanguageClientContext } from "../checker/context";
+import type { CompletionItem } from "../checker/worker/protocol";
 import { MonacoContext } from "./context";
-import { parseEditorPath } from "./editor-paths";
 
 /**
  * Map TypeScript `ScriptElementKind` strings to Monaco `CompletionItemKind`.
@@ -60,14 +59,14 @@ function toCompletionItemKind(
 }
 
 function toMonacoCompletion(
-  entry: CheckerCompletionItem,
+  entry: CompletionItem,
   range: Monaco.IRange,
   monaco: typeof Monaco,
 ): Monaco.languages.CompletionItem {
   return {
-    label: entry.name,
+    label: entry.label,
     kind: toCompletionItemKind(entry.kind, monaco),
-    insertText: entry.insertText ?? entry.name,
+    insertText: entry.insertText ?? entry.label,
     sortText: entry.sortText,
     range,
   };
@@ -75,7 +74,7 @@ function toMonacoCompletion(
 
 const CompletionSyncInner = () => {
   const { monaco } = use(use(MonacoContext));
-  const { getCompletions } = use(CheckerContext);
+  const { requestCompletion } = use(LanguageClientContext);
 
   useEffect(() => {
     const disposable = monaco.languages.registerCompletionItemProvider(
@@ -84,17 +83,9 @@ const CompletionSyncInner = () => {
         triggerCharacters: ["."],
 
         async provideCompletionItems(model, position) {
-          const parsed = parseEditorPath(model.uri.toString());
-          if (!parsed) {
-            return { suggestions: [] };
-          }
-
+          const uri = model.uri.toString();
           const offset = model.getOffsetAt(position);
-          const result = await getCompletions(
-            parsed.itemType,
-            parsed.itemId,
-            offset,
-          );
+          const result = await requestCompletion(uri, offset);
 
           const word = model.getWordUntilPosition(position);
           const range: Monaco.IRange = {
@@ -114,12 +105,12 @@ const CompletionSyncInner = () => {
     );
 
     return () => disposable.dispose();
-  }, [monaco, getCompletions]);
+  }, [monaco, requestCompletion]);
 
   return null;
 };
 
-/** Renders nothing visible — registers a Monaco CompletionItemProvider backed by the checker worker. */
+/** Renders nothing visible — registers a Monaco CompletionItemProvider backed by the language server. */
 export const CompletionSync: React.FC = () => (
   <Suspense fallback={null}>
     <CompletionSyncInner />
diff --git a/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
index 9bd907d02d5..adcbffcbcbe 100644
--- a/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
@@ -1,44 +1,46 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect, useRef } from "react";
 
-import { CheckerContext } from "../checker/context";
-import type { CheckerDiagnostic } from "../checker/worker/protocol";
+import { LanguageClientContext } from "../checker/context";
+import type { Diagnostic } from "../checker/worker/protocol";
 import { MonacoContext } from "./context";
-import { getEditorPath } from "./editor-paths";
 
 const OWNER = "checker";
 
-/** Convert ts.DiagnosticCategory number to Monaco MarkerSeverity. */
+/**
+ * Convert LSP-like severity to Monaco MarkerSeverity.
+ * LSP: 1=Error, 2=Warning, 3=Information, 4=Hint
+ */
 function toMarkerSeverity(
-  category: number,
+  severity: number,
   monaco: typeof Monaco,
 ): Monaco.MarkerSeverity {
-  switch (category) {
-    case 0:
-      return monaco.MarkerSeverity.Warning;
+  switch (severity) {
     case 1:
       return monaco.MarkerSeverity.Error;
     case 2:
-      return monaco.MarkerSeverity.Hint;
+      return monaco.MarkerSeverity.Warning;
     case 3:
       return monaco.MarkerSeverity.Info;
+    case 4:
+      return monaco.MarkerSeverity.Hint;
     default:
       return monaco.MarkerSeverity.Error;
   }
 }
 
-/** Convert CheckerDiagnostic[] to IMarkerData[] using the model for offset→position. */
+/** Convert Diagnostic[] to IMarkerData[] using the model for offset→position. */
 function diagnosticsToMarkers(
   model: Monaco.editor.ITextModel,
-  diagnostics: CheckerDiagnostic[],
+  diagnostics: Diagnostic[],
   monaco: typeof Monaco,
 ): Monaco.editor.IMarkerData[] {
   return diagnostics.map((diag) => {
     const start = model.getPositionAt(diag.start ?? 0);
     const end = model.getPositionAt((diag.start ?? 0) + (diag.length ?? 0));
     return {
-      severity: toMarkerSeverity(diag.category, monaco),
-      message: diag.messageText,
+      severity: toMarkerSeverity(diag.severity, monaco),
+      message: diag.message,
       startLineNumber: start.lineNumber,
       startColumn: start.column,
       endLineNumber: end.lineNumber,
@@ -50,57 +52,52 @@ function diagnosticsToMarkers(
 
 const DiagnosticsSyncInner = () => {
   const { monaco } = use(use(MonacoContext));
-  const { checkResult } = use(CheckerContext);
-  const prevPathsRef = useRef<Set<string>>(new Set());
+  const { diagnosticsByUri } = use(LanguageClientContext);
+  const prevUrisRef = useRef<Set<string>>(new Set());
 
   useEffect(() => {
-    const currentPaths = new Set<string>();
+    const currentUris = new Set<string>();
 
-    for (const item of checkResult.itemDiagnostics) {
-      const path = getEditorPath(item.itemType, item.itemId);
-      const uri = monaco.Uri.parse(path);
-      const model = monaco.editor.getModel(uri);
+    for (const [uri, diagnostics] of diagnosticsByUri) {
+      const monacoUri = monaco.Uri.parse(uri);
+      const model = monaco.editor.getModel(monacoUri);
       if (model) {
-        const markers = diagnosticsToMarkers(model, item.diagnostics, monaco);
+        const markers = diagnosticsToMarkers(model, diagnostics, monaco);
         monaco.editor.setModelMarkers(model, OWNER, markers);
       }
-      currentPaths.add(path);
+      currentUris.add(uri);
     }
 
     // Clear markers from models that no longer have diagnostics
-    for (const path of prevPathsRef.current) {
-      if (!currentPaths.has(path)) {
-        const uri = monaco.Uri.parse(path);
-        const model = monaco.editor.getModel(uri);
+    for (const uri of prevUrisRef.current) {
+      if (!currentUris.has(uri)) {
+        const monacoUri = monaco.Uri.parse(uri);
+        const model = monaco.editor.getModel(monacoUri);
         if (model) {
           monaco.editor.setModelMarkers(model, OWNER, []);
         }
       }
     }
 
-    prevPathsRef.current = currentPaths;
+    prevUrisRef.current = currentUris;
 
     // Handle models created after diagnostics arrived
     const disposable = monaco.editor.onDidCreateModel((model) => {
       const modelUri = model.uri.toString();
-      const item = checkResult.itemDiagnostics.find(
-        (i) =>
-          monaco.Uri.parse(getEditorPath(i.itemType, i.itemId)).toString() ===
-          modelUri,
-      );
-      if (item) {
-        const markers = diagnosticsToMarkers(model, item.diagnostics, monaco);
+      const diags = diagnosticsByUri.get(modelUri);
+      if (diags) {
+        const markers = diagnosticsToMarkers(model, diags, monaco);
         monaco.editor.setModelMarkers(model, OWNER, markers);
       }
     });
 
     return () => disposable.dispose();
-  }, [checkResult, monaco]);
+  }, [diagnosticsByUri, monaco]);
 
   return null;
 };
 
-/** Renders nothing visible — syncs CheckerContext diagnostics to Monaco model markers. */
+/** Renders nothing visible — syncs diagnostics from LanguageClientContext to Monaco model markers. */
 export const DiagnosticsSync: React.FC = () => (
   <Suspense fallback={null}>
     <DiagnosticsSyncInner />
diff --git a/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts b/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
index f6f167e5ef6..956b3d78b2a 100644
--- a/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
+++ b/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
@@ -1,10 +1,7 @@
-import type { CheckerItemDiagnostics } from "../checker/worker/protocol";
+import type { ItemType } from "../checker/lib/checker";
 
-/** Generates the Monaco model path for a given SDCPN item. */
-export function getEditorPath(
-  itemType: CheckerItemDiagnostics["itemType"],
-  itemId: string,
-): string {
+/** Generates the document URI for a given SDCPN item (used as Monaco model URI). */
+export function getDocumentUri(itemType: ItemType, itemId: string): string {
   switch (itemType) {
     case "transition-lambda":
       return `inmemory://sdcpn/transitions/${itemId}/lambda.ts`;
@@ -22,9 +19,9 @@ const TRANSITION_KERNEL_RE =
 const DIFFERENTIAL_EQUATION_RE =
   /^inmemory:\/\/sdcpn\/differential-equations\/([^/]+)\.ts$/;
 
-/** Extract `(itemType, itemId)` from a Monaco model URI string. */
-export function parseEditorPath(uri: string): {
-  itemType: CheckerItemDiagnostics["itemType"];
+/** Extract `(itemType, itemId)` from a document URI string. */
+export function parseDocumentUri(uri: string): {
+  itemType: ItemType;
   itemId: string;
 } | null {
   let match = TRANSITION_LAMBDA_RE.exec(uri);
diff --git a/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
index 06b3bd14d87..e3cb0aeab27 100644
--- a/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
@@ -1,24 +1,19 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect } from "react";
 
-import { CheckerContext } from "../checker/context";
+import { LanguageClientContext } from "../checker/context";
 import { MonacoContext } from "./context";
-import { parseEditorPath } from "./editor-paths";
 
 const HoverSyncInner = () => {
   const { monaco } = use(use(MonacoContext));
-  const { getQuickInfo } = use(CheckerContext);
+  const { requestHover } = use(LanguageClientContext);
 
   useEffect(() => {
     const disposable = monaco.languages.registerHoverProvider("typescript", {
       async provideHover(model, position) {
-        const parsed = parseEditorPath(model.uri.toString());
-        if (!parsed) {
-          return null;
-        }
-
+        const uri = model.uri.toString();
         const offset = model.getOffsetAt(position);
-        const info = await getQuickInfo(parsed.itemType, parsed.itemId, offset);
+        const info = await requestHover(uri, offset);
 
         if (!info) {
           return null;
@@ -45,12 +40,12 @@ const HoverSyncInner = () => {
     });
 
     return () => disposable.dispose();
-  }, [monaco, getQuickInfo]);
+  }, [monaco, requestHover]);
 
   return null;
 };
 
-/** Renders nothing visible — registers a Monaco HoverProvider backed by the checker worker. */
+/** Renders nothing visible — registers a Monaco HoverProvider backed by the language server. */
 export const HoverSync: React.FC = () => (
   <Suspense fallback={null}>
     <HoverSyncInner />
diff --git a/libs/@hashintel/petrinaut/src/monaco/provider.tsx b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
index 6488438e526..249629de23c 100644
--- a/libs/@hashintel/petrinaut/src/monaco/provider.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
@@ -63,14 +63,19 @@ async function initMonaco(): Promise<MonacoContextValue> {
   return { monaco, Editor: monacoReact.default };
 }
 
+/** Module-level lazy singleton — initialized once, reused across renders. */
+let monacoPromise: Promise<MonacoContextValue> | null = null;
+function getMonacoPromise(): Promise<MonacoContextValue> {
+  return (monacoPromise ??= initMonaco());
+}
+
 export const MonacoProvider: React.FC<{ children: React.ReactNode }> = ({
   children,
 }) => {
-  // Stable promise reference — created once, never changes.
-  const monacoPromise = initMonaco();
+  const promise = getMonacoPromise();
 
   return (
-    <MonacoContext.Provider value={monacoPromise}>
+    <MonacoContext.Provider value={promise}>
       <DiagnosticsSync />
       <CompletionSync />
       <HoverSync />
diff --git a/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
index 54f71f59f8b..6fc77d04abf 100644
--- a/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
@@ -1,13 +1,12 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect } from "react";
 
-import { CheckerContext } from "../checker/context";
-import type { CheckerSignatureHelpResult } from "../checker/worker/protocol";
+import { LanguageClientContext } from "../checker/context";
+import type { SignatureHelp } from "../checker/worker/protocol";
 import { MonacoContext } from "./context";
-import { parseEditorPath } from "./editor-paths";
 
 function toMonacoSignatureHelp(
-  result: NonNullable<CheckerSignatureHelpResult>,
+  result: NonNullable<SignatureHelp>,
 ): Monaco.languages.SignatureHelp {
   return {
     activeSignature: result.activeSignature,
@@ -25,7 +24,7 @@ function toMonacoSignatureHelp(
 
 const SignatureHelpSyncInner = () => {
   const { monaco } = use(use(MonacoContext));
-  const { getSignatureHelp } = use(CheckerContext);
+  const { requestSignatureHelp } = use(LanguageClientContext);
 
   useEffect(() => {
     const disposable = monaco.languages.registerSignatureHelpProvider(
@@ -35,17 +34,9 @@ const SignatureHelpSyncInner = () => {
         signatureHelpRetriggerCharacters: [","],
 
         async provideSignatureHelp(model, position) {
-          const parsed = parseEditorPath(model.uri.toString());
-          if (!parsed) {
-            return null;
-          }
-
+          const uri = model.uri.toString();
           const offset = model.getOffsetAt(position);
-          const result = await getSignatureHelp(
-            parsed.itemType,
-            parsed.itemId,
-            offset,
-          );
+          const result = await requestSignatureHelp(uri, offset);
 
           if (!result) {
             return null;
@@ -60,12 +51,12 @@ const SignatureHelpSyncInner = () => {
     );
 
     return () => disposable.dispose();
-  }, [monaco, getSignatureHelp]);
+  }, [monaco, requestSignatureHelp]);
 
   return null;
 };
 
-/** Renders nothing visible — registers a Monaco SignatureHelpProvider backed by the checker worker. */
+/** Renders nothing visible — registers a Monaco SignatureHelpProvider backed by the language server. */
 export const SignatureHelpSync: React.FC = () => (
   <Suspense fallback={null}>
     <SignatureHelpSyncInner />
diff --git a/libs/@hashintel/petrinaut/src/petrinaut.tsx b/libs/@hashintel/petrinaut/src/petrinaut.tsx
index df237136b33..8b06fa79f52 100644
--- a/libs/@hashintel/petrinaut/src/petrinaut.tsx
+++ b/libs/@hashintel/petrinaut/src/petrinaut.tsx
@@ -1,7 +1,7 @@
 import "reactflow/dist/style.css";
 import "./index.css";
 
-import { CheckerProvider } from "./checker/provider";
+import { LanguageClientProvider } from "./checker/provider";
 import type {
   Color,
   DifferentialEquation,
@@ -98,7 +98,7 @@ export const Petrinaut = ({
   return (
     <NotificationsProvider>
       <SDCPNProvider {...rest}>
-        <CheckerProvider>
+        <LanguageClientProvider>
           <MonacoProvider>
             <SimulationProvider>
               <PlaybackProvider>
@@ -110,7 +110,7 @@ export const Petrinaut = ({
               </PlaybackProvider>
             </SimulationProvider>
           </MonacoProvider>
-        </CheckerProvider>
+        </LanguageClientProvider>
       </SDCPNProvider>
     </NotificationsProvider>
   );
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx
index e82175b8e04..a92fca2955a 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx
@@ -3,7 +3,7 @@ import { refractive } from "@hashintel/refractive";
 import { use, useCallback, useEffect } from "react";
 import { FaChevronDown, FaChevronUp } from "react-icons/fa6";
 
-import { CheckerContext } from "../../../../checker/context";
+import { LanguageClientContext } from "../../../../checker/context";
 import {
   EditorContext,
   type EditorState,
@@ -69,7 +69,7 @@ export const BottomBar: React.FC<BottomBarProps> = ({
     bottomPanelHeight,
   } = use(EditorContext);
 
-  const { totalDiagnosticsCount } = use(CheckerContext);
+  const { totalDiagnosticsCount } = use(LanguageClientContext);
   const hasDiagnostics = totalDiagnosticsCount > 0;
 
   const showDiagnostics = useCallback(() => {
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx
index 648a2b0ffa4..7bca84a9515 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx
@@ -2,7 +2,7 @@ import { css, cva } from "@hashintel/ds-helpers/css";
 import { use } from "react";
 import { FaCheck, FaXmark } from "react-icons/fa6";
 
-import { CheckerContext } from "../../../../checker/context";
+import { LanguageClientContext } from "../../../../checker/context";
 import { ToolbarButton } from "./toolbar-button";
 
 const iconContainerStyle = cva({
@@ -47,7 +47,7 @@ export const DiagnosticsIndicator: React.FC<DiagnosticsIndicatorProps> = ({
   onClick,
   isExpanded,
 }) => {
-  const { totalDiagnosticsCount } = use(CheckerContext);
+  const { totalDiagnosticsCount } = use(LanguageClientContext);
 
   const hasErrors = totalDiagnosticsCount > 0;
 
