Fix: detect reasoning end with accepted MTP tokens

[karthiksenv]

Summary

Fixes reasoning-end detection when structured output is used together with reasoning mode and MTP/speculative decoding.

In the speculative decoding path, num_computed_tokens can be advanced before the newly accepted output tokens are appended to request.all_token_ids. Because StructuredOutputManager.should_advance() derived the token delta from num_computed_tokens, the derived slice could be empty even when the current accepted tokens contained the reasoning end marker. As a result, reasoning_ended was not set at the correct time and structured output constraints were not applied after the thinking phase.

This change passes the actual accepted output token IDs from the scheduler into should_advance() and uses those tokens for reasoning-end streaming detection. The previous derived-slice behavior is preserved as a fallback for existing callers.

Fixes #34650.

Changes

• Updated StructuredOutputManager.should_advance() to accept optional newly accepted token IDs.
• Updated the V1 scheduler to pass the current accepted output tokens into should_advance().
• Added a regression test covering the MTP-style pre-increment case where the old derived slice is empty but the accepted tokens include the reasoning end marker.

Verification

Passed locally:

python -m compileall vllm/v1/structured_output vllm/v1/core/sched tests/v1/structured_output/test_reasoning_structured_output.py
git diff --check

Could not run locally:

python -m pytest tests/v1/structured_output/test_reasoning_structured_output.py
python -m ruff check ...

The local environment is missing pytest/ruff, and importing the test stack is blocked by missing torch. I expect CI to provide the full dependency environment.

Related PR

There is an overlapping PR, #44927. I checked the upstream context before opening this PR. This PR focuses on passing the accepted MTP/speculative decoding tokens directly into should_advance() and includes a focused regression test for the pre-incremented num_computed_tokens case.

[claude]

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[karthiksenv]

I opened a PR for this issue: #47617 (comment)

The fix passes the actual accepted MTP/speculative decoding output tokens into StructuredOutputManager.should_advance() so reasoning-end streaming detection no longer depends only on the derived slice from num_computed_tokens.

I also added a regression test for the case where num_computed_tokens has already been advanced and the old derived slice is empty, but the accepted tokens contain the reasoning end marker.

[mergify]

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

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

[arpera]

The fix was already merged to main in PR #44993. So, I propose to close this PR then.

[vadiklyutiy]

Fixed in #44993.