Fix Qwen3 streaming content routing

[xy3xy3]

Purpose

Fix a Qwen3 streaming routing bug in the OpenAI-compatible /v1/chat/completions
endpoint when --reasoning-parser qwen3 is enabled and
chat_template_kwargs.enable_thinking=false.

This PR is related to #40816.

Before this change:

• Non-streaming requests correctly returned the answer in message.content
• Streaming requests could incorrectly emit the answer in
  choices[0].delta.reasoning
• OpenAI-compatible streaming clients that only read delta.content would miss
  the final answer

Root cause:

• The streaming path relied on res.prompt_token_ids to determine whether the
  prompt had already ended the reasoning block
• For Qwen3 with enable_thinking=false, the rendered prompt already contains
  the empty reasoning terminator
• Some streaming RequestOutput chunks do not carry prompt_token_ids, so the
  answer tokens could be misrouted into delta.reasoning

Fix:

• Capture rendered prompt_token_ids before streaming starts
• Pass them into chat_completion_stream_generator
• Initialize prompt_is_reasoning_end_arr from those prompt tokens up front
• Only fall back to res.prompt_token_ids when needed

This makes streaming behavior consistent with non-streaming behavior for
Qwen3/Qwen3.5 requests with thinking disabled.

Test Plan

Code-level regression coverage:

pytest -q tests/entrypoints/openai/chat_completion/test_thinking_token_budget.py \
  -k streaming_with_thinking_disabled_stays_in_content

Manual validation against a running container:

curl -sS http://localhost:8000/v1/chat/completions \
  -H 'Content-Type: application/json' \
  -d '{
    "model":"qwen3.6-35b-nvfp4",
    "messages":[{"role":"user","content":"Which is larger, 4 or 12? Output exactly one token: 4 or 12."}],
    "temperature":0.1,
    "max_tokens":16,
    "chat_template_kwargs":{"enable_thinking":false}
  }'

curl -N -sS http://localhost:8000/v1/chat/completions \
  -H 'Content-Type: application/json' \
  -d '{
    "model":"qwen3.6-35b-nvfp4",
    "messages":[{"role":"user","content":"Which is larger, 4 or 12? Output exactly one token: 4 or 12."}],
    "stream":true,
    "temperature":0.1,
    "max_tokens":16,
    "chat_template_kwargs":{"enable_thinking":false},
    "stream_options":{"include_usage":true}
  }'

Test Result

Environment used for manual verification:

• Model: qwen3.6-35b-nvfp4
• Server args include:
  • --reasoning-parser qwen3
  • --default-chat-template-kwargs '{"enable_thinking": false}'

Before fix:

• Non-streaming response returned message.content: "12"
• Streaming response emitted:
  • delta.reasoning: "1"
  • delta.reasoning: "2"
  • no usable delta.content

After fix:

• Non-streaming response returns:

{
  "choices": [
    {
      "message": {
        "content": "12",
        "reasoning": null
      }
    }
  ]
}

• Streaming response emits:

data: {"choices":[{"delta":{"role":"assistant","content":""},"finish_reason":null}]}
data: {"choices":[{"delta":{"content":"1"},"finish_reason":null}]}
data: {"choices":[{"delta":{"content":"2"},"finish_reason":null}]}
data: {"choices":[{"delta":{},"finish_reason":"stop"}]}
data: [DONE]

Observed result:

• Answer tokens now stay in delta.content
• No delta.reasoning is emitted for the disabled-thinking request

Documentation update:

• No documentation update required for this bug fix

---

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[claude]

Claude Code Review

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[gemini-code-assist]

Code Review

This pull request modifies the OpenAI serving entrypoint to ensure that streaming outputs are correctly routed to the content field when thinking is disabled. By passing prompt token IDs to the stream generator, the system can detect if the reasoning phase has already concluded within the prompt. A new test case verifies this behavior for both standard and streaming completions. A review comment identifies that the current implementation for capturing prompt token IDs is fragile because it overwrites the variable within a loop, which would only correctly handle the last prompt in a multi-prompt scenario.

[gemini-code-assist]

The variable stream_prompt_token_ids is assigned inside a loop over engine_inputs. If engine_inputs contains multiple prompts, stream_prompt_token_ids will only capture the token IDs of the last prompt. While the current streaming implementation asserts that there is only one generator (and thus one prompt), this logic is fragile and could lead to incorrect reasoning routing if multi-prompt streaming is supported in the future. Consider capturing the token IDs more robustly or explicitly handling the single-prompt assumption.

[sfeng33]

Thank you for the work, I moved the fix to abstract_parser to keep serving logic lean, and added some more tests.

[mergify]

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