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx
index 6556c3f2712..90bcd3b2a71 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/differential-equation-properties.tsx
@@ -19,7 +19,7 @@ import type {
   Place,
 } from "../../../../core/types/sdcpn";
 import { CodeEditor } from "../../../../monaco/code-editor";
-import { getEditorPath } from "../../../../monaco/editor-paths";
+import { getDocumentUri } from "../../../../monaco/editor-paths";
 import { useIsReadOnly } from "../../../../state/use-is-read-only";
 
 const containerStyle = css({
@@ -477,7 +477,10 @@ export const DifferentialEquationProperties: React.FC<
           )}
         </div>
         <CodeEditor
-          path={getEditorPath("differential-equation", differentialEquation.id)}
+          path={getDocumentUri(
+            "differential-equation",
+            differentialEquation.id,
+          )}
           language="typescript"
           value={differentialEquation.code}
           height="100%"
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
index b803b51c5fd..cbcbb5cd243 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/panels/PropertiesPanel/transition-properties.tsx
@@ -16,7 +16,7 @@ import {
 } from "../../../../core/default-codes";
 import type { Color, Place, Transition } from "../../../../core/types/sdcpn";
 import { CodeEditor } from "../../../../monaco/code-editor";
-import { getEditorPath } from "../../../../monaco/editor-paths";
+import { getDocumentUri } from "../../../../monaco/editor-paths";
 import { EditorContext } from "../../../../state/editor-context";
 import { SDCPNContext } from "../../../../state/sdcpn-context";
 import { useIsReadOnly } from "../../../../state/use-is-read-only";
@@ -389,7 +389,7 @@ export const TransitionProperties: React.FC<TransitionPropertiesProps> = ({
           )}
         </div>
         <CodeEditor
-          path={getEditorPath("transition-lambda", transition.id)}
+          path={getDocumentUri("transition-lambda", transition.id)}
           key={`lambda-${transition.lambdaType}-${transition.inputArcs
             .map((a) => `${a.placeId}:${a.weight}`)
             .join("-")}`}
@@ -492,7 +492,7 @@ export const TransitionProperties: React.FC<TransitionPropertiesProps> = ({
             )}
           </div>
           <CodeEditor
-            path={getEditorPath("transition-kernel", transition.id)}
+            path={getDocumentUri("transition-kernel", transition.id)}
             language="typescript"
             value={transition.transitionKernelCode || ""}
             height={400}
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
index 0eac7e15e5c..a93580da56f 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
@@ -2,9 +2,10 @@ import { css } from "@hashintel/ds-helpers/css";
 import { use, useCallback, useMemo, useState } from "react";
 import { FaChevronDown, FaChevronRight } from "react-icons/fa6";
 
-import { CheckerContext } from "../../../checker/context";
-import type { CheckerDiagnostic } from "../../../checker/worker/protocol";
+import { LanguageClientContext } from "../../../checker/context";
+import type { Diagnostic } from "../../../checker/worker/protocol";
 import type { SubView } from "../../../components/sub-view/types";
+import { parseDocumentUri } from "../../../monaco/editor-paths";
 import { EditorContext } from "../../../state/editor-context";
 import { SDCPNContext } from "../../../state/sdcpn-context";
 
@@ -103,7 +104,7 @@ interface GroupedDiagnostics {
   errorCount: number;
   items: Array<{
     subType: "lambda" | "kernel" | null;
-    diagnostics: CheckerDiagnostic[];
+    diagnostics: Diagnostic[];
   }>;
 }
 
@@ -111,7 +112,9 @@ interface GroupedDiagnostics {
  * DiagnosticsContent shows the full list of diagnostics grouped by entity.
  */
 const DiagnosticsContent: React.FC = () => {
-  const { checkResult, totalDiagnosticsCount } = use(CheckerContext);
+  const { diagnosticsByUri, totalDiagnosticsCount } = use(
+    LanguageClientContext,
+  );
   const { petriNetDefinition } = use(SDCPNContext);
   const { setSelectedResourceId } = use(EditorContext);
   // Track collapsed entities (all expanded by default)
@@ -131,13 +134,18 @@ const DiagnosticsContent: React.FC = () => {
   const groupedDiagnostics = useMemo(() => {
     const groups = new Map<string, GroupedDiagnostics>();
 
-    for (const item of checkResult.itemDiagnostics) {
-      const entityId = item.itemId;
+    for (const [uri, diagnostics] of diagnosticsByUri) {
+      const parsed = parseDocumentUri(uri);
+      if (!parsed) {
+        continue;
+      }
+
+      const entityId = parsed.itemId;
       let entityType: EntityType;
       let entityName: string;
       let subType: "lambda" | "kernel" | null;
 
-      if (item.itemType === "differential-equation") {
+      if (parsed.itemType === "differential-equation") {
         entityType = "differential-equation";
         const de = petriNetDefinition.differentialEquations.find(
           (deItem) => deItem.id === entityId,
@@ -150,7 +158,7 @@ const DiagnosticsContent: React.FC = () => {
           (tr) => tr.id === entityId,
         );
         entityName = transition?.name ?? entityId;
-        subType = item.itemType === "transition-lambda" ? "lambda" : "kernel";
+        subType = parsed.itemType === "transition-lambda" ? "lambda" : "kernel";
       }
 
       const key = `${entityType}:${entityId}`;
@@ -165,15 +173,15 @@ const DiagnosticsContent: React.FC = () => {
       }
 
       const group = groups.get(key)!;
-      group.errorCount += item.diagnostics.length;
+      group.errorCount += diagnostics.length;
       group.items.push({
         subType,
-        diagnostics: item.diagnostics,
+        diagnostics,
       });
     }
 
     return Array.from(groups.values());
-  }, [checkResult, petriNetDefinition]);
+  }, [diagnosticsByUri, petriNetDefinition]);
 
   const toggleEntity = useCallback((entityKey: string) => {
     setCollapsedEntities((prev) => {
@@ -253,7 +261,7 @@ const DiagnosticsContent: React.FC = () => {
                             className={diagnosticButtonStyle}
                           >
                             <span className={bulletStyle}>•</span>
-                            {diagnostic.messageText}
+                            {diagnostic.message}
                             {diagnostic.start !== undefined && (
                               <span className={positionStyle}>
                                 (pos: {diagnostic.start})

From 201124e8474eee9b5af7963fb5df78c27ec407b2 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 13:25:51 +0100
Subject: [PATCH 11/22] H-5839: Adopt vscode-languageserver-types for standard
 LSP protocol types

Replace hand-rolled Diagnostic, CompletionItem, Hover, and SignatureHelp types
with official LSP types from vscode-languageserver-types. Protocol now uses
LSP Position (line/character) instead of raw offsets, with offset<->position
conversion utilities bridging the TS LanguageService (offset-based) and LSP
(Position-based) worlds.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 libs/@hashintel/petrinaut/package.json        |   1 +
 .../petrinaut/src/checker/context.ts          |  11 +-
 .../lib/create-sdcpn-language-service.ts      |  15 +-
 .../src/checker/lib/position-utils.test.ts    |  68 +++++++
 .../src/checker/lib/position-utils.ts         |  38 ++++
 .../src/checker/worker/checker.worker.ts      | 180 +++++++++++++-----
 .../petrinaut/src/checker/worker/protocol.ts  | 132 ++++---------
 .../src/checker/worker/use-language-client.ts |  30 ++-
 .../petrinaut/src/monaco/completion-sync.tsx  |  67 +++----
 .../petrinaut/src/monaco/diagnostics-sync.tsx |  46 ++---
 .../petrinaut/src/monaco/hover-sync.tsx       |  79 ++++++--
 .../src/monaco/signature-help-sync.tsx        |  46 +++--
 .../src/views/Editor/subviews/diagnostics.tsx |  16 +-
 yarn.lock                                     |   8 +
 14 files changed, 468 insertions(+), 269 deletions(-)
 create mode 100644 libs/@hashintel/petrinaut/src/checker/lib/position-utils.test.ts
 create mode 100644 libs/@hashintel/petrinaut/src/checker/lib/position-utils.ts

diff --git a/libs/@hashintel/petrinaut/package.json b/libs/@hashintel/petrinaut/package.json
index c07cdbc5008..85d9c0a1069 100644
--- a/libs/@hashintel/petrinaut/package.json
+++ b/libs/@hashintel/petrinaut/package.json
@@ -55,6 +55,7 @@
     "reactflow": "11.11.4",
     "typescript": "5.9.3",
     "uuid": "13.0.0",
+    "vscode-languageserver-types": "3.17.5",
     "web-worker": "1.4.1"
   },
   "devDependencies": {
diff --git a/libs/@hashintel/petrinaut/src/checker/context.ts b/libs/@hashintel/petrinaut/src/checker/context.ts
index eeacedf8c1f..e3c07ea5114 100644
--- a/libs/@hashintel/petrinaut/src/checker/context.ts
+++ b/libs/@hashintel/petrinaut/src/checker/context.ts
@@ -5,6 +5,7 @@ import type {
   Diagnostic,
   DocumentUri,
   Hover,
+  Position,
   SignatureHelp,
 } from "./worker/protocol";
 
@@ -18,22 +19,22 @@ export interface LanguageClientContextValue {
   /** Request completions at a position within a document. */
   requestCompletion: (
     uri: DocumentUri,
-    offset: number,
+    position: Position,
   ) => Promise<CompletionList>;
   /** Request hover info at a position within a document. */
-  requestHover: (uri: DocumentUri, offset: number) => Promise<Hover>;
+  requestHover: (uri: DocumentUri, position: Position) => Promise<Hover | null>;
   /** Request signature help at a position within a document. */
   requestSignatureHelp: (
     uri: DocumentUri,
-    offset: number,
-  ) => Promise<SignatureHelp>;
+    position: Position,
+  ) => Promise<SignatureHelp | null>;
 }
 
 const DEFAULT_CONTEXT_VALUE: LanguageClientContextValue = {
   diagnosticsByUri: new Map(),
   totalDiagnosticsCount: 0,
   notifyDocumentChanged: () => {},
-  requestCompletion: () => Promise.resolve({ items: [] }),
+  requestCompletion: () => Promise.resolve({ isIncomplete: false, items: [] }),
   requestHover: () => Promise.resolve(null),
   requestSignatureHelp: () => Promise.resolve(null),
 };
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
index e790d0effc5..778c54bae76 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
@@ -278,10 +278,10 @@ export class SDCPNLanguageServer {
       if (!this.controller.hasFile(name)) {
         this.controller.addFile(name, newFile);
       } else {
-        const existing = this.controller.getFile(name);
+        const existing = this.controller.getFile(name)!;
         if (
-          existing?.content !== newFile.content ||
-          existing?.prefix !== newFile.prefix
+          existing.content !== newFile.content ||
+          existing.prefix !== newFile.prefix
         ) {
           this.controller.updateFile(name, newFile);
         }
@@ -294,6 +294,15 @@ export class SDCPNLanguageServer {
     this.controller.updateContent(fileName, content);
   }
 
+  /** Get the full text content (prefix + user content) of a virtual file. */
+  getFileContent(fileName: string): string | undefined {
+    const file = this.controller.getFile(fileName);
+    if (!file) {
+      return undefined;
+    }
+    return (file.prefix ?? "") + file.content;
+  }
+
   getSemanticDiagnostics(fileName: string): ts.Diagnostic[] {
     const entry = this.controller.getFile(fileName);
     const prefixLength = entry?.prefix?.length ?? 0;
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/position-utils.test.ts b/libs/@hashintel/petrinaut/src/checker/lib/position-utils.test.ts
new file mode 100644
index 00000000000..894cfea8327
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/checker/lib/position-utils.test.ts
@@ -0,0 +1,68 @@
+import { describe, expect, it } from "vitest";
+import { Position } from "vscode-languageserver-types";
+
+import { offsetToPosition, positionToOffset } from "./position-utils";
+
+describe("offsetToPosition", () => {
+  it("returns 0:0 for offset 0", () => {
+    expect(offsetToPosition("hello", 0)).toEqual(Position.create(0, 0));
+  });
+
+  it("returns correct position within a single line", () => {
+    expect(offsetToPosition("hello", 3)).toEqual(Position.create(0, 3));
+  });
+
+  it("returns start of second line after newline", () => {
+    expect(offsetToPosition("ab\ncd", 3)).toEqual(Position.create(1, 0));
+  });
+
+  it("returns correct position on second line", () => {
+    expect(offsetToPosition("ab\ncd", 4)).toEqual(Position.create(1, 1));
+  });
+
+  it("handles multiple newlines", () => {
+    const text = "line1\nline2\nline3";
+    // 'l' in line3 is at offset 12
+    expect(offsetToPosition(text, 12)).toEqual(Position.create(2, 0));
+    expect(offsetToPosition(text, 15)).toEqual(Position.create(2, 3));
+  });
+
+  it("clamps to end of text", () => {
+    expect(offsetToPosition("ab", 100)).toEqual(Position.create(0, 2));
+  });
+});
+
+describe("positionToOffset", () => {
+  it("returns 0 for position 0:0", () => {
+    expect(positionToOffset("hello", Position.create(0, 0))).toBe(0);
+  });
+
+  it("returns correct offset within a single line", () => {
+    expect(positionToOffset("hello", Position.create(0, 3))).toBe(3);
+  });
+
+  it("returns offset at start of second line", () => {
+    expect(positionToOffset("ab\ncd", Position.create(1, 0))).toBe(3);
+  });
+
+  it("returns correct offset on second line", () => {
+    expect(positionToOffset("ab\ncd", Position.create(1, 1))).toBe(4);
+  });
+
+  it("handles multiple newlines", () => {
+    const text = "line1\nline2\nline3";
+    expect(positionToOffset(text, Position.create(2, 0))).toBe(12);
+    expect(positionToOffset(text, Position.create(2, 3))).toBe(15);
+  });
+});
+
+describe("roundtrip", () => {
+  it("offsetToPosition → positionToOffset is identity", () => {
+    const text = "function foo() {\n  return 42;\n}\n";
+    for (let offset = 0; offset <= text.length; offset++) {
+      const pos = offsetToPosition(text, offset);
+      const recovered = positionToOffset(text, pos);
+      expect(recovered).toBe(offset);
+    }
+  });
+});
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/position-utils.ts b/libs/@hashintel/petrinaut/src/checker/lib/position-utils.ts
new file mode 100644
index 00000000000..27b5097cd23
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/checker/lib/position-utils.ts
@@ -0,0 +1,38 @@
+import { Position } from "vscode-languageserver-types";
+
+/**
+ * Convert a character offset to an LSP Position (zero-based line and character).
+ */
+export function offsetToPosition(text: string, offset: number): Position {
+  let line = 0;
+  let character = 0;
+  const clamped = Math.min(offset, text.length);
+
+  for (let i = 0; i < clamped; i++) {
+    if (text[i] === "\n") {
+      line++;
+      character = 0;
+    } else {
+      character++;
+    }
+  }
+
+  return Position.create(line, character);
+}
+
+/**
+ * Convert an LSP Position (zero-based line and character) to a character offset.
+ */
+export function positionToOffset(text: string, position: Position): number {
+  let line = 0;
+  let i = 0;
+
+  while (i < text.length && line < position.line) {
+    if (text[i] === "\n") {
+      line++;
+    }
+    i++;
+  }
+
+  return i + position.character;
+}
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
index df0f1ef56f5..4f065c86b40 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
+++ b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
@@ -10,22 +10,29 @@
  * The LanguageService is created once and reused across SDCPN changes.
  */
 import ts from "typescript";
+import {
+  CompletionItemKind,
+  DiagnosticSeverity,
+  MarkupKind,
+  Range,
+  type CompletionItem,
+  type CompletionList,
+  type Diagnostic,
+  type DocumentUri,
+  type Hover,
+  type SignatureHelp,
+  type SignatureInformation,
+} from "vscode-languageserver-types";
 
 import type { SDCPN } from "../../core/types/sdcpn";
 import { checkSDCPN } from "../lib/checker";
 import { SDCPNLanguageServer } from "../lib/create-sdcpn-language-service";
 import { getItemFilePath } from "../lib/file-paths";
+import { offsetToPosition, positionToOffset } from "../lib/position-utils";
 import type {
   ClientMessage,
-  CompletionItem,
-  CompletionList,
-  Diagnostic,
-  DocumentUri,
-  Hover,
   PublishDiagnosticsParams,
   ServerMessage,
-  SignatureHelp,
-  SignatureInformation,
 } from "./protocol";
 
 // ---------------------------------------------------------------------------
@@ -87,36 +94,92 @@ function filePathToUri(filePath: string): DocumentUri | null {
 }
 
