common/parser: add proper reasoning tag prefill reading

[pwilkin]

This changes the erroneous behavior of the autoparser that ascribed thinking behavior to templates. As people rightly mentioned, some models have dynamic or hybrid reasoning - they can reason or not depending on some switches and even the template behavior can change due to this (i.e. inserting <think> in assistant prefill after a "no_think" appears in a user message).

Therefore, the FORCED_OPEN and FORCED_CLOSED formats are gone. The parser will now just detect models with tagged reasoning, i.e. an opening and closing reasoning marker (deleted DELIMITER also since it's a special case with the opening marker being empty). However, it will check the assistant prefill for those markers and will append them to the input for the grammar and the parser, so that they are taken into account, therefore just simplifying the parsing mechanism since it doesn't now have to differentiate whether the <think>' / ` was added by the template or generated by the model.

[pwilkin]

Fixes #20356
Fixes #20325
Fixes #20265

This also clears the ground for disabling grammar triggers inside reasoning loops in a subsequent PR, which would resolve #20260

[aldehir]

Dumb question, why not find the start of the assistant message and prepend that?

I agree it would be easier to parse if we had a "prefill" of some sort that normalizes the input, such that we can handle the logic in the grammar and not through flags. However, if we're going this route I would look into prepending the start of the entire assistant message. This will also open the door for parsing output from requests with an assistant prefill.

[pwilkin]

Yeah, that would be the logical conclusion, but for now it's easier for me just to extract the reasoning markers since finding the actual start of the assistant message is nontrivial.

[aldehir]

Qwen3.5 uses

<think>\n\n</think>\n\n

{%- if enable_thinking is defined and enable_thinking is false %}
{{- '<think>\n\n</think>\n\n' }}
{%- else %}
{{- '<think>\n' }}
{%- endif %}

however,

      "reasoning_prefill": "<think></think>\n\n",

It probably doesn't matter for this model, but it is technically not adhering to the template.

[aldehir]

    {
      "id": 248045,
      "piece": "<|im_start|>"
    },
    {
      "id": 74455,
      "piece": "assistant"
    },
    {
      "id": 198,
      "piece": "\n"
    },
    {
      "id": 248068,
      "piece": "<think>"
    },
    {
      "id": 271,
      "piece": "\n\n"
    },
    {
      "id": 248069,
      "piece": "</think>"
    },
    {
      "id": 271,
      "piece": "\n\n"
    }

Maybe set reasoning_prefill from the start of the opening tag to the end of the prompt?

[aldehir]

finding the actual start of the assistant message is nontrivial.

Run the template once with add_generation_prompt = false, capture the size, run again with true, extract the string content that spans the delta? I think that would work in most cases.

[pwilkin]

That usually works, yeah 😀 I can try that and see what the results are (this is what calculate_diff_split from the analyzer does BTW). I'm just worried about some weird edge cases.

[bsdice]

Nice patch! With model https://huggingface.co/mradermacher/Qwen3.5-40B-Claude-4.5-Opus-High-Reasoning-Thinking-GGUF the patches fix webui getting confused on /think and not splitting correctly reasoning and generation part. Build llama.cpp-cuda-git-b8334.r9.710878a7dd-1.

[pwilkin]

@aldehir changed the prefill extraction behavior to the differential one you mentioned.

[aldehir]

Think we name this generation_prompt? It lines up with the add_generation_prompt flag.