[Bugfix] fix(reasoning): route streaming deltas as content when prompt_is_reasoning_end and no tool parser

[MMostafa-Hub]

Summary

Fixes #40816 — Qwen3 streaming with enable_thinking=False returns all tokens in delta.reasoning instead of delta.content.

Root cause

DelegatingParser._in_reasoning_phase had a bug when only a reasoning parser is configured (no tool parser). The old code:

def _in_reasoning_phase(self, state: StreamState) -> bool:
    if self._reasoning_parser is None:
        return False
    if self._tool_parser is None:
        return True   # ← always True, ignores state.reasoning_ended
    return not state.reasoning_ended

When self._tool_parser is None the method unconditionally returned True, completely ignoring state.reasoning_ended. This means that even after the serving layer called prompt_is_reasoning_end (which sets state.reasoning_ended = True when </think> is present in the prompt — i.e. when enable_thinking=False), all subsequent output tokens were still sent to extract_reasoning_streaming and returned as DeltaMessage(reasoning=...).

Fix

Remove the dead if self._tool_parser is None: return True branch — it was never correct:

def _in_reasoning_phase(self, state: StreamState) -> bool:
    if self._reasoning_parser is None:
        return False
    return not state.reasoning_ended

Also add an early-exit branch in parse_delta: when reasoning has already ended and there is no tool parser, emit the delta directly as DeltaMessage(content=...) without going through the reasoning parser at all.

Test

Added test_prompt_is_reasoning_end_routes_to_content in tests/reasoning/test_qwen3_reasoning_parser.py — a regression test that:

1. Builds a _WrappedParser with only a qwen3 reasoning parser (no tool parser).
2. Passes prompt_token_ids containing end_token_id on the first parse_delta call (simulating the serving layer's prompt_is_reasoning_end path).
3. Asserts that all subsequent deltas land in delta.content, not delta.reasoning.

Regression introduced by

PR #39446 (April 14 2026) migrated chat-completion streaming to the unified DelegatingParser.parse_delta path. The if self._tool_parser is None: return True shortcut was written as a micro-optimisation for the common case where a tool parser is absent, but it broke the state.reasoning_ended contract.

Known workarounds (before this fix)

Two workarounds exist that avoid the broken _in_reasoning_phase branch:

1. Remove --reasoning-parser from the vllm serve command entirely — no reasoning parser means _in_reasoning_phase returns False immediately and tokens flow to content.
2. Add --tool-call-parser + --enable-auto-tool-choice — with a tool parser present, the if self._tool_parser is None shortcut is never taken and state.reasoning_ended is correctly consulted.

Neither is a satisfying fix for users who want reasoning parsing enabled but thinking disabled per-request.

---

Note

A note to reviewers: I encountered this bug while deploying Qwen3.6 with enable_thinking=False and observed that streaming responses returned an empty content field with all tokens landing in reasoning. The root-cause analysis and the fix were developed with AI assistance (Claude Sonnet 4.6 by Anthropic). I don't have deep knowledge of vLLM internals, so please review the fix carefully and let me know if the approach is sound or if there's a better way to handle this. Happy to iterate based on feedback.

Signed-off-by: Mohamed Mostafa moh.mostafa.ibra@gmail.com

[claude]

Claude Code Review

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[gemini-code-assist]

Code Review

This pull request addresses a bug in Qwen3 streaming where final answers were incorrectly categorized as reasoning when thinking was disabled. The changes modify vllm/parser/abstract_parser.py to ensure that if reasoning has ended and no tool parser is active, subsequent deltas are routed directly as content. Additionally, a regression test has been added to verify this behavior. I have no feedback to provide.

[sfeng33]

Closing as fixed in #40820