 // ---------------------------------------------------------------------------
-// Serialization helpers
+// TS → LSP type conversions
 // ---------------------------------------------------------------------------
 
 /**
- * Map `ts.DiagnosticCategory` to LSP-like severity.
+ * Map `ts.DiagnosticCategory` to `DiagnosticSeverity`.
  * TS: 0=Warning, 1=Error, 2=Suggestion, 3=Message
  * LSP: 1=Error, 2=Warning, 3=Information, 4=Hint
  */
-function toLspSeverity(category: number): number {
+function toLspSeverity(category: number): DiagnosticSeverity {
   switch (category) {
     case 0:
-      return 2; // Warning
+      return DiagnosticSeverity.Warning;
     case 1:
-      return 1; // Error
+      return DiagnosticSeverity.Error;
     case 2:
-      return 4; // Hint (Suggestion)
+      return DiagnosticSeverity.Hint;
     case 3:
-      return 3; // Information (Message)
+      return DiagnosticSeverity.Information;
     default:
-      return 1;
+      return DiagnosticSeverity.Error;
   }
 }
 
-function serializeDiagnostic(diag: ts.Diagnostic): Diagnostic {
+/**
+ * Map TS `ScriptElementKind` strings to LSP `CompletionItemKind`.
+ */
+function toCompletionItemKind(kind: string): CompletionItemKind {
+  switch (kind) {
+    case "method":
+    case "construct":
+      return CompletionItemKind.Method;
+    case "function":
+    case "local function":
+      return CompletionItemKind.Function;
+    case "constructor":
+      return CompletionItemKind.Constructor;
+    case "property":
+    case "getter":
+    case "setter":
+      return CompletionItemKind.Property;
+    case "parameter":
+    case "var":
+    case "local var":
+    case "let":
+    case "const":
+      return CompletionItemKind.Variable;
+    case "class":
+      return CompletionItemKind.Class;
+    case "interface":
+      return CompletionItemKind.Interface;
+    case "type":
+    case "type parameter":
+    case "primitive type":
+    case "alias":
+      return CompletionItemKind.TypeParameter;
+    case "enum":
+      return CompletionItemKind.Enum;
+    case "enum member":
+      return CompletionItemKind.EnumMember;
+    case "module":
+    case "external module name":
+      return CompletionItemKind.Module;
+    case "keyword":
+      return CompletionItemKind.Keyword;
+    case "string":
+      return CompletionItemKind.Value;
+    default:
+      return CompletionItemKind.Text;
+  }
+}
+
+function serializeDiagnostic(
+  diag: ts.Diagnostic,
+  fileContent: string,
+): Diagnostic {
+  const start = diag.start ?? 0;
+  const end = start + (diag.length ?? 0);
   return {
     severity: toLspSeverity(diag.category),
-    code: diag.code,
+    range: Range.create(
+      offsetToPosition(fileContent, start),
+      offsetToPosition(fileContent, end),
+    ),
     message: ts.flattenDiagnosticMessageText(diag.messageText, "\n"),
-    start: diag.start,
-    length: diag.length,
+    code: diag.code,
+    source: "ts",
   };
 }
 
@@ -152,9 +215,12 @@ function publishAllDiagnostics(sdcpn: SDCPN): void {
   const params: PublishDiagnosticsParams[] = result.itemDiagnostics.map(
     (item) => {
       const uri = filePathToUri(item.filePath);
+      const fileContent = server!.getFileContent(item.filePath) ?? "";
       return {
         uri: uri ?? item.filePath,
-        diagnostics: item.diagnostics.map(serializeDiagnostic),
+        diagnostics: item.diagnostics.map((diag) =>
+          serializeDiagnostic(diag, fileContent),
+        ),
       };
     },
   );
@@ -190,9 +256,7 @@ self.onmessage = ({ data }: MessageEvent<ClientMessage>) => {
       case "sdcpn/didChange": {
         const { sdcpn } = data.params;
         lastSDCPN = sdcpn;
-        if (!server) {
-          server = new SDCPNLanguageServer();
-        }
+        server ??= new SDCPNLanguageServer();
         server.syncFiles(sdcpn);
         publishAllDiagnostics(sdcpn);
         break;
@@ -218,32 +282,41 @@ self.onmessage = ({ data }: MessageEvent<ClientMessage>) => {
       case "textDocument/completion": {
         const { id } = data;
         if (!server) {
-          respond(id, { items: [] } satisfies CompletionList);
+          respond(id, {
+            isIncomplete: false,
+            items: [],
+          } satisfies CompletionList);
           break;
         }
 
         const filePath = uriToFilePath(data.params.textDocument.uri);
         if (!filePath) {
-          respond(id, { items: [] } satisfies CompletionList);
+          respond(id, {
+            isIncomplete: false,
+            items: [],
+          } satisfies CompletionList);
           break;
         }
 
+        const fileContent = server.getFileContent(filePath) ?? "";
+        const offset = positionToOffset(fileContent, data.params.position);
+
         const completions = server.getCompletionsAtPosition(
           filePath,
-          data.params.offset,
+          offset,
           undefined,
         );
 
         const items: CompletionItem[] = (completions?.entries ?? []).map(
           (entry) => ({
             label: entry.name,
-            kind: entry.kind,
+            kind: toCompletionItemKind(entry.kind),
             sortText: entry.sortText,
             insertText: entry.insertText,
           }),
         );
 
-        respond(id, { items } satisfies CompletionList);
+        respond(id, { isIncomplete: false, items } satisfies CompletionList);
         break;
       }
 
@@ -260,17 +333,29 @@ self.onmessage = ({ data }: MessageEvent<ClientMessage>) => {
           break;
         }
 
-        const info = server.getQuickInfoAtPosition(
-          filePath,
-          data.params.offset,
-        );
+        const fileContent = server.getFileContent(filePath) ?? "";
+        const offset = positionToOffset(fileContent, data.params.position);
 
-        const result: Hover = info
+        const info = server.getQuickInfoAtPosition(filePath, offset);
+
+        const result: Hover | null = info
           ? {
-              displayParts: ts.displayPartsToString(info.displayParts),
-              documentation: ts.displayPartsToString(info.documentation),
-              start: info.textSpan.start,
-              length: info.textSpan.length,
+              contents: {
+                kind: MarkupKind.Markdown,
+                value: [
+                  `\`\`\`typescript\n${ts.displayPartsToString(info.displayParts)}\n\`\`\``,
+                  ts.displayPartsToString(info.documentation),
+                ]
+                  .filter(Boolean)
+                  .join("\n\n"),
+              },
+              range: Range.create(
+                offsetToPosition(fileContent, info.textSpan.start),
+                offsetToPosition(
+                  fileContent,
+                  info.textSpan.start + info.textSpan.length,
+                ),
+              ),
             }
           : null;
 
@@ -291,13 +376,12 @@ self.onmessage = ({ data }: MessageEvent<ClientMessage>) => {
           break;
         }
 
-        const help = server.getSignatureHelpItems(
-          filePath,
-          data.params.offset,
-          undefined,
-        );
+        const fileContent = server.getFileContent(filePath) ?? "";
+        const offset = positionToOffset(fileContent, data.params.position);
+
+        const help = server.getSignatureHelpItems(filePath, offset, undefined);
 
-        const result: SignatureHelp = help
+        const result: SignatureHelp | null = help
           ? {
               activeSignature: help.selectedItemIndex,
               activeParameter: help.argumentIndex,
@@ -313,10 +397,16 @@ self.onmessage = ({ data }: MessageEvent<ClientMessage>) => {
                   ]
                     .map((part) => part.text)
                     .join(""),
-                  documentation: ts.displayPartsToString(item.documentation),
+                  documentation: {
+                    kind: MarkupKind.PlainText,
+                    value: ts.displayPartsToString(item.documentation),
+                  },
                   parameters: item.parameters.map((param) => ({
                     label: ts.displayPartsToString(param.displayParts),
-                    documentation: ts.displayPartsToString(param.documentation),
+                    documentation: {
+                      kind: MarkupKind.PlainText,
+                      value: ts.displayPartsToString(param.documentation),
+                    },
                   })),
                 }),
               ),
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts b/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
index 057336a6dae..a56971ac090 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
+++ b/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
@@ -1,121 +1,53 @@
 /**
  * LSP-inspired protocol types for the checker WebWorker.
  *
- * Uses JSON-RPC 2.0 with LSP-like method names and structures.
- * Diagnostic types are serializable (no `ts.SourceFile` references) so they
- * can cross the postMessage boundary via structured clone.
+ * Uses JSON-RPC 2.0 with standard LSP types from `vscode-languageserver-types`.
+ * All diagnostic, completion, hover, and signature help types are the official
+ * LSP types, serializable for postMessage structured clone.
  *
- * Deviations from LSP:
- * - Uses character offsets instead of line/character positions (both Monaco
- *   and the TS LanguageService work in offsets internally).
- * - Uses `sdcpn/didChange` for structural model changes (not per-document).
+ * Custom extensions:
+ * - `sdcpn/didChange` for structural model changes (not per-document).
  */
-import type { SDCPN } from "../../core/types/sdcpn";
+import type {
+  CompletionItem,
+  CompletionList,
+  Diagnostic,
+  DocumentUri,
+  Hover,
+  Position,
+  SignatureHelp,
+  TextDocumentIdentifier,
+} from "vscode-languageserver-types";
 
-// ---------------------------------------------------------------------------
-// Document identification
-// ---------------------------------------------------------------------------
-
-/** URI identifying a text document (e.g., "inmemory://sdcpn/transitions/t1/lambda.ts"). */
-export type DocumentUri = string;
+import type { SDCPN } from "../../core/types/sdcpn";
 
-export type TextDocumentIdentifier = {
-  uri: DocumentUri;
+// Re-export LSP types used by consumers
+export type {
+  CompletionItem,
+  CompletionList,
+  Diagnostic,
+  DocumentUri,
+  Hover,
+  Position,
+  SignatureHelp,
+  TextDocumentIdentifier,
 };
 
 /**
- * Position in a text document, specified as a character offset.
- *
- * Unlike standard LSP (which uses line/character), we use offsets since both
- * Monaco and the TS LanguageService work in offsets internally.
+ * Parameters for `textDocument/publishDiagnostics` notification.
+ * Defined here rather than pulling in `vscode-languageserver-protocol`.
  */
-export type TextDocumentPositionParams = {
-  textDocument: TextDocumentIdentifier;
-  offset: number;
-};
-
-// ---------------------------------------------------------------------------
-// Diagnostics
-// ---------------------------------------------------------------------------
-
-/** A single diagnostic, safe for structured clone. */
-export type Diagnostic = {
-  /** Severity: 1=Error, 2=Warning, 3=Info, 4=Hint. */
-  severity: number;
-  /** TypeScript error code (e.g. 2322 for type mismatch). */
-  code: number;
-  /** Human-readable message, pre-flattened from `ts.DiagnosticMessageChain`. */
-  message: string;
-  /** Character offset in user code where the diagnostic starts. */
-  start: number | undefined;
-  /** Length of the diagnostic span in characters. */
-  length: number | undefined;
-};
-
-/** Diagnostics for a single document. */
 export type PublishDiagnosticsParams = {
   uri: DocumentUri;
   diagnostics: Diagnostic[];
 };
 
-// ---------------------------------------------------------------------------
-// Completions
-// ---------------------------------------------------------------------------
-
-/** A single completion suggestion, safe for structured clone. */
-export type CompletionItem = {
-  label: string;
-  /** @see ts.ScriptElementKind */
-  kind: string;
-  sortText: string;
-  insertText?: string;
-};
-
-/** Result of requesting completions at a position. */
-export type CompletionList = {
-  items: CompletionItem[];
-};
-
-// ---------------------------------------------------------------------------
-// Hover
-// ---------------------------------------------------------------------------
-
-/** Result of requesting hover info at a position. */
-export type Hover = {
-  /** Type/signature display string. */
-  displayParts: string;
-  /** JSDoc documentation string. */
-  documentation: string;
-  /** Offset in user code where the hovered symbol starts. */
-  start: number;
-  /** Length of the hovered symbol span. */
-  length: number;
-} | null;
-
-// ---------------------------------------------------------------------------
-// Signature Help
-// ---------------------------------------------------------------------------
-
-/** A single parameter in a signature. */
-export type SignatureParameter = {
-  label: string;
-  documentation: string;
-};
-
-/** A single signature (overload). */
-export type SignatureInformation = {
-  label: string;
-  documentation: string;
-  parameters: SignatureParameter[];
+/** Position in a text document (LSP standard: line/character based). */
+export type TextDocumentPositionParams = {
+  textDocument: TextDocumentIdentifier;
+  position: Position;
 };
 
-/** Result of requesting signature help at a position. */
-export type SignatureHelp = {
-  signatures: SignatureInformation[];
-  activeSignature: number;
-  activeParameter: number;
-} | null;
-
 // ---------------------------------------------------------------------------
 // JSON-RPC 2.0 messages: main thread → worker
 // ---------------------------------------------------------------------------
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/use-language-client.ts b/libs/@hashintel/petrinaut/src/checker/worker/use-language-client.ts
index 0cc12567b95..a5522658ca4 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/use-language-client.ts
+++ b/libs/@hashintel/petrinaut/src/checker/worker/use-language-client.ts
@@ -6,6 +6,7 @@ import type {
   CompletionList,
   DocumentUri,
   Hover,
+  Position,
   PublishDiagnosticsParams,
   ServerMessage,
   SignatureHelp,
@@ -27,15 +28,15 @@ export type LanguageClientApi = {
   /** Request completions at a position within a document. */
   requestCompletion: (
     uri: DocumentUri,
-    offset: number,
+    position: Position,
   ) => Promise<CompletionList>;
   /** Request hover info at a position within a document. */
-  requestHover: (uri: DocumentUri, offset: number) => Promise<Hover>;
+  requestHover: (uri: DocumentUri, position: Position) => Promise<Hover | null>;
   /** Request signature help at a position within a document. */
   requestSignatureHelp: (
     uri: DocumentUri,
-    offset: number,
-  ) => Promise<SignatureHelp>;
+    position: Position,
+  ) => Promise<SignatureHelp | null>;
   /** Register a callback for diagnostics pushed from the server. */
   onDiagnostics: (
     callback: (params: PublishDiagnosticsParams[]) => void,
@@ -75,10 +76,7 @@ export function useLanguageClient(): LanguageClientApi {
         } else {
           pending.resolve(msg.result as never);
         }
-      } else if (
-        "method" in msg &&
-        msg.method === "textDocument/publishDiagnostics"
-      ) {
+      } else if ("method" in msg) {
         // Server-pushed notification
         diagnosticsCallbackRef.current?.(msg.params);
       }
@@ -154,39 +152,39 @@ export function useLanguageClient(): LanguageClientApi {
   }, []);
 
   const requestCompletion = useCallback(
-    (uri: DocumentUri, offset: number): Promise<CompletionList> => {
+    (uri: DocumentUri, position: Position): Promise<CompletionList> => {
       const id = nextId.current++;
       return sendRequest<CompletionList>({
         jsonrpc: "2.0",
         id,
         method: "textDocument/completion",
-        params: { textDocument: { uri }, offset },
+        params: { textDocument: { uri }, position },
       });
     },
     [sendRequest],
   );
 
   const requestHover = useCallback(
-    (uri: DocumentUri, offset: number): Promise<Hover> => {
+    (uri: DocumentUri, position: Position): Promise<Hover | null> => {
       const id = nextId.current++;
-      return sendRequest<Hover>({
+      return sendRequest<Hover | null>({
         jsonrpc: "2.0",
         id,
         method: "textDocument/hover",
-        params: { textDocument: { uri }, offset },
+        params: { textDocument: { uri }, position },
       });
     },
     [sendRequest],
   );
 
   const requestSignatureHelp = useCallback(
-    (uri: DocumentUri, offset: number): Promise<SignatureHelp> => {
+    (uri: DocumentUri, position: Position): Promise<SignatureHelp | null> => {
       const id = nextId.current++;
-      return sendRequest<SignatureHelp>({
+      return sendRequest<SignatureHelp | null>({
         jsonrpc: "2.0",
         id,
         method: "textDocument/signatureHelp",
-        params: { textDocument: { uri }, offset },
+        params: { textDocument: { uri }, position },
       });
     },
     [sendRequest],
diff --git a/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
index dfdb5624cca..c633c6ae006 100644
--- a/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
@@ -1,57 +1,44 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect } from "react";
+import { CompletionItemKind, Position } from "vscode-languageserver-types";
+import type { CompletionItem } from "vscode-languageserver-types";
 
 import { LanguageClientContext } from "../checker/context";
-import type { CompletionItem } from "../checker/worker/protocol";
 import { MonacoContext } from "./context";
 
