[SpecDec + Reasoning] Fix race condition when <channel|> reasoning-end

[SvenLorenz]

Hello,

Setup

I investigated further from my issue #38106. It's actually the combination of speculative decoding, reasoning and tool_choice="required". If either one of these three is disabled the bug doesn't happen. I reproduced this with these two configs:

The configs

cyankiwi/gemma-4-31B-it-AWQ-8bit
  --tool-call-parser=gemma4
  --reasoning-parser=gemma4
  --enable-auto-tool-choice
  --speculative-config '{"model":"google/gemma-4-31B-it-assistant","num_speculative_tokens":4}'
lukealonso/Qwen3.5-397B-A17B-NVFP4
    --enable-auto-tool-choice
    --tool-call-parser qwen3_coder
    --reasoning-parser qwen3
    --speculative-config '{"method":"qwen3_next_mtp","num_speculative_tokens":3}'

And the following request fails in roughly 75% of cases before the bugfix for me:

The request

curl -X POST http://localhost:8001/v1/chat/completions \
                          -H "Content-Type: application/json" \
                          -d '{
                        "messages": [
                            {
                                "role": "user",
                                "content": [
                                    {
                                        "text": "Detect if there is a bunny in this video.",
                                        "type": "text"
                                    },
                                    {
                                        "type": "video_url",
                                        "video_url": {
                                            "url": "https://test-videos.co.uk/vids/bigbuckbunny/mp4/h264/1080/Big_Buck_Bunny_1080_10s_1MB.mp4"
                                        }
                                    }
                                ]
                            }
                        ],
                        "model": "ai_model",
                        "max_completion_tokens": 16000,
                        "presence_penalty": 1.5,
                        "reasoning_effort": "low",
                        "stream": false,
                        "temperature": 1.0,
                        "tool_choice": "required",
                        "tools": [
                            {
                                "type": "function",
                                "function": {
                                    "name": "final_result",
                                    "description": "The final response which ends this conversation",
                                    "parameters": {
                                        "properties": {
                                            "chain_of_thought": {
                                                "description": "The chain of thought that results in the conclusion for any potential bunnies.",
                                                "type": "string"
                                            },
                                            "conclusion": {
                                                "description": "The conclusion based on the occurrences.",
                                                "enum": ["detection", "no detection", "unsure"],
                                                "type": "string"
                                            }
                                        },
                                        "required": ["chain_of_thought", "conclusion"],
                                        "title": "Bunnies",
                                        "type": "object"
                                    }
                                }
                            }
                        ],
                        "top_p": 0.95,
                        "top_k": 20,
                        "priority": 0.0,
                        "chat_template_kwargs": { "enable_thinking": true }
                    }'

The bug

When speculative decoding generates the reasoning-end token as a draft token, the old code unconditionally set reasoning_ended=True and force-fed the unconstrained bonus token to the grammar, corrupting its state. All subsequent outputs were then constrained by a corrupted grammar leading to unconstrained generation of the model, which more often than not lead to the model generating native tool calls. This then breaks the tool_choice="required" path in _parse_tool_calls_from_content, because it expects the tool calls to be a json of format list[FunctionDefinition]. The model generating native tool calls could be fixed by the suggestion I had in the issue above, but since the model could also generate something else afaik, this is not a true fix.
This bug is also stochastic, because if the reasoning-end token gets generated as the bonus token everything works.

The fix

grammar_bitmask() now does per-token reasoning-end detection: when apply_bitmask=False, each draft token is checked against the reasoner. If reasoning-end token is found mid-batch, apply_bitmask flips to True for subsequent positions (draft tokens after the end token + bonus slot), and bonus_requires_grammar=True is set.
update_from_output() has a new elif branch for bonus_requires_grammar. The bonus token is fed to the grammar:

• Accepted: bonus was generated under constraint → set reasoning_ended=True, constrain all future tokens
• Rejected: bonus was generated without constraint (spec decode rejected reasoning-end token draft) → reset bonus_requires_grammar=False, leave reasoning_ended=False, grammar state untouched. The model continues reasoning naturally and reasoning-end token will appear in a future batch.
  accept_tokens() has a suppress_accept_errors flag to avoid ERROR log spam from expected rejections in this path.

Test Plan

Four unit tests were added to tests/v1/structured_output/test_reasoning_structured_output.py, using mocked reasoners and grammars to validate the grammar_bitmask() per-token reasoning-end detection logic:

1. test_grammar_bitmask_reasoning_ends_mid_batch — <channel|> is the last draft token ([10, 20, 30, 99]). Verifies: only the bonus slot (idx 4) gets a constrained bitmask, bonus_requires_grammar=True is set, reasoning_ended stays False, and no accept_tokens calls are made for draft positions.
2. test_grammar_bitmask_reasoning_ends_mid_draft — <channel|> is at position 1 ([10, 99, 30, 40]). Verifies: positions 2, 3, and bonus (idx 2-4) get constrained bitmasks, accept_tokens is skipped for all positions (draft tokens after <channel|> were generated unconstrained), bonus_requires_grammar=True, no rollback called.
3. test_grammar_bitmask_no_reasoning_end — No end token in batch ([10, 20, 30, 40]). Verifies: all positions unconstrained, reasoning_ended unchanged, no fill_bitmask calls.
4. test_grammar_bitmask_reasoning_already_ended — reasoning_ended=True before the batch. Verifies: all 5 positions (4 draft + 1 bonus) constrained, 4 accept_tokens calls made, rollback called — the normal pre-existing path is unchanged.

Test Result

All tests for tests/v1/structured_output are passing.

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