diff --git a/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java b/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java
index 66cc304d6..2a5e42a1d 100644
--- a/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java
+++ b/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java
@@ -160,7 +160,7 @@ public StdioMcpSessionTransport() {
 		@Override
 		public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
 
-			return Mono.zip(inboundReady.asMono(), outboundReady.asMono()).then(Mono.defer(() -> {
+			return Mono.when(inboundReady.asMono(), outboundReady.asMono()).then(Mono.defer(() -> {
 				if (outboundSink.tryEmitNext(message).isSuccess()) {
 					return Mono.empty();
 				}