 /**
- * Map TypeScript `ScriptElementKind` strings to Monaco `CompletionItemKind`.
- * @see https://github.com/microsoft/TypeScript/blob/main/src/services/types.ts
+ * Map LSP `CompletionItemKind` to Monaco `CompletionItemKind`.
  */
-function toCompletionItemKind(
-  kind: string,
+function toMonacoCompletionKind(
+  kind: CompletionItemKind | undefined,
   monaco: typeof Monaco,
 ): Monaco.languages.CompletionItemKind {
   switch (kind) {
-    case "method":
-    case "construct":
+    case CompletionItemKind.Method:
       return monaco.languages.CompletionItemKind.Method;
-    case "function":
-    case "local function":
+    case CompletionItemKind.Function:
       return monaco.languages.CompletionItemKind.Function;
-    case "constructor":
+    case CompletionItemKind.Constructor:
       return monaco.languages.CompletionItemKind.Constructor;
-    case "property":
-    case "getter":
-    case "setter":
+    case CompletionItemKind.Property:
       return monaco.languages.CompletionItemKind.Property;
-    case "parameter":
-    case "var":
-    case "local var":
-    case "let":
+    case CompletionItemKind.Variable:
       return monaco.languages.CompletionItemKind.Variable;
-    case "const":
-      return monaco.languages.CompletionItemKind.Variable;
-    case "class":
+    case CompletionItemKind.Class:
       return monaco.languages.CompletionItemKind.Class;
-    case "interface":
+    case CompletionItemKind.Interface:
       return monaco.languages.CompletionItemKind.Interface;
-    case "type":
-    case "type parameter":
-    case "primitive type":
-    case "alias":
+    case CompletionItemKind.TypeParameter:
       return monaco.languages.CompletionItemKind.TypeParameter;
-    case "enum":
+    case CompletionItemKind.Enum:
       return monaco.languages.CompletionItemKind.Enum;
-    case "enum member":
+    case CompletionItemKind.EnumMember:
       return monaco.languages.CompletionItemKind.EnumMember;
-    case "module":
-    case "external module name":
+    case CompletionItemKind.Module:
       return monaco.languages.CompletionItemKind.Module;
-    case "keyword":
+    case CompletionItemKind.Keyword:
       return monaco.languages.CompletionItemKind.Keyword;
-    case "string":
+    case CompletionItemKind.Value:
       return monaco.languages.CompletionItemKind.Value;
     default:
       return monaco.languages.CompletionItemKind.Text;
@@ -65,7 +52,7 @@ function toMonacoCompletion(
 ): Monaco.languages.CompletionItem {
   return {
     label: entry.label,
-    kind: toCompletionItemKind(entry.kind, monaco),
+    kind: toMonacoCompletionKind(entry.kind, monaco),
     insertText: entry.insertText ?? entry.label,
     sortText: entry.sortText,
     range,
@@ -82,15 +69,19 @@ const CompletionSyncInner = () => {
       {
         triggerCharacters: ["."],
 
-        async provideCompletionItems(model, position) {
+        async provideCompletionItems(model, monacoPosition) {
           const uri = model.uri.toString();
-          const offset = model.getOffsetAt(position);
-          const result = await requestCompletion(uri, offset);
+          // Convert Monaco 1-based position to LSP 0-based Position
+          const position = Position.create(
+            monacoPosition.lineNumber - 1,
+            monacoPosition.column - 1,
+          );
+          const result = await requestCompletion(uri, position);
 
-          const word = model.getWordUntilPosition(position);
+          const word = model.getWordUntilPosition(monacoPosition);
           const range: Monaco.IRange = {
-            startLineNumber: position.lineNumber,
-            endLineNumber: position.lineNumber,
+            startLineNumber: monacoPosition.lineNumber,
+            endLineNumber: monacoPosition.lineNumber,
             startColumn: word.startColumn,
             endColumn: word.endColumn,
           };
diff --git a/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
index adcbffcbcbe..08dd0f69769 100644
--- a/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
@@ -1,53 +1,49 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect, useRef } from "react";
+import { DiagnosticSeverity } from "vscode-languageserver-types";
+import type { Diagnostic } from "vscode-languageserver-types";
 
 import { LanguageClientContext } from "../checker/context";
-import type { Diagnostic } from "../checker/worker/protocol";
 import { MonacoContext } from "./context";
 
 const OWNER = "checker";
 
 /**
- * Convert LSP-like severity to Monaco MarkerSeverity.
- * LSP: 1=Error, 2=Warning, 3=Information, 4=Hint
+ * Convert LSP `DiagnosticSeverity` to Monaco `MarkerSeverity`.
  */
 function toMarkerSeverity(
-  severity: number,
+  severity: DiagnosticSeverity | undefined,
   monaco: typeof Monaco,
 ): Monaco.MarkerSeverity {
   switch (severity) {
-    case 1:
+    case DiagnosticSeverity.Error:
       return monaco.MarkerSeverity.Error;
-    case 2:
+    case DiagnosticSeverity.Warning:
       return monaco.MarkerSeverity.Warning;
-    case 3:
+    case DiagnosticSeverity.Information:
       return monaco.MarkerSeverity.Info;
-    case 4:
+    case DiagnosticSeverity.Hint:
       return monaco.MarkerSeverity.Hint;
     default:
       return monaco.MarkerSeverity.Error;
   }
 }
 
-/** Convert Diagnostic[] to IMarkerData[] using the model for offset→position. */
+/** Convert LSP Diagnostic[] to Monaco IMarkerData[]. */
 function diagnosticsToMarkers(
-  model: Monaco.editor.ITextModel,
   diagnostics: Diagnostic[],
   monaco: typeof Monaco,
 ): Monaco.editor.IMarkerData[] {
-  return diagnostics.map((diag) => {
-    const start = model.getPositionAt(diag.start ?? 0);
-    const end = model.getPositionAt((diag.start ?? 0) + (diag.length ?? 0));
-    return {
-      severity: toMarkerSeverity(diag.severity, monaco),
-      message: diag.message,
-      startLineNumber: start.lineNumber,
-      startColumn: start.column,
-      endLineNumber: end.lineNumber,
-      endColumn: end.column,
-      code: String(diag.code),
-    };
-  });
+  return diagnostics.map((diag) => ({
+    severity: toMarkerSeverity(diag.severity, monaco),
+    message: diag.message,
+    // Monaco uses 1-based line/column, LSP uses 0-based
+    startLineNumber: diag.range.start.line + 1,
+    startColumn: diag.range.start.character + 1,
+    endLineNumber: diag.range.end.line + 1,
+    endColumn: diag.range.end.character + 1,
+    code: diag.code != null ? String(diag.code) : undefined,
+  }));
 }
 
 const DiagnosticsSyncInner = () => {
@@ -62,7 +58,7 @@ const DiagnosticsSyncInner = () => {
       const monacoUri = monaco.Uri.parse(uri);
       const model = monaco.editor.getModel(monacoUri);
       if (model) {
-        const markers = diagnosticsToMarkers(model, diagnostics, monaco);
+        const markers = diagnosticsToMarkers(diagnostics, monaco);
         monaco.editor.setModelMarkers(model, OWNER, markers);
       }
       currentUris.add(uri);
@@ -86,7 +82,7 @@ const DiagnosticsSyncInner = () => {
       const modelUri = model.uri.toString();
       const diags = diagnosticsByUri.get(modelUri);
       if (diags) {
-        const markers = diagnosticsToMarkers(model, diags, monaco);
+        const markers = diagnosticsToMarkers(diags, monaco);
         monaco.editor.setModelMarkers(model, OWNER, markers);
       }
     });
diff --git a/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
index e3cb0aeab27..4dea49a0b5d 100644
--- a/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
@@ -1,39 +1,82 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect } from "react";
+import { MarkupKind, Position } from "vscode-languageserver-types";
+import type { Hover, MarkupContent } from "vscode-languageserver-types";
 
 import { LanguageClientContext } from "../checker/context";
 import { MonacoContext } from "./context";
 
+/** Extract display string from LSP Hover contents. */
+function hoverContentsToMarkdown(hover: Hover): Monaco.IMarkdownString[] {
+  const { contents } = hover;
+
+  // MarkupContent
+  if (typeof contents === "object" && "kind" in contents) {
+    const mc = contents as MarkupContent;
+    if (mc.kind === MarkupKind.Markdown) {
+      return [{ value: mc.value }];
+    }
+    // PlainText — wrap in code block for Monaco
+    return [{ value: mc.value }];
+  }
+
+  // string
+  if (typeof contents === "string") {
+    return [{ value: contents }];
+  }
+
+  // MarkedString[]
+  if (Array.isArray(contents)) {
+    return contents.map((item) => {
+      if (typeof item === "string") {
+        return { value: item };
+      }
+      return { value: `\`\`\`${item.language}\n${item.value}\n\`\`\`` };
+    });
+  }
+
+  // MarkedString { language, value }
+  if ("language" in contents) {
+    return [
+      {
+        value: `\`\`\`${contents.language}\n${contents.value}\n\`\`\``,
+      },
+    ];
+  }
+
+  return [];
+}
+
 const HoverSyncInner = () => {
   const { monaco } = use(use(MonacoContext));
   const { requestHover } = use(LanguageClientContext);
 
   useEffect(() => {
     const disposable = monaco.languages.registerHoverProvider("typescript", {
-      async provideHover(model, position) {
+      async provideHover(model, monacoPosition) {
         const uri = model.uri.toString();
-        const offset = model.getOffsetAt(position);
-        const info = await requestHover(uri, offset);
+        // Convert Monaco 1-based position to LSP 0-based Position
+        const position = Position.create(
+          monacoPosition.lineNumber - 1,
+          monacoPosition.column - 1,
+        );
+        const info = await requestHover(uri, position);
 
         if (!info) {
           return null;
         }
 
-        const startPos = model.getPositionAt(info.start);
-        const endPos = model.getPositionAt(info.start + info.length);
-        const range: Monaco.IRange = {
-          startLineNumber: startPos.lineNumber,
-          startColumn: startPos.column,
-          endLineNumber: endPos.lineNumber,
-          endColumn: endPos.column,
-        };
-
-        const contents: Monaco.IMarkdownString[] = [
-          { value: `\`\`\`typescript\n${info.displayParts}\n\`\`\`` },
-        ];
-        if (info.documentation) {
-          contents.push({ value: info.documentation });
-        }
+        const contents = hoverContentsToMarkdown(info);
+
+        // Convert LSP 0-based range to Monaco 1-based range
+        const range: Monaco.IRange | undefined = info.range
+          ? {
+              startLineNumber: info.range.start.line + 1,
+              startColumn: info.range.start.character + 1,
+              endLineNumber: info.range.end.line + 1,
+              endColumn: info.range.end.character + 1,
+            }
+          : undefined;
 
         return { range, contents };
       },
diff --git a/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
index 6fc77d04abf..8fe8a1d28c3 100644
--- a/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
@@ -1,22 +1,42 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect } from "react";
+import { MarkupKind, Position } from "vscode-languageserver-types";
+import type { MarkupContent, SignatureHelp } from "vscode-languageserver-types";
 
 import { LanguageClientContext } from "../checker/context";
-import type { SignatureHelp } from "../checker/worker/protocol";
 import { MonacoContext } from "./context";
 
+/** Extract documentation string from LSP MarkupContent or plain string. */
+function extractDocumentation(
+  doc: string | MarkupContent | undefined,
+): string | undefined {
+  if (!doc) {
+    return undefined;
+  }
+  if (typeof doc === "string") {
+    return doc || undefined;
+  }
+  if (doc.kind === MarkupKind.Markdown || doc.kind === MarkupKind.PlainText) {
+    return doc.value || undefined;
+  }
+  return undefined;
+}
+
 function toMonacoSignatureHelp(
-  result: NonNullable<SignatureHelp>,
+  result: SignatureHelp,
 ): Monaco.languages.SignatureHelp {
   return {
-    activeSignature: result.activeSignature,
-    activeParameter: result.activeParameter,
+    activeSignature: result.activeSignature ?? 0,
+    activeParameter: result.activeParameter ?? 0,
     signatures: result.signatures.map((sig) => ({
       label: sig.label,
-      documentation: sig.documentation || undefined,
-      parameters: sig.parameters.map((param) => ({
-        label: param.label,
-        documentation: param.documentation || undefined,
+      documentation: extractDocumentation(sig.documentation),
+      parameters: (sig.parameters ?? []).map((param) => ({
+        label:
+          typeof param.label === "string"
+            ? param.label
+            : [param.label[0], param.label[1]],
+        documentation: extractDocumentation(param.documentation),
       })),
     })),
   };
@@ -33,10 +53,14 @@ const SignatureHelpSyncInner = () => {
         signatureHelpTriggerCharacters: ["(", ","],
         signatureHelpRetriggerCharacters: [","],
 
-        async provideSignatureHelp(model, position) {
+        async provideSignatureHelp(model, monacoPosition) {
           const uri = model.uri.toString();
-          const offset = model.getOffsetAt(position);
-          const result = await requestSignatureHelp(uri, offset);
+          // Convert Monaco 1-based position to LSP 0-based Position
+          const position = Position.create(
+            monacoPosition.lineNumber - 1,
+            monacoPosition.column - 1,
+          );
+          const result = await requestSignatureHelp(uri, position);
 
           if (!result) {
             return null;
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
index a93580da56f..f389291be52 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
@@ -2,8 +2,9 @@ import { css } from "@hashintel/ds-helpers/css";
 import { use, useCallback, useMemo, useState } from "react";
 import { FaChevronDown, FaChevronRight } from "react-icons/fa6";
 
+import type { Diagnostic } from "vscode-languageserver-types";
+
 import { LanguageClientContext } from "../../../checker/context";
-import type { Diagnostic } from "../../../checker/worker/protocol";
 import type { SubView } from "../../../components/sub-view/types";
 import { parseDocumentUri } from "../../../monaco/editor-paths";
 import { EditorContext } from "../../../state/editor-context";
@@ -251,8 +252,8 @@ const DiagnosticsContent: React.FC = () => {
                       {itemGroup.diagnostics.map((diagnostic, index) => (
                         <li
                           key={`${group.entityId}-${itemGroup.subType}-${
-                            diagnostic.start ?? index
-                          }`}
+                            diagnostic.range.start.line
+                          }-${diagnostic.range.start.character}-${index}`}
                           className={diagnosticItemStyle}
                         >
                           <button
@@ -262,11 +263,10 @@ const DiagnosticsContent: React.FC = () => {
                           >
                             <span className={bulletStyle}>•</span>
                             {diagnostic.message}
-                            {diagnostic.start !== undefined && (
-                              <span className={positionStyle}>
-                                (pos: {diagnostic.start})
-                              </span>
-                            )}
+                            <span className={positionStyle}>
+                              (Ln {diagnostic.range.start.line + 1}, Col{" "}
+                              {diagnostic.range.start.character + 1})
+                            </span>
                           </button>
                         </li>
                       ))}
diff --git a/yarn.lock b/yarn.lock
index 408032f7a54..1115c3550dc 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -7829,6 +7829,7 @@ __metadata:
     vite: "npm:8.0.0-beta.14"
     vite-plugin-dts: "npm:4.5.4"
     vitest: "npm:4.0.18"
+    vscode-languageserver-types: "npm:3.17.5"
     web-worker: "npm:1.4.1"
   peerDependencies:
     "@hashintel/ds-components": "workspace:^"
@@ -45394,6 +45395,13 @@ __metadata:
   languageName: node
   linkType: hard
 
+"vscode-languageserver-types@npm:3.17.5":
+  version: 3.17.5
+  resolution: "vscode-languageserver-types@npm:3.17.5"
+  checksum: 10c0/1e1260de79a2cc8de3e46f2e0182cdc94a7eddab487db5a3bd4ee716f67728e685852707d72c059721ce500447be9a46764a04f0611e94e4321ffa088eef36f8
+  languageName: node
+  linkType: hard
+
 "vscode-uri@npm:^3.0.8":
   version: 3.1.0
   resolution: "vscode-uri@npm:3.1.0"

From f98b7d2ead1797479f3ddaf123a86565895816df Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 13:52:11 +0100
Subject: [PATCH 12/22] H-5839: Add changeset for LSP language service layer

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .changeset/bright-ideas-flow.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/bright-ideas-flow.md

diff --git a/.changeset/bright-ideas-flow.md b/.changeset/bright-ideas-flow.md
new file mode 100644
index 00000000000..b3d25c3acdc
--- /dev/null
+++ b/.changeset/bright-ideas-flow.md
@@ -0,0 +1,5 @@
+---
+"@hashintel/petrinaut": minor
+---
+
+Add LSP-based language service layer for Monaco code editors with diagnostics, completions, hover, and signature help

From b1ef02aa1d8e59f8577fe5e0f6117019b3a87db8 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 15:42:51 +0100
Subject: [PATCH 13/22] H-5839: Modernize LanguageClientProvider to React 19
 patterns

