Fix Qwen3 reasoning tool calls embedded inside think

[ZenoAFfectionate]

Summary

This PR fixes a Qwen3/Qwen3.5 non-streaming compatibility issue when using:

• --reasoning-parser qwen3
• --tool-call-parser qwen3_coder

Qwen models can emit XML tool calls inside <think> ... </think>. The current
non-streaming pipeline extracts reasoning first and only parses tool calls from
content, so valid XML tool calls embedded in reasoning are lost.

This patch updates qwen3_reasoning_parser to promote valid XML tool-call
blocks out of reasoning into content, allowing the existing qwen3_coder
tool parser to recover them without changing the generic serving stack.

Why this scope

This PR fixes parser recovery, not model generation behavior. It does not try to
prevent Qwen3.5 from emitting tool calls inside <think>; it makes vLLM robust
when that output pattern appears.

Tests

Added tests cover:

• unchanged behavior for normal reasoning extraction
• embedded tool call promotion from reasoning to content
• successful parsing by qwen3_coder
• truncated reasoning recovery without </think>
• preservation of post-</think> content

Limitation

This change fixes the non-streaming path. Streaming recovery would require
additional serving-layer changes and is intentionally left out of this minimal
patch.

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[mergify]

Documentation preview: https://vllm--39055.org.readthedocs.build/en/39055/

[gemini-code-assist]

Code Review

This pull request introduces a mechanism to recover Qwen3 XML tool calls that are emitted inside the <think> reasoning block by promoting them to the message content field. The review feedback correctly identifies a bug where prepending these tool calls to the content causes the Qwen3CoderToolParser to discard any existing response text; it is recommended to append them instead. Additionally, the test suite should be updated to verify that trailing text is preserved in the final API response after tool call extraction.

[gemini-code-assist]

Prepending promoted tool calls to the content field causes the Qwen3CoderToolParser to discard any existing text in content. This occurs because the tool parser extracts content by taking everything before the first tool call marker (<tool_call> or <function=).

By prepending the promoted tool calls, any original response text now follows a tool call and will be lost in the final API response. Appending the promoted tool calls to the end of content preserves the original text while still allowing the tool parser to find and extract the tool calls correctly.

Suggested change

	content_parts = ["\n\n".join(extracted_blocks)]
	if content:
	content_parts.append(content)
	merged_content = "\n\n".join(part for part in content_parts if part) or None
	content_parts = []
	if content:
	content_parts.append(content)
	content_parts.append("\n\n".join(extracted_blocks))
	merged_content = "\n\n".join(part for part in content_parts if part) or None

[gemini-code-assist]

The test should verify that the response text is preserved after tool call extraction. Currently, it only checks the intermediate content string returned by the reasoning parser, which masks the data loss issue where the downstream tool parser discards text following a tool call.

Suggested change

	tool_call_info = tool_parser.extract_tool_calls(content, request=request)
	assert tool_call_info.tools_called is True
	assert len(tool_call_info.tool_calls) == 1
	tool_call = tool_call_info.tool_calls[0]
	assert tool_call.function.name == "Finish"
	assert json.loads(tool_call.function.arguments) == {"answer": "204"}
	tool_call_info = tool_parser.extract_tool_calls(content, request=request)
	assert tool_call_info.tools_called is True
	assert len(tool_call_info.tool_calls) == 1
	tool_call = tool_call_info.tool_calls[0]
	assert tool_call.function.name == "Finish"
	assert json.loads(tool_call.function.arguments) == {"answer": "204"}
	# Verify that trailing text is preserved in the final extracted content
	assert tool_call_info.content is not None
	assert "assistant trailing text" in tool_call_info.content

[ZenoAFfectionate]

Reviewer summary:

• Repro: vLLM 0.19 + --reasoning-parser qwen3 + --tool-call-parser qwen3_coder, non-streaming path.
• Failure mode: if Qwen3/Qwen3.5 emits XML <tool_call>...</tool_call> inside <think>, the OpenAI-compatible response can contain populated reasoning but empty tool_calls.
• Root cause: qwen3_reasoning_parser moves all pre-</think> text into reasoning, while downstream tool parsing only looks at content.
• Fix in this PR: in non-streaming qwen3_reasoning_parser, promote embedded XML tool-call blocks from reasoning into content so the existing qwen3_coder parser can recover them.
• Scope: minimal, one parser file changed; no generic serving-stack change; non-streaming only.
• Validation: added tests for no-regression on normal reasoning extraction, embedded tool-call promotion, successful parsing by qwen3_coder, truncated reasoning recovery, and preservation of post-</think> content.

Related issue: #39056

[ZenoAFfectionate]

Update: this issue was specifically observed and reproduced on:

• Qwen/Qwen3.5-35B-A3B-FP8

The fix is intended to address the parser interaction for this confirmed model/configuration:

• --reasoning-parser qwen3
• --tool-call-parser qwen3_coder
• non-streaming path

It may also help other Qwen3/Qwen3.5 variants using the same parser combination, but the confirmed reproduction for this PR is Qwen/Qwen3.5-35B-A3B-FP8.

Related issue: #39056

[ZenoAFfectionate]

Maintainers: could someone please apply ready or verified to this PR?

