fix: bypass reasoning gate for completions requests

[JMPSequeira]

Problem

When --reasoning-parser is configured on the server (e.g. glm45 for GLM-5.2 or qwen3 for Qwen3.5), vLLM copies it into structured_outputs_config.reasoning_parser, which activates a reasoning gate in StructuredOutputManager.should_fill_bitmask. The gate defers grammar bitmask enforcement until is_reasoning_end_for_prompt returns True.

For chat requests, the renderer populates reasoning_parser_kwargs with chat_template_kwargs (e.g. {"enable_thinking": false}), so the parser knows whether thinking is active and returns the correct reasoning_ended state.

For completions requests, reasoning_parser_kwargs is None. The reasoning parsers (Qwen3Parser, Glm47MoeParser) default thinking_enabled=True when no kwargs are provided, so is_reasoning_end_for_prompt returns False forever, and should_fill_bitmask returns False forever. The bitmask is filled with the all-ones mask (no constraint) and grammar is silently ignored.

Root cause

Qwen3Parser.is_reasoning_end_for_prompt returns not self.thinking_enabled, and thinking_enabled defaults to True when no chat_template_kwargs are provided (completions path). Same for Glm47MoeParser.

Fix

Bypass the reasoning gate in should_fill_bitmask and should_advance when reasoning_parser_kwargs is empty (i.e. a completions request with no reasoning phase). Chat requests always have reasoning_parser_kwargs set by the renderer, so they are unaffected.

Verification

• Qwen3.5-9B (V1 model runner): completions root ::= "hello" → "hello" ✓, choice grammar → valid choice ✓, LLang GBNF (344 lines, 75 nonterminals) → valid LLang ✓
• GLM-5.2-504B (V2 model runner + MTP speculative decoding): completions root ::= "hello" → "hello" ✓, choice grammar → "red" ✓, LLang GBNF → valid LLang ✓
• Chat requests with enable_thinking: false still work (grammar applied to content)
• Chat requests with enable_thinking: true still work (grammar deferred until reasoning ends)

Notes

This supersedes #53 — the _backend selection hypothesis was a red herring. I empirically confirmed SamplingParams.structured_outputs._backend survives the EngineCore IPC roundtrip (msgspec) intact. The real blocker was the reasoning gate.

Summary by CodeRabbit

• Bug Fixes
  • Improved structured output handling for completion-style requests without reasoning settings.
  • Grammar/bitmask processing now starts immediately in these cases, instead of waiting for reasoning-related checks.
  • FSM progression now advances correctly for completions that do not include a reasoning phase.

[github-actions]

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[coderabbitai]

📝 Walkthrough

Walkthrough

In vllm/v1/structured_output/__init__.py, both should_fill_bitmask and should_advance gain early-return bypasses: when reasoning_parser_kwargs is absent on a structured output request, each method returns True immediately, skipping the reasoning-ended gate logic.

Changes

Reasoning gate bypass for completion requests

Layer / File(s)	Summary
Early bypass in should_fill_bitmask and should_advance vllm/v1/structured_output/__init__.py	Both methods return True before reasoning-ended checks when reasoning_parser_kwargs is missing/falsey, so completion requests are not gated on reasoning phase state.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~5 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main fix: bypassing the reasoning gate for completions requests.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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• Create PR with unit tests

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[coderabbitai]

🧹 Nitpick comments (1)

vllm/v1/structured_output/__init__.py (1)

314-322: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Add a regression test with an actual reasoner configured.

The sampled downstream fixture only covers the reasoner is None path, so it would not catch the parser-default thinking_enabled=True behavior this change is fixing. Please add a completion-shaped request with reasoning_parser_kwargs=None and a configured reasoner_cls, then assert both should_fill_bitmask() and should_advance() return True.

Also applies to: 356-360

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@vllm/v1/structured_output/__init__.py` around lines 314 - 322, Add a
regression test around the structured output reasoning gate in
structured_output/__init__.py that uses a completion-shaped request with
reasoning_parser_kwargs=None and a real reasoner_cls configured, so it exercises
the parser default behavior instead of the reasoner-is-None path. Verify the
relevant helper path (including should_fill_bitmask() and should_advance())
returns True for this case, and place the assertion near the existing logic for
is_reasoning_end_for_prompt / should_advance to cover the bypass behavior for
completions.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@vllm/v1/structured_output/__init__.py`:
- Around line 314-322: Add a regression test around the structured output
reasoning gate in structured_output/__init__.py that uses a completion-shaped
request with reasoning_parser_kwargs=None and a real reasoner_cls configured, so
it exercises the parser default behavior instead of the reasoner-is-None path.
Verify the relevant helper path (including should_fill_bitmask() and
should_advance()) returns True for this case, and place the assertion near the
existing logic for is_reasoning_end_for_prompt / should_advance to cover the
bypass behavior for completions.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 53bf3e1a-e868-4e9b-8425-df27c4f02293

📥 Commits

Reviewing files that changed from the base of the PR and between 2d040b9 and a91a3f8.

📒 Files selected for processing (1)

• vllm/v1/structured_output/__init__.py