Use React 19 Context shorthand, derive totalDiagnosticsCount directly
instead of useMemo, and extract pure helper functions for building the
diagnostics map.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../petrinaut/src/checker/provider.tsx        | 46 +++++++++++--------
 .../petrinaut/src/monaco/diagnostics-sync.tsx |  2 +-
 2 files changed, 28 insertions(+), 20 deletions(-)

diff --git a/libs/@hashintel/petrinaut/src/checker/provider.tsx b/libs/@hashintel/petrinaut/src/checker/provider.tsx
index a2c95cb3137..39c982f6554 100644
--- a/libs/@hashintel/petrinaut/src/checker/provider.tsx
+++ b/libs/@hashintel/petrinaut/src/checker/provider.tsx
@@ -1,4 +1,4 @@
-import { use, useCallback, useEffect, useMemo, useRef, useState } from "react";
+import { use, useCallback, useEffect, useRef, useState } from "react";
 
 import { SDCPNContext } from "../state/sdcpn-context";
 import { LanguageClientContext } from "./context";
@@ -9,6 +9,28 @@ import type {
 } from "./worker/protocol";
 import { useLanguageClient } from "./worker/use-language-client";
 
+/** Build an immutable diagnostics map, excluding empty entries. */
+function buildDiagnosticsMap(
+  allParams: PublishDiagnosticsParams[],
+): Map<DocumentUri, Diagnostic[]> {
+  return new Map(
+    allParams
+      .filter((param) => param.diagnostics.length > 0)
+      .map((param) => [param.uri, param.diagnostics]),
+  );
+}
+
+/** Count total diagnostics across all URIs. */
+function countDiagnostics(
+  diagnosticsByUri: Map<DocumentUri, Diagnostic[]>,
+): number {
+  let count = 0;
+  for (const diagnostics of diagnosticsByUri.values()) {
+    count += diagnostics.length;
+  }
+  return count;
+}
+
 export const LanguageClientProvider: React.FC<{
   children: React.ReactNode;
 }> = ({ children }) => {
@@ -22,15 +44,7 @@ export const LanguageClientProvider: React.FC<{
   // Subscribe to diagnostics pushed from the server
   const handleDiagnostics = useCallback(
     (allParams: PublishDiagnosticsParams[]) => {
-      setDiagnosticsByUri(() => {
-        const next = new Map<DocumentUri, Diagnostic[]>();
-        for (const param of allParams) {
-          if (param.diagnostics.length > 0) {
-            next.set(param.uri, param.diagnostics);
-          }
-        }
-        return next;
-      });
+      setDiagnosticsByUri(buildDiagnosticsMap(allParams));
     },
     [],
   );
@@ -50,16 +64,10 @@ export const LanguageClientProvider: React.FC<{
     }
   }, [petriNetDefinition, client]);
 
-  const totalDiagnosticsCount = useMemo(() => {
-    let count = 0;
-    for (const diagnostics of diagnosticsByUri.values()) {
-      count += diagnostics.length;
-    }
-    return count;
-  }, [diagnosticsByUri]);
+  const totalDiagnosticsCount = countDiagnostics(diagnosticsByUri);
 
   return (
-    <LanguageClientContext.Provider
+    <LanguageClientContext
       value={{
         diagnosticsByUri,
         totalDiagnosticsCount,
@@ -70,6 +78,6 @@ export const LanguageClientProvider: React.FC<{
       }}
     >
       {children}
-    </LanguageClientContext.Provider>
+    </LanguageClientContext>
   );
 };
diff --git a/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
index 08dd0f69769..c08a65433c8 100644
--- a/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
@@ -1,7 +1,7 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect, useRef } from "react";
-import { DiagnosticSeverity } from "vscode-languageserver-types";
 import type { Diagnostic } from "vscode-languageserver-types";
+import { DiagnosticSeverity } from "vscode-languageserver-types";
 
 import { LanguageClientContext } from "../checker/context";
 import { MonacoContext } from "./context";

From c36007d59006380983cc0348b6d3d4db8f23971e Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 15:59:00 +0100
Subject: [PATCH 14/22] H-5839: Fix ESLint errors across checker and Monaco
 sync components

Fix import sorting, remove unused imports, simplify unnecessary type
assertions and conditions, avoid array index keys, and fix no-return-assign
in provider singleton.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../petrinaut/src/checker/worker/checker.worker.ts   |  8 ++++----
 .../petrinaut/src/monaco/completion-sync.tsx         |  2 +-
 libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx  |  4 ++--
 libs/@hashintel/petrinaut/src/monaco/provider.tsx    |  3 ++-
 .../petrinaut/src/monaco/signature-help-sync.tsx     | 12 ++++++------
 .../src/views/Editor/subviews/diagnostics.tsx        |  7 ++-----
 6 files changed, 17 insertions(+), 19 deletions(-)

diff --git a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
index 4f065c86b40..5f8a3d2ee96 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
+++ b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
@@ -11,15 +11,15 @@
  */
 import ts from "typescript";
 import {
-  CompletionItemKind,
-  DiagnosticSeverity,
-  MarkupKind,
-  Range,
   type CompletionItem,
+  CompletionItemKind,
   type CompletionList,
   type Diagnostic,
+  DiagnosticSeverity,
   type DocumentUri,
   type Hover,
+  MarkupKind,
+  Range,
   type SignatureHelp,
   type SignatureInformation,
 } from "vscode-languageserver-types";
diff --git a/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
index c633c6ae006..546ca7f09a9 100644
--- a/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
@@ -1,7 +1,7 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect } from "react";
-import { CompletionItemKind, Position } from "vscode-languageserver-types";
 import type { CompletionItem } from "vscode-languageserver-types";
+import { CompletionItemKind, Position } from "vscode-languageserver-types";
 
 import { LanguageClientContext } from "../checker/context";
 import { MonacoContext } from "./context";
diff --git a/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
index 4dea49a0b5d..0cb92f7e1a5 100644
--- a/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
@@ -1,7 +1,7 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect } from "react";
+import type { Hover } from "vscode-languageserver-types";
 import { MarkupKind, Position } from "vscode-languageserver-types";
-import type { Hover, MarkupContent } from "vscode-languageserver-types";
 
 import { LanguageClientContext } from "../checker/context";
 import { MonacoContext } from "./context";
@@ -12,7 +12,7 @@ function hoverContentsToMarkdown(hover: Hover): Monaco.IMarkdownString[] {
 
   // MarkupContent
   if (typeof contents === "object" && "kind" in contents) {
-    const mc = contents as MarkupContent;
+    const mc = contents;
     if (mc.kind === MarkupKind.Markdown) {
       return [{ value: mc.value }];
     }
diff --git a/libs/@hashintel/petrinaut/src/monaco/provider.tsx b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
index 249629de23c..e36ac9f819b 100644
--- a/libs/@hashintel/petrinaut/src/monaco/provider.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/provider.tsx
@@ -66,7 +66,8 @@ async function initMonaco(): Promise<MonacoContextValue> {
 /** Module-level lazy singleton — initialized once, reused across renders. */
 let monacoPromise: Promise<MonacoContextValue> | null = null;
 function getMonacoPromise(): Promise<MonacoContextValue> {
-  return (monacoPromise ??= initMonaco());
+  monacoPromise ??= initMonaco();
+  return monacoPromise;
 }
 
 export const MonacoProvider: React.FC<{ children: React.ReactNode }> = ({
diff --git a/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
index 8fe8a1d28c3..582b5cb03c2 100644
--- a/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
@@ -1,7 +1,10 @@
 import type * as Monaco from "monaco-editor";
 import { Suspense, use, useEffect } from "react";
-import { MarkupKind, Position } from "vscode-languageserver-types";
-import type { MarkupContent, SignatureHelp } from "vscode-languageserver-types";
+import {
+  type MarkupContent,
+  Position,
+  type SignatureHelp,
+} from "vscode-languageserver-types";
 
 import { LanguageClientContext } from "../checker/context";
 import { MonacoContext } from "./context";
@@ -16,10 +19,7 @@ function extractDocumentation(
   if (typeof doc === "string") {
     return doc || undefined;
   }
-  if (doc.kind === MarkupKind.Markdown || doc.kind === MarkupKind.PlainText) {
-    return doc.value || undefined;
-  }
-  return undefined;
+  return doc.value || undefined;
 }
 
 function toMonacoSignatureHelp(
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
index f389291be52..6086cf61ff3 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
@@ -1,7 +1,6 @@
 import { css } from "@hashintel/ds-helpers/css";
 import { use, useCallback, useMemo, useState } from "react";
 import { FaChevronDown, FaChevronRight } from "react-icons/fa6";
-
 import type { Diagnostic } from "vscode-languageserver-types";
 
 import { LanguageClientContext } from "../../../checker/context";
@@ -249,11 +248,9 @@ const DiagnosticsContent: React.FC = () => {
 
                     {/* Diagnostics list */}
                     <ul className={diagnosticsListStyle}>
-                      {itemGroup.diagnostics.map((diagnostic, index) => (
+                      {itemGroup.diagnostics.map((diagnostic) => (
                         <li
-                          key={`${group.entityId}-${itemGroup.subType}-${
-                            diagnostic.range.start.line
-                          }-${diagnostic.range.start.character}-${index}`}
+                          key={`${group.entityId}-${itemGroup.subType}-${diagnostic.range.start.line}:${diagnostic.range.start.character}-${diagnostic.code}`}
                           className={diagnosticItemStyle}
                         >
                           <button

From edf90bf29f0c1788fe70e4cbc2457b3efb6e54e6 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 16:17:48 +0100
Subject: [PATCH 15/22] H-5839: Consolidate document URI mapping into single
 shared module

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../src/checker/lib/document-uris.test.ts     | 128 ++++++++++++++++++
 .../src/checker/lib/document-uris.ts          | 107 +++++++++++++++
 .../src/checker/worker/checker.worker.ts      |  61 +--------
 .../petrinaut/src/monaco/editor-paths.ts      |  48 +------
 4 files changed, 241 insertions(+), 103 deletions(-)
 create mode 100644 libs/@hashintel/petrinaut/src/checker/lib/document-uris.test.ts
 create mode 100644 libs/@hashintel/petrinaut/src/checker/lib/document-uris.ts

diff --git a/libs/@hashintel/petrinaut/src/checker/lib/document-uris.test.ts b/libs/@hashintel/petrinaut/src/checker/lib/document-uris.test.ts
new file mode 100644
index 00000000000..ba21fc144e9
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/checker/lib/document-uris.test.ts
@@ -0,0 +1,128 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  filePathToUri,
+  getDocumentUri,
+  parseDocumentUri,
+  uriToFilePath,
+} from "./document-uris";
+
+describe("getDocumentUri", () => {
+  it("builds a transition-lambda URI", () => {
+    expect(getDocumentUri("transition-lambda", "t1")).toBe(
+      "inmemory://sdcpn/transitions/t1/lambda.ts",
+    );
+  });
+
+  it("builds a transition-kernel URI", () => {
+    expect(getDocumentUri("transition-kernel", "t1")).toBe(
+      "inmemory://sdcpn/transitions/t1/kernel.ts",
+    );
+  });
+
+  it("builds a differential-equation URI", () => {
+    expect(getDocumentUri("differential-equation", "de1")).toBe(
+      "inmemory://sdcpn/differential-equations/de1.ts",
+    );
+  });
+});
+
+describe("parseDocumentUri", () => {
+  it("parses a transition-lambda URI", () => {
+    expect(
+      parseDocumentUri("inmemory://sdcpn/transitions/t1/lambda.ts"),
+    ).toEqual({ itemType: "transition-lambda", itemId: "t1" });
+  });
+
+  it("parses a transition-kernel URI", () => {
+    expect(
+      parseDocumentUri("inmemory://sdcpn/transitions/t1/kernel.ts"),
+    ).toEqual({ itemType: "transition-kernel", itemId: "t1" });
+  });
+
+  it("parses a differential-equation URI", () => {
+    expect(
+      parseDocumentUri("inmemory://sdcpn/differential-equations/de1.ts"),
+    ).toEqual({ itemType: "differential-equation", itemId: "de1" });
+  });
+
+  it("returns null for an unknown URI", () => {
+    expect(parseDocumentUri("inmemory://sdcpn/unknown/foo.ts")).toBeNull();
+  });
+
+  it("returns null for a completely unrelated string", () => {
+    expect(parseDocumentUri("https://example.com")).toBeNull();
+  });
+});
+
+describe("uriToFilePath", () => {
+  it("converts a transition-lambda URI to a file path", () => {
+    expect(uriToFilePath("inmemory://sdcpn/transitions/t1/lambda.ts")).toBe(
+      "/transitions/t1/lambda/code.ts",
+    );
+  });
+
+  it("converts a transition-kernel URI to a file path", () => {
+    expect(uriToFilePath("inmemory://sdcpn/transitions/t1/kernel.ts")).toBe(
+      "/transitions/t1/kernel/code.ts",
+    );
+  });
+
+  it("converts a differential-equation URI to a file path", () => {
+    expect(
+      uriToFilePath("inmemory://sdcpn/differential-equations/de1.ts"),
+    ).toBe("/differential_equations/de1/code.ts");
+  });
+
+  it("returns null for an unknown URI", () => {
+    expect(uriToFilePath("inmemory://sdcpn/unknown/foo.ts")).toBeNull();
+  });
+});
+
+describe("filePathToUri", () => {
+  it("converts a transition-lambda file path to a URI", () => {
+    expect(filePathToUri("/transitions/t1/lambda/code.ts")).toBe(
+      "inmemory://sdcpn/transitions/t1/lambda.ts",
+    );
+  });
+
+  it("converts a transition-kernel file path to a URI", () => {
+    expect(filePathToUri("/transitions/t1/kernel/code.ts")).toBe(
+      "inmemory://sdcpn/transitions/t1/kernel.ts",
+    );
+  });
+
+  it("converts a differential-equation file path to a URI", () => {
+    expect(filePathToUri("/differential_equations/de1/code.ts")).toBe(
+      "inmemory://sdcpn/differential-equations/de1.ts",
+    );
+  });
+
+  it("returns null for an unknown file path", () => {
+    expect(filePathToUri("/unknown/foo/code.ts")).toBeNull();
+  });
+});
+
+describe("roundtrip", () => {
+  const cases = [
+    { itemType: "transition-lambda" as const, itemId: "t1" },
+    { itemType: "transition-kernel" as const, itemId: "t2" },
+    { itemType: "differential-equation" as const, itemId: "de1" },
+  ];
+
+  for (const { itemType, itemId } of cases) {
+    it(`URI → filePath → URI roundtrips for ${itemType}`, () => {
+      const uri = getDocumentUri(itemType, itemId);
+      const filePath = uriToFilePath(uri);
+      expect(filePath).not.toBeNull();
+      expect(filePathToUri(filePath!)).toBe(uri);
+    });
+
+    it(`URI → parse → rebuild roundtrips for ${itemType}`, () => {
+      const uri = getDocumentUri(itemType, itemId);
+      const parsed = parseDocumentUri(uri);
+      expect(parsed).not.toBeNull();
+      expect(getDocumentUri(parsed!.itemType, parsed!.itemId)).toBe(uri);
+    });
+  }
+});
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/document-uris.ts b/libs/@hashintel/petrinaut/src/checker/lib/document-uris.ts
new file mode 100644
index 00000000000..0d2c9333989
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/checker/lib/document-uris.ts
@@ -0,0 +1,107 @@
+/**
+ * Single source of truth for mapping between:
+ * - Document URIs (`inmemory://sdcpn/...`) used by Monaco and the LSP protocol
+ * - Internal file paths (`/transitions/.../code.ts`) used by the TS LanguageService
+ * - Item types + IDs used by the SDCPN domain model
+ */
+import type { ItemType } from "./checker";
+import { getItemFilePath } from "./file-paths";
+
+// ---------------------------------------------------------------------------
+// URI construction
+// ---------------------------------------------------------------------------
+
+/** Build a document URI for a given SDCPN item (used as Monaco model URI). */
+export function getDocumentUri(itemType: ItemType, itemId: string): string {
+  switch (itemType) {
+    case "transition-lambda":
+      return `inmemory://sdcpn/transitions/${itemId}/lambda.ts`;
+    case "transition-kernel":
+      return `inmemory://sdcpn/transitions/${itemId}/kernel.ts`;
+    case "differential-equation":
+      return `inmemory://sdcpn/differential-equations/${itemId}.ts`;
+  }
+}
+
+// ---------------------------------------------------------------------------
+// URI parsing
+// ---------------------------------------------------------------------------
+
+const TRANSITION_LAMBDA_URI_RE =
+  /^inmemory:\/\/sdcpn\/transitions\/([^/]+)\/lambda\.ts$/;
+const TRANSITION_KERNEL_URI_RE =
+  /^inmemory:\/\/sdcpn\/transitions\/([^/]+)\/kernel\.ts$/;
+const DE_URI_RE = /^inmemory:\/\/sdcpn\/differential-equations\/([^/]+)\.ts$/;
+
+/** Extract `(itemType, itemId)` from a document URI string. */
+export function parseDocumentUri(
+  uri: string,
+): { itemType: ItemType; itemId: string } | null {
+  let match = TRANSITION_LAMBDA_URI_RE.exec(uri);
+  if (match) {
+    return { itemType: "transition-lambda", itemId: match[1]! };
+  }
+
+  match = TRANSITION_KERNEL_URI_RE.exec(uri);
+  if (match) {
+    return { itemType: "transition-kernel", itemId: match[1]! };
+  }
+
+  match = DE_URI_RE.exec(uri);
+  if (match) {
+    return { itemType: "differential-equation", itemId: match[1]! };
+  }
+
+  return null;
+}
+
+// ---------------------------------------------------------------------------
+// URI ↔ internal file path conversion
+// ---------------------------------------------------------------------------
+
+/** Convert a document URI to the internal virtual file path used by the TS LanguageService. */
+export function uriToFilePath(uri: string): string | null {
+  const parsed = parseDocumentUri(uri);
+  if (!parsed) {
+    return null;
+  }
+
+  switch (parsed.itemType) {
+    case "transition-lambda":
+      return getItemFilePath("transition-lambda-code", {
+        transitionId: parsed.itemId,
+      });
+    case "transition-kernel":
+      return getItemFilePath("transition-kernel-code", {
+        transitionId: parsed.itemId,
+      });
+    case "differential-equation":
+      return getItemFilePath("differential-equation-code", {
+        id: parsed.itemId,
+      });
+  }
+}
+
+const TRANSITION_LAMBDA_PATH_RE = /^\/transitions\/([^/]+)\/lambda\/code\.ts$/;
+const TRANSITION_KERNEL_PATH_RE = /^\/transitions\/([^/]+)\/kernel\/code\.ts$/;
+const DE_PATH_RE = /^\/differential_equations\/([^/]+)\/code\.ts$/;
+
+/** Convert an internal file path to a document URI. */
+export function filePathToUri(filePath: string): string | null {
+  let match = TRANSITION_LAMBDA_PATH_RE.exec(filePath);
+  if (match) {
+    return getDocumentUri("transition-lambda", match[1]!);
+  }
+
+  match = TRANSITION_KERNEL_PATH_RE.exec(filePath);
+  if (match) {
+    return getDocumentUri("transition-kernel", match[1]!);
+  }
+
+  match = DE_PATH_RE.exec(filePath);
+  if (match) {
+    return getDocumentUri("differential-equation", match[1]!);
+  }
+
+  return null;
+}
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
index 5f8a3d2ee96..718be67db5e 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
+++ b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
@@ -16,7 +16,6 @@ import {
   type CompletionList,
   type Diagnostic,
   DiagnosticSeverity,
-  type DocumentUri,
   type Hover,
   MarkupKind,
   Range,
@@ -27,7 +26,7 @@ import {
 import type { SDCPN } from "../../core/types/sdcpn";
 import { checkSDCPN } from "../lib/checker";
 import { SDCPNLanguageServer } from "../lib/create-sdcpn-language-service";
-import { getItemFilePath } from "../lib/file-paths";
+import { filePathToUri, uriToFilePath } from "../lib/document-uris";
 import { offsetToPosition, positionToOffset } from "../lib/position-utils";
 import type {
   ClientMessage,
@@ -35,64 +34,6 @@ import type {
   ServerMessage,
 } from "./protocol";
 