Current CI failure is gate-only (pre-run-check), not code lint/test:

• failing job: 70052668260
• reason: author has < 4 merged PRs and PR lacks ready/verified

Once labeled, the workflow should proceed to pre-commit/tests normally. Thanks!

[epheien]

I have encountered similar issues with both 27b and 397b, but I have always used them in a streaming manner.

Is this fix only for non-streaming output?

[jogoossens]

hitting this problem all the time, very hard to get qwen stable on vllm

[meitalbensinai]

Also happens for me with the new Qwen 3.6 30b

[Sandermage]

@ZenoAFfectionate — thank you for this PR. It was one of the first patches we tried when investigating tool-call corruption on our Qwen3.6 setup, and the ~20% clean-rate improvement it gave us was crucial — that delta confirmed parser-level fixes were on the right track and motivated us to keep investigating the parser layer in parallel with the model layer.

What we tested

Backported the _split_embedded_tool_calls helper + the extract_reasoning integration on a Qwen3.6-35B-A3B-FP8 production rig (2× A5000, vLLM 0.19.2rc1.dev205+g07351e088).

Empirical impact: standalone improvement from ~20% baseline to ~40% clean (n=20)

The improvement is real but smaller than we initially expected, and the reason turned out to be informative. Our setup uses enable_thinking=false in the chat template, which inserts an empty <think></think> block at the start of the prompt — model output usually doesn't contain <think> blocks at all in this mode. Your PR's main payoff (extracting <tool_call> XML from REASONING content) only fires when the model emits <think> itself even with enable_thinking=false — which it does sometimes, hence the ~20% improvement we measured.

For setups that actually use enable_thinking=true (which is many production deployments), the improvement should be substantially larger because the path your PR fixes will fire on every reasoning-bearing response.

Composition with our existing patches

Our existing patch tree already had P12 (mirroring an earlier fix) which handles the </think>-absent case via implicit <tool_call>-as-reasoning-end. Your PR composes cleanly with it — your patch handles the orthogonal case where </think> IS present and <tool_call> is nested inside reasoning. Layering both gives broader coverage than either alone.

Backport reference + credit

Kept in our tree as opt-in research artifact: patch_59_qwen3_reasoning_tool_call_recovery.py (env flag GENESIS_ENABLE_P59_QWEN3_TOOL_RECOVERY=1). Credit to you in the docstring + CREDITS.md.

Thanks again. The PR design (regex-based extraction that prepends to content rather than replacing reasoning) is clean — backporting it took 5 sub-patches but no architectural changes, which is the best kind of PR to backport.

[ExtReMLapin]

Superseded by #40783 IMO

[naroam1]

Hi maintainers — chiming in as a downstream user.

This PR addresses #39056, which is one of the open items blocking our planned migration to Qwen3.5-9B/27B for production agentic workloads (--reasoning-parser qwen3 + --tool-call-parser combos with thinking enabled). Without this fix, agent loops randomly fail because XML <tool_call> blocks emitted inside <think> are lost during reasoning extraction.

Per the authors comment from Apr 6, the only blocker is the ready / verified label gate (author has <4 merged PRs, so CI cannot run). The actual diff is small (3 files, +251 −2) and well-scoped to the non-streaming path with explicit limitation called out for streaming (covered separately by #40783).

Could a maintainer apply the ready label so CI can run, and a code-owner for vllm/reasoning/qwen3_reasoning_parser.py take a look? Happy to validate against our production stack post-merge.

Thanks @ZenoAFfectionate for the work!

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @ZenoAFfectionate.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[nuk3s]

Confirmed against Intel/Qwen3.5-122B-A10B-int4-AutoRound on vLLM 0.18.1rc1. Two things.

Append fix — @gemini-code-assist's concern is real. When the model emits text after </think> and a leaked tool call inside <think>, prepending produces:

content = "<tool_call>...</tool_call>\n\nassistant trailing text"

qwen3_coder's content_index = model_output.find("<tool_call>") returns 0, so content = "" and the trailing text drops. Append preserves it:

-    content_parts = ["\n\n".join(extracted_blocks)]
-    if content:
-        content_parts.append(content)
+    content_parts = []
+    if content:
+        content_parts.append(content)
+    content_parts.append("\n\n".join(extracted_blocks))

Worth extending test_promoted_qwen3_reasoning_tool_call_remains_parseable with assert "assistant trailing text" in content — fails with prepend, passes with append.

Streaming still broken on stable 0.18.x without an is_reasoning_end override. Before the vllm/parser/abstract_parser.py refactor #40783 targets: the parser correctly moves text to content, but chat_completion/serving.py:938 gates the transition to tool-parser invocation on is_reasoning_end(output_token_ids) — which only returns True when </think> appears, which it doesn't here. Client sees the XML in content, tool_calls=[].

Two overrides on the parser keyed on a per-request _tool_call_promoted flag fix it:

def is_reasoning_end(self, input_ids):
    if self._tool_call_promoted: return True
    return super().is_reasoning_end(input_ids)

def extract_content_ids(self, input_ids):
    if self._tool_call_promoted: return list(input_ids)
    return super().extract_content_ids(input_ids)

Orthogonal to this PR (non-streaming) and #40783 (newer architecture). Worth a note in docs/design/qwen3_reasoning_tool_call_recovery.md that 0.18.x streaming users need both pieces. Can send as a separate PR — scoped to qwen3_reasoning_parser.py, no serving-layer touches.