fix(structured_output): pass new_token_ids to should_advance() to fix MTP spec-decode off-by-one

[nac7]

Fixes #34650.

Problem

When Multi-Token Prediction (MTP) speculative decoding accepts a draft token, _update_after_schedule pre-increments num_computed_tokens by (1 main + N spec) tokens before model execution. After update_from_output appends the accepted tokens, the index-based delta in should_advance() starts one position past the main token.

This means a </think> token emitted as the main token is silently missed: reasoning_ended is never set to True, and grammar constraints (JSON mode, structured output) are permanently disabled for the rest of the request. Users running Qwen3 models with MTP enabled see unstructured output with no error or warning.

Root Cause

should_advance() infers which new tokens to scan by computing a delta from num_computed_tokens:

delta_from = request.num_computed_tokens - request.num_output_placeholders
start = delta_from if delta_from >= 0 else max(len(all_token_ids) + delta_from, 0)

After an MTP step, num_computed_tokens has already been pre-incremented to include the spec tokens, so start lands past the </think> token — it is never examined by is_reasoning_end_streaming.

Fix

Add an optional new_token_ids: list[int] | None = None parameter to should_advance(). The one call site inside update_from_output() already holds the accepted token list, so passing it directly bypasses the broken index arithmetic:

# scheduler.py — caller passes accepted tokens directly
if new_token_ids and self.structured_output_manager.should_advance(
    request, new_token_ids
):

# structured_output/__init__.py — use provided delta when available
if new_token_ids is not None:
    delta: Iterable[int] = new_token_ids
else:
    # existing path for non-MTP callers: async-scheduling-aware delta
    delta_from = request.num_computed_tokens - request.num_output_placeholders
    ...
    delta = itertools.islice(all_token_ids, start, None)
if reasoner.is_reasoning_end_streaming(all_token_ids, delta):

All other call sites (update_draft_token_ids, update_draft_token_ids_in_output) omit the new parameter and continue using the existing async-scheduling-aware delta, which is correct for those contexts.

Tests

tests/v1/structured_output/test_reasoning_structured_output.py — added test_mtp_reasoning_grammar_not_disabled:

• Simulates an MTP step by pre-incrementing num_computed_tokens as _update_after_schedule would
• Calls update_from_output with a </think> token appended
• Asserts reasoning_ended is True (was False before fix)
• Asserts should_advance() returns True, confirming grammar is still enabled

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[estrella159]

This + #44927/#45477 is what gates re-enabling MTP for us on Qwen3.6-27B-FP8 (prefix-cache +
reasoning, TP=1, fp8 KV). Happy to test an RC. Thanks.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @nac7.

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

[abmfy]

Hello, thanks for the contribution!
This appears to be superseded by #44993, which includes the same new_token_ids fix and also handles advancing the grammar through post-</think> tokens in the same speculative batch. Since #44993 has merged, should this PR be closed?

[nac7]

Closing — I found while rebasing that this exact fix (passing new_token_ids through to should_advance() to correct the MTP spec-decode off-by-one) has already been merged into main independently, along with an updated test suite in tests/v1/structured_output/test_reasoning_structured_output.py. No remaining diff to contribute here.