-// ---------------------------------------------------------------------------
-// URI ↔ internal file path mapping
-// ---------------------------------------------------------------------------
-
-const TRANSITION_LAMBDA_URI_RE =
-  /^inmemory:\/\/sdcpn\/transitions\/([^/]+)\/lambda\.ts$/;
-const TRANSITION_KERNEL_URI_RE =
-  /^inmemory:\/\/sdcpn\/transitions\/([^/]+)\/kernel\.ts$/;
-const DE_URI_RE = /^inmemory:\/\/sdcpn\/differential-equations\/([^/]+)\.ts$/;
-
-/** Convert a document URI to the internal virtual file path used by the TS LanguageService. */
-function uriToFilePath(uri: DocumentUri): string | null {
-  let match = TRANSITION_LAMBDA_URI_RE.exec(uri);
-  if (match) {
-    return getItemFilePath("transition-lambda-code", {
-      transitionId: match[1]!,
-    });
-  }
-
-  match = TRANSITION_KERNEL_URI_RE.exec(uri);
-  if (match) {
-    return getItemFilePath("transition-kernel-code", {
-      transitionId: match[1]!,
-    });
-  }
-
-  match = DE_URI_RE.exec(uri);
-  if (match) {
-    return getItemFilePath("differential-equation-code", { id: match[1]! });
-  }
-
-  return null;
-}
-
-const TRANSITION_LAMBDA_PATH_RE = /^\/transitions\/([^/]+)\/lambda\/code\.ts$/;
-const TRANSITION_KERNEL_PATH_RE = /^\/transitions\/([^/]+)\/kernel\/code\.ts$/;
-const DE_PATH_RE = /^\/differential_equations\/([^/]+)\/code\.ts$/;
-
-/** Convert an internal file path to the document URI used by Monaco. */
-function filePathToUri(filePath: string): DocumentUri | null {
-  let match = TRANSITION_LAMBDA_PATH_RE.exec(filePath);
-  if (match) {
-    return `inmemory://sdcpn/transitions/${match[1]!}/lambda.ts`;
-  }
-
-  match = TRANSITION_KERNEL_PATH_RE.exec(filePath);
-  if (match) {
-    return `inmemory://sdcpn/transitions/${match[1]!}/kernel.ts`;
-  }
-
-  match = DE_PATH_RE.exec(filePath);
-  if (match) {
-    return `inmemory://sdcpn/differential-equations/${match[1]!}.ts`;
-  }
-
-  return null;
-}
-
 // ---------------------------------------------------------------------------
 // TS → LSP type conversions
 // ---------------------------------------------------------------------------
diff --git a/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts b/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
index 956b3d78b2a..305df6aa2e8 100644
--- a/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
+++ b/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
@@ -1,43 +1,5 @@
-import type { ItemType } from "../checker/lib/checker";
-
-/** Generates the document URI for a given SDCPN item (used as Monaco model URI). */
-export function getDocumentUri(itemType: ItemType, itemId: string): string {
-  switch (itemType) {
-    case "transition-lambda":
-      return `inmemory://sdcpn/transitions/${itemId}/lambda.ts`;
-    case "transition-kernel":
-      return `inmemory://sdcpn/transitions/${itemId}/kernel.ts`;
-    case "differential-equation":
-      return `inmemory://sdcpn/differential-equations/${itemId}.ts`;
-  }
-}
-
-const TRANSITION_LAMBDA_RE =
-  /^inmemory:\/\/sdcpn\/transitions\/([^/]+)\/lambda\.ts$/;
-const TRANSITION_KERNEL_RE =
-  /^inmemory:\/\/sdcpn\/transitions\/([^/]+)\/kernel\.ts$/;
-const DIFFERENTIAL_EQUATION_RE =
-  /^inmemory:\/\/sdcpn\/differential-equations\/([^/]+)\.ts$/;
-
-/** Extract `(itemType, itemId)` from a document URI string. */
-export function parseDocumentUri(uri: string): {
-  itemType: ItemType;
-  itemId: string;
-} | null {
-  let match = TRANSITION_LAMBDA_RE.exec(uri);
-  if (match) {
-    return { itemType: "transition-lambda", itemId: match[1]! };
-  }
-
-  match = TRANSITION_KERNEL_RE.exec(uri);
-  if (match) {
-    return { itemType: "transition-kernel", itemId: match[1]! };
-  }
-
-  match = DIFFERENTIAL_EQUATION_RE.exec(uri);
-  if (match) {
-    return { itemType: "differential-equation", itemId: match[1]! };
-  }
-
-  return null;
-}
+/**
+ * Re-exports from the centralized document-uris module.
+ * Monaco components import from here for convenience.
+ */
+export { getDocumentUri, parseDocumentUri } from "../checker/lib/document-uris";

From 441087fc3322676227b6059267108cc3bcb41917 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 16:31:30 +0100
Subject: [PATCH 16/22] H-5839: Extract TS-to-LSP conversion functions into
 testable module

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../src/checker/lib/document-uris.test.ts     |  16 +-
 .../src/checker/lib/ts-to-lsp.test.ts         | 162 ++++++++++++++++++
 .../petrinaut/src/checker/lib/ts-to-lsp.ts    | 102 +++++++++++
 .../src/checker/worker/checker.worker.ts      |  94 +---------
 4 files changed, 275 insertions(+), 99 deletions(-)
 create mode 100644 libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.test.ts
 create mode 100644 libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.ts

diff --git a/libs/@hashintel/petrinaut/src/checker/lib/document-uris.test.ts b/libs/@hashintel/petrinaut/src/checker/lib/document-uris.test.ts
index ba21fc144e9..c73ba724652 100644
--- a/libs/@hashintel/petrinaut/src/checker/lib/document-uris.test.ts
+++ b/libs/@hashintel/petrinaut/src/checker/lib/document-uris.test.ts
@@ -110,19 +110,23 @@ describe("roundtrip", () => {
     { itemType: "differential-equation" as const, itemId: "de1" },
   ];
 
-  for (const { itemType, itemId } of cases) {
-    it(`URI → filePath → URI roundtrips for ${itemType}`, () => {
+  it.for(cases)(
+    "URI → filePath → URI roundtrips for $itemType",
+    ({ itemType, itemId }) => {
       const uri = getDocumentUri(itemType, itemId);
       const filePath = uriToFilePath(uri);
       expect(filePath).not.toBeNull();
       expect(filePathToUri(filePath!)).toBe(uri);
-    });
+    },
+  );
 
-    it(`URI → parse → rebuild roundtrips for ${itemType}`, () => {
+  it.for(cases)(
+    "URI → parse → rebuild roundtrips for $itemType",
+    ({ itemType, itemId }) => {
       const uri = getDocumentUri(itemType, itemId);
       const parsed = parseDocumentUri(uri);
       expect(parsed).not.toBeNull();
       expect(getDocumentUri(parsed!.itemType, parsed!.itemId)).toBe(uri);
-    });
-  }
+    },
+  );
 });
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.test.ts b/libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.test.ts
new file mode 100644
index 00000000000..6c95c75ecfb
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.test.ts
@@ -0,0 +1,162 @@
+import ts from "typescript";
+import { describe, expect, it } from "vitest";
+import {
+  CompletionItemKind,
+  DiagnosticSeverity,
+} from "vscode-languageserver-types";
+
+import {
+  serializeDiagnostic,
+  toCompletionItemKind,
+  toLspSeverity,
+} from "./ts-to-lsp";
+
+describe("toLspSeverity", () => {
+  it.for([
+    { category: 0, expected: DiagnosticSeverity.Warning, label: "Warning" },
+    { category: 1, expected: DiagnosticSeverity.Error, label: "Error" },
+    { category: 2, expected: DiagnosticSeverity.Hint, label: "Suggestion" },
+    {
+      category: 3,
+      expected: DiagnosticSeverity.Information,
+      label: "Message",
+    },
+    { category: 99, expected: DiagnosticSeverity.Error, label: "unknown" },
+  ])(
+    "maps TS $label ($category) to LSP severity $expected",
+    ({ category, expected }) => {
+      expect(toLspSeverity(category)).toBe(expected);
+    },
+  );
+});
+
+describe("toCompletionItemKind", () => {
+  it.for([
+    { tsKind: "method", expected: CompletionItemKind.Method },
+    { tsKind: "construct", expected: CompletionItemKind.Method },
+    { tsKind: "function", expected: CompletionItemKind.Function },
+    { tsKind: "local function", expected: CompletionItemKind.Function },
+    { tsKind: "constructor", expected: CompletionItemKind.Constructor },
+    { tsKind: "property", expected: CompletionItemKind.Property },
+    { tsKind: "getter", expected: CompletionItemKind.Property },
+    { tsKind: "setter", expected: CompletionItemKind.Property },
+    { tsKind: "parameter", expected: CompletionItemKind.Variable },
+    { tsKind: "var", expected: CompletionItemKind.Variable },
+    { tsKind: "local var", expected: CompletionItemKind.Variable },
+    { tsKind: "let", expected: CompletionItemKind.Variable },
+    { tsKind: "const", expected: CompletionItemKind.Variable },
+    { tsKind: "class", expected: CompletionItemKind.Class },
+    { tsKind: "interface", expected: CompletionItemKind.Interface },
+    { tsKind: "type", expected: CompletionItemKind.TypeParameter },
+    { tsKind: "type parameter", expected: CompletionItemKind.TypeParameter },
+    { tsKind: "primitive type", expected: CompletionItemKind.TypeParameter },
+    { tsKind: "alias", expected: CompletionItemKind.TypeParameter },
+    { tsKind: "enum", expected: CompletionItemKind.Enum },
+    { tsKind: "enum member", expected: CompletionItemKind.EnumMember },
+    { tsKind: "module", expected: CompletionItemKind.Module },
+    { tsKind: "external module name", expected: CompletionItemKind.Module },
+    { tsKind: "keyword", expected: CompletionItemKind.Keyword },
+    { tsKind: "string", expected: CompletionItemKind.Value },
+    { tsKind: "unknown-kind", expected: CompletionItemKind.Text },
+  ])(
+    'maps "$tsKind" to CompletionItemKind $expected',
+    ({ tsKind, expected }) => {
+      expect(toCompletionItemKind(tsKind)).toBe(expected);
+    },
+  );
+});
+
+describe("serializeDiagnostic", () => {
+  const fileContent = "const x = 1;\nconst y = 2;\n";
+
+  function makeTsDiagnostic(
+    overrides: Partial<ts.Diagnostic> = {},
+  ): ts.Diagnostic {
+    return {
+      file: undefined,
+      start: 0,
+      length: 5,
+      messageText: "Test error",
+      category: ts.DiagnosticCategory.Error,
+      code: 2304,
+      ...overrides,
+    };
+  }
+
+  it("converts a simple TS diagnostic to LSP format", () => {
+    const diag = makeTsDiagnostic({ start: 0, length: 5 });
+    const result = serializeDiagnostic(diag, fileContent);
+
+    expect(result.severity).toBe(DiagnosticSeverity.Error);
+    expect(result.message).toBe("Test error");
+    expect(result.code).toBe(2304);
+    expect(result.source).toBe("ts");
+    expect(result.range.start).toEqual({ line: 0, character: 0 });
+    expect(result.range.end).toEqual({ line: 0, character: 5 });
+  });
+
+  it("handles a diagnostic on the second line", () => {
+    // "const y" starts at offset 13 (after "const x = 1;\n")
+    const diag = makeTsDiagnostic({ start: 13, length: 7 });
+    const result = serializeDiagnostic(diag, fileContent);
+
+    expect(result.range.start).toEqual({ line: 1, character: 0 });
+    expect(result.range.end).toEqual({ line: 1, character: 7 });
+  });
+
+  it("handles a diagnostic spanning across lines", () => {
+    // From offset 6 ("= 1;\nconst") to offset 19
+    const diag = makeTsDiagnostic({ start: 6, length: 13 });
+    const result = serializeDiagnostic(diag, fileContent);
+
+    expect(result.range.start).toEqual({ line: 0, character: 6 });
+    expect(result.range.end).toEqual({ line: 1, character: 6 });
+  });
+
+  it("defaults start to 0 when undefined", () => {
+    const diag = makeTsDiagnostic({ start: undefined, length: 3 });
+    const result = serializeDiagnostic(diag, fileContent);
+
+    expect(result.range.start).toEqual({ line: 0, character: 0 });
+    expect(result.range.end).toEqual({ line: 0, character: 3 });
+  });
+
+  it("defaults length to 0 when undefined", () => {
+    const diag = makeTsDiagnostic({ start: 6, length: undefined });
+    const result = serializeDiagnostic(diag, fileContent);
+
+    // start and end should be the same position
+    expect(result.range.start).toEqual(result.range.end);
+  });
+
+  it("flattens chained diagnostic messages", () => {
+    const diag = makeTsDiagnostic({
+      messageText: {
+        messageText: "Outer error",
+        category: ts.DiagnosticCategory.Error,
+        code: 1000,
+        next: [
+          {
+            messageText: "Inner cause",
+            category: ts.DiagnosticCategory.Error,
+            code: 1001,
+            next: undefined,
+          },
+        ],
+      },
+    });
+    const result = serializeDiagnostic(diag, fileContent);
+
+    expect(result.message).toContain("Outer error");
+    expect(result.message).toContain("Inner cause");
+  });
+
+  it("maps TS warning category to LSP warning severity", () => {
+    const diag = makeTsDiagnostic({
+      category: ts.DiagnosticCategory.Warning,
+    });
+    const result = serializeDiagnostic(diag, fileContent);
+
+    expect(result.severity).toBe(DiagnosticSeverity.Warning);
+  });
+});
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.ts b/libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.ts
new file mode 100644
index 00000000000..f10c93baa53
--- /dev/null
+++ b/libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.ts
@@ -0,0 +1,102 @@
+/**
+ * Pure conversion functions from TypeScript LanguageService types to LSP types.
+ *
+ * These define the stable contract between the TS LanguageService and all LSP
+ * consumers (Monaco sync components, diagnostics panel, etc.).
+ */
+import ts from "typescript";
+import {
+  CompletionItemKind,
+  type Diagnostic,
+  DiagnosticSeverity,
+  Range,
+} from "vscode-languageserver-types";
+
+import { offsetToPosition } from "./position-utils";
+
+/**
+ * Map `ts.DiagnosticCategory` to `DiagnosticSeverity`.
+ * TS: 0=Warning, 1=Error, 2=Suggestion, 3=Message
+ * LSP: 1=Error, 2=Warning, 3=Information, 4=Hint
+ */
+export function toLspSeverity(category: number): DiagnosticSeverity {
+  switch (category) {
+    case 0:
+      return DiagnosticSeverity.Warning;
+    case 1:
+      return DiagnosticSeverity.Error;
+    case 2:
+      return DiagnosticSeverity.Hint;
+    case 3:
+      return DiagnosticSeverity.Information;
+    default:
+      return DiagnosticSeverity.Error;
+  }
+}
+
+/**
+ * Map TS `ScriptElementKind` strings to LSP `CompletionItemKind`.
+ */
+export function toCompletionItemKind(kind: string): CompletionItemKind {
+  switch (kind) {
+    case "method":
+    case "construct":
+      return CompletionItemKind.Method;
+    case "function":
+    case "local function":
+      return CompletionItemKind.Function;
+    case "constructor":
+      return CompletionItemKind.Constructor;
+    case "property":
+    case "getter":
+    case "setter":
+      return CompletionItemKind.Property;
+    case "parameter":
+    case "var":
+    case "local var":
+    case "let":
+    case "const":
+      return CompletionItemKind.Variable;
+    case "class":
+      return CompletionItemKind.Class;
+    case "interface":
+      return CompletionItemKind.Interface;
+    case "type":
+    case "type parameter":
+    case "primitive type":
+    case "alias":
+      return CompletionItemKind.TypeParameter;
+    case "enum":
+      return CompletionItemKind.Enum;
+    case "enum member":
+      return CompletionItemKind.EnumMember;
+    case "module":
+    case "external module name":
+      return CompletionItemKind.Module;
+    case "keyword":
+      return CompletionItemKind.Keyword;
+    case "string":
+      return CompletionItemKind.Value;
+    default:
+      return CompletionItemKind.Text;
+  }
+}
+
+/** Convert a TS diagnostic to an LSP Diagnostic with position-based ranges. */
+export function serializeDiagnostic(
+  diag: ts.Diagnostic,
+  fileContent: string,
+): Diagnostic {
+  const start = diag.start ?? 0;
+  const end = start + (diag.length ?? 0);
+  return {
+    severity: toLspSeverity(diag.category),
+    range: Range.create(
+      offsetToPosition(fileContent, start),
+      offsetToPosition(fileContent, end),
+    ),
+    message: ts.flattenDiagnosticMessageText(diag.messageText, "\n"),
+    code: diag.code,
+    source: "ts",
+  };
+}
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
index 718be67db5e..ab3357d29f0 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
+++ b/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
@@ -12,10 +12,7 @@
 import ts from "typescript";
 import {
   type CompletionItem,
-  CompletionItemKind,
   type CompletionList,
-  type Diagnostic,
-  DiagnosticSeverity,
   type Hover,
   MarkupKind,
   Range,
@@ -28,102 +25,13 @@ import { checkSDCPN } from "../lib/checker";
 import { SDCPNLanguageServer } from "../lib/create-sdcpn-language-service";
 import { filePathToUri, uriToFilePath } from "../lib/document-uris";
 import { offsetToPosition, positionToOffset } from "../lib/position-utils";
+import { serializeDiagnostic, toCompletionItemKind } from "../lib/ts-to-lsp";
 import type {
   ClientMessage,
   PublishDiagnosticsParams,
   ServerMessage,
 } from "./protocol";
 
-// ---------------------------------------------------------------------------
-// TS → LSP type conversions
-// ---------------------------------------------------------------------------
-
-/**
- * Map `ts.DiagnosticCategory` to `DiagnosticSeverity`.
- * TS: 0=Warning, 1=Error, 2=Suggestion, 3=Message
- * LSP: 1=Error, 2=Warning, 3=Information, 4=Hint
- */
-function toLspSeverity(category: number): DiagnosticSeverity {
-  switch (category) {
-    case 0:
-      return DiagnosticSeverity.Warning;
-    case 1:
-      return DiagnosticSeverity.Error;
-    case 2:
-      return DiagnosticSeverity.Hint;
-    case 3:
-      return DiagnosticSeverity.Information;
-    default:
-      return DiagnosticSeverity.Error;
-  }
-}
-
-/**
- * Map TS `ScriptElementKind` strings to LSP `CompletionItemKind`.
- */
-function toCompletionItemKind(kind: string): CompletionItemKind {
-  switch (kind) {
-    case "method":
-    case "construct":
-      return CompletionItemKind.Method;
-    case "function":
-    case "local function":
-      return CompletionItemKind.Function;
-    case "constructor":
-      return CompletionItemKind.Constructor;
-    case "property":
-    case "getter":
-    case "setter":
-      return CompletionItemKind.Property;
-    case "parameter":
-    case "var":
-    case "local var":
-    case "let":
-    case "const":
-      return CompletionItemKind.Variable;
-    case "class":
-      return CompletionItemKind.Class;
-    case "interface":
-      return CompletionItemKind.Interface;
-    case "type":
-    case "type parameter":
-    case "primitive type":
-    case "alias":
-      return CompletionItemKind.TypeParameter;
-    case "enum":
-      return CompletionItemKind.Enum;
-    case "enum member":
-      return CompletionItemKind.EnumMember;
-    case "module":
-    case "external module name":
-      return CompletionItemKind.Module;
-    case "keyword":
-      return CompletionItemKind.Keyword;
-    case "string":
-      return CompletionItemKind.Value;
-    default:
-      return CompletionItemKind.Text;
-  }
-}
-
-function serializeDiagnostic(
-  diag: ts.Diagnostic,
-  fileContent: string,
-): Diagnostic {
-  const start = diag.start ?? 0;
-  const end = start + (diag.length ?? 0);
-  return {
-    severity: toLspSeverity(diag.category),
-    range: Range.create(
-      offsetToPosition(fileContent, start),
-      offsetToPosition(fileContent, end),
-    ),
-    message: ts.flattenDiagnosticMessageText(diag.messageText, "\n"),
-    code: diag.code,
-    source: "ts",
-  };
-}
-
 // ---------------------------------------------------------------------------
 // Server state
 // ---------------------------------------------------------------------------

From e6188e6ff207598695416991b400153910df6717 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 16:54:46 +0100
Subject: [PATCH 17/22] H-5839: Rename checker/ to lsp/ and checker.worker to
 language-server.worker

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../petrinaut/src/{checker => lsp}/context.ts         |  0
 .../src/{checker => lsp}/lib/checker.test.ts          |  0
 .../petrinaut/src/{checker => lsp}/lib/checker.ts     |  0
 .../lib/create-language-service-host.ts               |  0
 .../lib/create-sdcpn-language-service.test.ts         |  0
 .../lib/create-sdcpn-language-service.ts              |  0
 .../src/{checker => lsp}/lib/document-uris.test.ts    |  0
 .../src/{checker => lsp}/lib/document-uris.ts         |  0
 .../petrinaut/src/{checker => lsp}/lib/file-paths.ts  |  0
 .../src/{checker => lsp}/lib/helper/create-sdcpn.ts   |  0
 .../src/{checker => lsp}/lib/position-utils.test.ts   |  0
 .../src/{checker => lsp}/lib/position-utils.ts        |  0
 .../src/{checker => lsp}/lib/ts-to-lsp.test.ts        |  0
 .../petrinaut/src/{checker => lsp}/lib/ts-to-lsp.ts   |  0
 .../petrinaut/src/{checker => lsp}/provider.tsx       |  0
 .../worker/language-server.worker.ts}                 |  2 +-
 .../petrinaut/src/{checker => lsp}/worker/protocol.ts |  2 +-
 .../{checker => lsp}/worker/use-language-client.ts    | 11 +++++++----
 .../petrinaut/src/monaco/completion-sync.tsx          |  2 +-
 .../petrinaut/src/monaco/diagnostics-sync.tsx         |  2 +-
 libs/@hashintel/petrinaut/src/monaco/editor-paths.ts  |  2 +-
 libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx   |  2 +-
 .../petrinaut/src/monaco/signature-help-sync.tsx      |  2 +-
 libs/@hashintel/petrinaut/src/petrinaut.tsx           |  2 +-
 .../views/Editor/components/BottomBar/bottom-bar.tsx  |  2 +-
 .../components/BottomBar/diagnostics-indicator.tsx    |  2 +-
 .../src/views/Editor/subviews/diagnostics.tsx         |  2 +-
 27 files changed, 18 insertions(+), 15 deletions(-)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/context.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/checker.test.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/checker.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/create-language-service-host.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/create-sdcpn-language-service.test.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/create-sdcpn-language-service.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/document-uris.test.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/document-uris.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/file-paths.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/helper/create-sdcpn.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/position-utils.test.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/position-utils.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/ts-to-lsp.test.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/lib/ts-to-lsp.ts (100%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/provider.tsx (100%)
 rename libs/@hashintel/petrinaut/src/{checker/worker/checker.worker.ts => lsp/worker/language-server.worker.ts} (99%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/worker/protocol.ts (97%)
 rename libs/@hashintel/petrinaut/src/{checker => lsp}/worker/use-language-client.ts (96%)

diff --git a/libs/@hashintel/petrinaut/src/checker/context.ts b/libs/@hashintel/petrinaut/src/lsp/context.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/context.ts
rename to libs/@hashintel/petrinaut/src/lsp/context.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/checker.test.ts b/libs/@hashintel/petrinaut/src/lsp/lib/checker.test.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/checker.test.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/checker.test.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/checker.ts b/libs/@hashintel/petrinaut/src/lsp/lib/checker.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/checker.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/checker.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts b/libs/@hashintel/petrinaut/src/lsp/lib/create-language-service-host.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/create-language-service-host.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/create-language-service-host.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.test.ts b/libs/@hashintel/petrinaut/src/lsp/lib/create-sdcpn-language-service.test.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.test.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/create-sdcpn-language-service.test.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts b/libs/@hashintel/petrinaut/src/lsp/lib/create-sdcpn-language-service.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/create-sdcpn-language-service.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/create-sdcpn-language-service.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/document-uris.test.ts b/libs/@hashintel/petrinaut/src/lsp/lib/document-uris.test.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/document-uris.test.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/document-uris.test.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/document-uris.ts b/libs/@hashintel/petrinaut/src/lsp/lib/document-uris.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/document-uris.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/document-uris.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/file-paths.ts b/libs/@hashintel/petrinaut/src/lsp/lib/file-paths.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/file-paths.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/file-paths.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/helper/create-sdcpn.ts b/libs/@hashintel/petrinaut/src/lsp/lib/helper/create-sdcpn.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/helper/create-sdcpn.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/helper/create-sdcpn.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/position-utils.test.ts b/libs/@hashintel/petrinaut/src/lsp/lib/position-utils.test.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/position-utils.test.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/position-utils.test.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/position-utils.ts b/libs/@hashintel/petrinaut/src/lsp/lib/position-utils.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/position-utils.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/position-utils.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.test.ts b/libs/@hashintel/petrinaut/src/lsp/lib/ts-to-lsp.test.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.test.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/ts-to-lsp.test.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.ts b/libs/@hashintel/petrinaut/src/lsp/lib/ts-to-lsp.ts
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/lib/ts-to-lsp.ts
rename to libs/@hashintel/petrinaut/src/lsp/lib/ts-to-lsp.ts
diff --git a/libs/@hashintel/petrinaut/src/checker/provider.tsx b/libs/@hashintel/petrinaut/src/lsp/provider.tsx
similarity index 100%
rename from libs/@hashintel/petrinaut/src/checker/provider.tsx
rename to libs/@hashintel/petrinaut/src/lsp/provider.tsx
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts b/libs/@hashintel/petrinaut/src/lsp/worker/language-server.worker.ts
similarity index 99%
rename from libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
rename to libs/@hashintel/petrinaut/src/lsp/worker/language-server.worker.ts
index ab3357d29f0..1400c32b5e2 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/checker.worker.ts
+++ b/libs/@hashintel/petrinaut/src/lsp/worker/language-server.worker.ts
@@ -1,6 +1,6 @@
 /* eslint-disable no-restricted-globals */
 /**
- * Checker WebWorker — runs TypeScript validation off the main thread.
+ * Language Server WebWorker — runs TypeScript validation off the main thread.
  *
  * Implements an LSP-inspired protocol over JSON-RPC 2.0:
  * - Notifications: `initialize`, `sdcpn/didChange`, `textDocument/didChange`
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts b/libs/@hashintel/petrinaut/src/lsp/worker/protocol.ts
similarity index 97%
rename from libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
rename to libs/@hashintel/petrinaut/src/lsp/worker/protocol.ts
index a56971ac090..685dd39c64a 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/protocol.ts
+++ b/libs/@hashintel/petrinaut/src/lsp/worker/protocol.ts
@@ -1,5 +1,5 @@
 /**
- * LSP-inspired protocol types for the checker WebWorker.
+ * LSP-inspired protocol types for the language server WebWorker.
  *
  * Uses JSON-RPC 2.0 with standard LSP types from `vscode-languageserver-types`.
  * All diagnostic, completion, hover, and signature help types are the official
diff --git a/libs/@hashintel/petrinaut/src/checker/worker/use-language-client.ts b/libs/@hashintel/petrinaut/src/lsp/worker/use-language-client.ts
similarity index 96%
rename from libs/@hashintel/petrinaut/src/checker/worker/use-language-client.ts
rename to libs/@hashintel/petrinaut/src/lsp/worker/use-language-client.ts
index a5522658ca4..b08b9aad72e 100644
--- a/libs/@hashintel/petrinaut/src/checker/worker/use-language-client.ts
+++ b/libs/@hashintel/petrinaut/src/lsp/worker/use-language-client.ts
@@ -44,7 +44,7 @@ export type LanguageClientApi = {
 };
 
 /**
- * Spawn a checker WebWorker and return an LSP-inspired API to interact with it.
+ * Spawn the language server WebWorker and return an LSP-inspired API to interact with it.
  * The worker is created on mount and terminated on unmount.
  */
 export function useLanguageClient(): LanguageClientApi {
@@ -56,9 +56,12 @@ export function useLanguageClient(): LanguageClientApi {
   >(null);
 
   useEffect(() => {
-    const worker = new Worker(new URL("./checker.worker.ts", import.meta.url), {
-      type: "module",
-    });
+    const worker = new Worker(
+      new URL("./language-server.worker.ts", import.meta.url),
+      {
+        type: "module",
+      },
+    );
 
     worker.onmessage = (event: MessageEvent<ServerMessage>) => {
       const msg = event.data;
diff --git a/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
index 546ca7f09a9..2359c3f8b1c 100644
--- a/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
@@ -3,7 +3,7 @@ import { Suspense, use, useEffect } from "react";
 import type { CompletionItem } from "vscode-languageserver-types";
 import { CompletionItemKind, Position } from "vscode-languageserver-types";
 
-import { LanguageClientContext } from "../checker/context";
+import { LanguageClientContext } from "../lsp/context";
 import { MonacoContext } from "./context";
 
 /**
diff --git a/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
index c08a65433c8..027e4c1744f 100644
--- a/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/diagnostics-sync.tsx
@@ -3,7 +3,7 @@ import { Suspense, use, useEffect, useRef } from "react";
 import type { Diagnostic } from "vscode-languageserver-types";
 import { DiagnosticSeverity } from "vscode-languageserver-types";
 
-import { LanguageClientContext } from "../checker/context";
+import { LanguageClientContext } from "../lsp/context";
 import { MonacoContext } from "./context";
 
 const OWNER = "checker";
diff --git a/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts b/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
index 305df6aa2e8..63e0b18f495 100644
--- a/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
+++ b/libs/@hashintel/petrinaut/src/monaco/editor-paths.ts
@@ -2,4 +2,4 @@
  * Re-exports from the centralized document-uris module.
  * Monaco components import from here for convenience.
  */
-export { getDocumentUri, parseDocumentUri } from "../checker/lib/document-uris";
+export { getDocumentUri, parseDocumentUri } from "../lsp/lib/document-uris";
diff --git a/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
index 0cb92f7e1a5..cd87813ee63 100644
--- a/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
@@ -3,7 +3,7 @@ import { Suspense, use, useEffect } from "react";
 import type { Hover } from "vscode-languageserver-types";
 import { MarkupKind, Position } from "vscode-languageserver-types";
 
-import { LanguageClientContext } from "../checker/context";
+import { LanguageClientContext } from "../lsp/context";
 import { MonacoContext } from "./context";
 
 /** Extract display string from LSP Hover contents. */
diff --git a/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
index 582b5cb03c2..a1e178ad7c3 100644
--- a/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
@@ -6,7 +6,7 @@ import {
   type SignatureHelp,
 } from "vscode-languageserver-types";
 
-import { LanguageClientContext } from "../checker/context";
+import { LanguageClientContext } from "../lsp/context";
 import { MonacoContext } from "./context";
 
 /** Extract documentation string from LSP MarkupContent or plain string. */
diff --git a/libs/@hashintel/petrinaut/src/petrinaut.tsx b/libs/@hashintel/petrinaut/src/petrinaut.tsx
index 8b06fa79f52..0cdeb052c82 100644
--- a/libs/@hashintel/petrinaut/src/petrinaut.tsx
+++ b/libs/@hashintel/petrinaut/src/petrinaut.tsx
@@ -1,7 +1,7 @@
 import "reactflow/dist/style.css";
 import "./index.css";
 
-import { LanguageClientProvider } from "./checker/provider";
+import { LanguageClientProvider } from "./lsp/provider";
 import type {
   Color,
   DifferentialEquation,
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx
index a92fca2955a..fa8fac7026c 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/bottom-bar.tsx
@@ -3,7 +3,7 @@ import { refractive } from "@hashintel/refractive";
 import { use, useCallback, useEffect } from "react";
 import { FaChevronDown, FaChevronUp } from "react-icons/fa6";
 
-import { LanguageClientContext } from "../../../../checker/context";
+import { LanguageClientContext } from "../../../../lsp/context";
 import {
   EditorContext,
   type EditorState,
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx
index 7bca84a9515..5102623505b 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/components/BottomBar/diagnostics-indicator.tsx
@@ -2,7 +2,7 @@ import { css, cva } from "@hashintel/ds-helpers/css";
 import { use } from "react";
 import { FaCheck, FaXmark } from "react-icons/fa6";
 
-import { LanguageClientContext } from "../../../../checker/context";
+import { LanguageClientContext } from "../../../../lsp/context";
 import { ToolbarButton } from "./toolbar-button";
 
 const iconContainerStyle = cva({
diff --git a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
index 6086cf61ff3..58c3725a3a2 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
@@ -3,7 +3,7 @@ import { use, useCallback, useMemo, useState } from "react";
 import { FaChevronDown, FaChevronRight } from "react-icons/fa6";
 import type { Diagnostic } from "vscode-languageserver-types";
 
-import { LanguageClientContext } from "../../../checker/context";
+import { LanguageClientContext } from "../../../lsp/context";
 import type { SubView } from "../../../components/sub-view/types";
 import { parseDocumentUri } from "../../../monaco/editor-paths";
 import { EditorContext } from "../../../state/editor-context";

From 65446c0a3914ffe3f2093137e16229373a4e8491 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 17:37:48 +0100
Subject: [PATCH 18/22] H-5839: Fix position mapping to use user content
 instead of prefix+content

The worker was converting Monaco positions against the full file content
(prefix + user code), but Monaco positions are relative to the visible
user code only. This caused double-counting of the prefix offset since
SDCPNLanguageServer methods already add prefixLength internally.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../lsp/lib/create-sdcpn-language-service.ts  |  5 +++
 .../src/lsp/worker/language-server.worker.ts  | 31 +++++++++++++------
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/libs/@hashintel/petrinaut/src/lsp/lib/create-sdcpn-language-service.ts b/libs/@hashintel/petrinaut/src/lsp/lib/create-sdcpn-language-service.ts
index 778c54bae76..7ad9a30b46a 100644
--- a/libs/@hashintel/petrinaut/src/lsp/lib/create-sdcpn-language-service.ts
+++ b/libs/@hashintel/petrinaut/src/lsp/lib/create-sdcpn-language-service.ts
@@ -303,6 +303,11 @@ export class SDCPNLanguageServer {
     return (file.prefix ?? "") + file.content;
   }
 
+  /** Get only the user-visible content of a virtual file (without the injected prefix). */
+  getUserContent(fileName: string): string | undefined {
+    return this.controller.getFile(fileName)?.content;
+  }
+
   getSemanticDiagnostics(fileName: string): ts.Diagnostic[] {
     const entry = this.controller.getFile(fileName);
     const prefixLength = entry?.prefix?.length ?? 0;
diff --git a/libs/@hashintel/petrinaut/src/lsp/worker/language-server.worker.ts b/libs/@hashintel/petrinaut/src/lsp/worker/language-server.worker.ts
index 1400c32b5e2..abb584c388f 100644
--- a/libs/@hashintel/petrinaut/src/lsp/worker/language-server.worker.ts
+++ b/libs/@hashintel/petrinaut/src/lsp/worker/language-server.worker.ts
@@ -64,11 +64,13 @@ function publishAllDiagnostics(sdcpn: SDCPN): void {
   const params: PublishDiagnosticsParams[] = result.itemDiagnostics.map(
     (item) => {
       const uri = filePathToUri(item.filePath);
-      const fileContent = server!.getFileContent(item.filePath) ?? "";
+      // Use user content (without prefix) because diagnostic offsets have
+      // already been adjusted to be relative to user content by adjustDiagnostics.
+      const userContent = server!.getUserContent(item.filePath) ?? "";
       return {
         uri: uri ?? item.filePath,
         diagnostics: item.diagnostics.map((diag) =>
-          serializeDiagnostic(diag, fileContent),
+          serializeDiagnostic(diag, userContent),
         ),
       };
     },
@@ -147,8 +149,11 @@ self.onmessage = ({ data }: MessageEvent<ClientMessage>) => {
           break;
         }
 
-        const fileContent = server.getFileContent(filePath) ?? "";
-        const offset = positionToOffset(fileContent, data.params.position);
+        // Use user content (without prefix) for position conversion since
+        // Monaco positions are relative to the visible user code only.
+        // SDCPNLanguageServer methods handle the prefix offset internally.
+        const userContent = server.getUserContent(filePath) ?? "";
+        const offset = positionToOffset(userContent, data.params.position);
 
         const completions = server.getCompletionsAtPosition(
           filePath,
@@ -182,11 +187,15 @@ self.onmessage = ({ data }: MessageEvent<ClientMessage>) => {
           break;
         }
 
-        const fileContent = server.getFileContent(filePath) ?? "";
-        const offset = positionToOffset(fileContent, data.params.position);
+        // Use user content (without prefix) for position conversion since
+        // Monaco positions are relative to the visible user code only.
+        const userContent = server.getUserContent(filePath) ?? "";
+        const offset = positionToOffset(userContent, data.params.position);
 
         const info = server.getQuickInfoAtPosition(filePath, offset);
 
+        // textSpan offsets from getQuickInfoAtPosition are already
+        // adjusted to be relative to user content (prefix subtracted).
         const result: Hover | null = info
           ? {
               contents: {
@@ -199,9 +208,9 @@ self.onmessage = ({ data }: MessageEvent<ClientMessage>) => {
                   .join("\n\n"),
               },
               range: Range.create(
-                offsetToPosition(fileContent, info.textSpan.start),
+                offsetToPosition(userContent, info.textSpan.start),
                 offsetToPosition(
-                  fileContent,
+                  userContent,
                   info.textSpan.start + info.textSpan.length,
                 ),
               ),
@@ -225,8 +234,10 @@ self.onmessage = ({ data }: MessageEvent<ClientMessage>) => {
           break;
         }
 
-        const fileContent = server.getFileContent(filePath) ?? "";
-        const offset = positionToOffset(fileContent, data.params.position);
+        // Use user content (without prefix) for position conversion since
+        // Monaco positions are relative to the visible user code only.
+        const userContent = server.getUserContent(filePath) ?? "";
+        const offset = positionToOffset(userContent, data.params.position);
 
         const help = server.getSignatureHelpItems(filePath, offset, undefined);
 

From 088e22ba25f90e150d6dd84d0d86821beb22eb8d Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 17:38:33 +0100
Subject: [PATCH 19/22] H-5839: Reset LanguageClientProvider when petriNetId
 changes

Add key={petriNetId} so the language client remounts with fresh state
when switching between Petri nets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 libs/@hashintel/petrinaut/src/petrinaut.tsx | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libs/@hashintel/petrinaut/src/petrinaut.tsx b/libs/@hashintel/petrinaut/src/petrinaut.tsx
index 0cdeb052c82..e78a7e04acf 100644
--- a/libs/@hashintel/petrinaut/src/petrinaut.tsx
+++ b/libs/@hashintel/petrinaut/src/petrinaut.tsx
@@ -1,7 +1,6 @@
 import "reactflow/dist/style.css";
 import "./index.css";
 
-import { LanguageClientProvider } from "./lsp/provider";
 import type {
   Color,
   DifferentialEquation,
@@ -12,6 +11,7 @@ import type {
   SDCPN,
   Transition,
 } from "./core/types/sdcpn";
+import { LanguageClientProvider } from "./lsp/provider";
 import { MonacoProvider } from "./monaco/provider";
 import { NotificationsProvider } from "./notifications/notifications-provider";
 import { PlaybackProvider } from "./playback/provider";
@@ -98,7 +98,7 @@ export const Petrinaut = ({
   return (
     <NotificationsProvider>
       <SDCPNProvider {...rest}>
-        <LanguageClientProvider>
+        <LanguageClientProvider key={rest.petriNetId}>
           <MonacoProvider>
             <SimulationProvider>
               <PlaybackProvider>

From 8c61cd07c7eee61a91d1127d051b46d2f29ce3c4 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 17:42:34 +0100
Subject: [PATCH 20/22] Lint

---
 .../petrinaut/src/views/Editor/subviews/diagnostics.tsx         | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
index 58c3725a3a2..1d96deb9252 100644
--- a/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
+++ b/libs/@hashintel/petrinaut/src/views/Editor/subviews/diagnostics.tsx
@@ -3,8 +3,8 @@ import { use, useCallback, useMemo, useState } from "react";
 import { FaChevronDown, FaChevronRight } from "react-icons/fa6";
 import type { Diagnostic } from "vscode-languageserver-types";
 
-import { LanguageClientContext } from "../../../lsp/context";
 import type { SubView } from "../../../components/sub-view/types";
+import { LanguageClientContext } from "../../../lsp/context";
 import { parseDocumentUri } from "../../../monaco/editor-paths";
 import { EditorContext } from "../../../state/editor-context";
 import { SDCPNContext } from "../../../state/sdcpn-context";

From fdc56a845c7f6cf93b3d24f921b67d382d15f983 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Thu, 26 Feb 2026 23:12:32 +0100
Subject: [PATCH 21/22] H-5839: Sync document content to worker before LSP
 requests

Monaco providers now call notifyDocumentChanged before each
completion/hover/signatureHelp request so the worker always has
up-to-date content when converting positions. Without this, typing
a trigger character (e.g. ".") could race the SDCPN state sync,
causing the worker to compute offsets against stale text.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../petrinaut/src/monaco/completion-sync.tsx        | 13 +++++++++++--
 libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx |  6 ++++--
 .../petrinaut/src/monaco/signature-help-sync.tsx    |  8 ++++++--
 3 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
index 2359c3f8b1c..b71da0d9deb 100644
--- a/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/completion-sync.tsx
@@ -61,7 +61,9 @@ function toMonacoCompletion(
 
 const CompletionSyncInner = () => {
   const { monaco } = use(use(MonacoContext));
-  const { requestCompletion } = use(LanguageClientContext);
+  const { notifyDocumentChanged, requestCompletion } = use(
+    LanguageClientContext,
+  );
 
   useEffect(() => {
     const disposable = monaco.languages.registerCompletionItemProvider(
@@ -71,6 +73,13 @@ const CompletionSyncInner = () => {
 
         async provideCompletionItems(model, monacoPosition) {
           const uri = model.uri.toString();
+          // TODO(FE-497): Sync current content to the worker before requesting
+          // completions. When a trigger character (e.g. ".") is typed, the
+          // Monaco model already has the new text but the worker may still have
+          // stale content (the SDCPN state sync goes through a React render
+          // cycle). Since the web worker processes messages in order, this
+          // ensures the content update is applied before the completion request.
+          notifyDocumentChanged(uri, model.getValue());
           // Convert Monaco 1-based position to LSP 0-based Position
           const position = Position.create(
             monacoPosition.lineNumber - 1,
@@ -96,7 +105,7 @@ const CompletionSyncInner = () => {
     );
 
     return () => disposable.dispose();
-  }, [monaco, requestCompletion]);
+  }, [monaco, notifyDocumentChanged, requestCompletion]);
 
   return null;
 };
diff --git a/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
index cd87813ee63..5fd454b65a1 100644
--- a/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/hover-sync.tsx
@@ -49,12 +49,14 @@ function hoverContentsToMarkdown(hover: Hover): Monaco.IMarkdownString[] {
 
 const HoverSyncInner = () => {
   const { monaco } = use(use(MonacoContext));
-  const { requestHover } = use(LanguageClientContext);
+  const { notifyDocumentChanged, requestHover } = use(LanguageClientContext);
 
   useEffect(() => {
     const disposable = monaco.languages.registerHoverProvider("typescript", {
       async provideHover(model, monacoPosition) {
         const uri = model.uri.toString();
+        // TODO(FE-497): Sync current content to ensure the worker has the latest text.
+        notifyDocumentChanged(uri, model.getValue());
         // Convert Monaco 1-based position to LSP 0-based Position
         const position = Position.create(
           monacoPosition.lineNumber - 1,
@@ -83,7 +85,7 @@ const HoverSyncInner = () => {
     });
 
     return () => disposable.dispose();
-  }, [monaco, requestHover]);
+  }, [monaco, notifyDocumentChanged, requestHover]);
 
   return null;
 };
diff --git a/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
index a1e178ad7c3..ad151ed3abc 100644
--- a/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
+++ b/libs/@hashintel/petrinaut/src/monaco/signature-help-sync.tsx
@@ -44,7 +44,9 @@ function toMonacoSignatureHelp(
 
 const SignatureHelpSyncInner = () => {
   const { monaco } = use(use(MonacoContext));
-  const { requestSignatureHelp } = use(LanguageClientContext);
+  const { notifyDocumentChanged, requestSignatureHelp } = use(
+    LanguageClientContext,
+  );
 
   useEffect(() => {
     const disposable = monaco.languages.registerSignatureHelpProvider(
@@ -55,6 +57,8 @@ const SignatureHelpSyncInner = () => {
 
         async provideSignatureHelp(model, monacoPosition) {
           const uri = model.uri.toString();
+          // TODO(FE-497): Sync current content to ensure the worker has the latest text.
+          notifyDocumentChanged(uri, model.getValue());
           // Convert Monaco 1-based position to LSP 0-based Position
           const position = Position.create(
             monacoPosition.lineNumber - 1,
@@ -75,7 +79,7 @@ const SignatureHelpSyncInner = () => {
     );
 
     return () => disposable.dispose();
-  }, [monaco, requestSignatureHelp]);
+  }, [monaco, notifyDocumentChanged, requestSignatureHelp]);
 
   return null;
 };

From d86a78889e266c0ab2113f7bb04b75d4b0b323a1 Mon Sep 17 00:00:00 2001
From: Chris Feijoo <chris@kube.io>
Date: Fri, 27 Feb 2026 00:25:35 +0100
Subject: [PATCH 22/22] H-5839: Clamp `positionToOffset` to `text.length` for
 symmetry with `offsetToPosition`

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 libs/@hashintel/petrinaut/src/lsp/lib/position-utils.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libs/@hashintel/petrinaut/src/lsp/lib/position-utils.ts b/libs/@hashintel/petrinaut/src/lsp/lib/position-utils.ts
index 27b5097cd23..dc7a5fa31f2 100644
--- a/libs/@hashintel/petrinaut/src/lsp/lib/position-utils.ts
+++ b/libs/@hashintel/petrinaut/src/lsp/lib/position-utils.ts
@@ -34,5 +34,5 @@ export function positionToOffset(text: string, position: Position): number {
     i++;
   }
 
-  return i + position.character;
+  return Math.min(i + position.character, text.length);
 }