[Bugfix] Preserve reasoning in streaming deltas spanning phase boundary

[ashwing]

Summary

• Fix: When speculative decoding (MTP) accepts a multi-token batch that spans the reasoning/content boundary, the tool parser in DelegatingParser.parse_delta() overwrites delta_message, discarding reasoning extracted from the same boundary delta. This patch saves reasoning before tool extraction and restores it afterward.
• Root cause: With single-token streaming, the </think> end token arrives alone so reasoning and tool-call phases never overlap in a single delta. With MTP accepting 3–6 tokens at once, a single delta can contain both reasoning text and post-think content, triggering the overwrite.
• Regression test: Parametrized test simulates multi-token chunked streaming (chunk sizes 3–6) and asserts reasoning is fully preserved.

Fixes #42781

Test plan

• test_parse_delta_spec_decode_boundary_preserves_reasoning[3] — PASS
• test_parse_delta_spec_decode_boundary_preserves_reasoning[4] — PASS (was FAIL without fix)
• test_parse_delta_spec_decode_boundary_preserves_reasoning[5] — PASS (was FAIL without fix)
• test_parse_delta_spec_decode_boundary_preserves_reasoning[6] — PASS
• All 5 existing test_streaming.py tests — PASS (no regressions)
• Tests run on Docker vllm/vllm-openai:latest with GPU

python -m pytest tests/parser/test_streaming.py -v --noconftest
# 9 passed

Duplicate-work check

gh pr list --repo vllm-project/vllm --state open --search "42781 in:body"
# No existing PRs

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[gemini-code-assist]

Code Review

This pull request addresses a bug where reasoning content could be lost during speculative decoding when a token batch spans the boundary between reasoning and content. It introduces a regression test and modifies the parse_delta method in abstract_parser.py to preserve the reasoning field when the tool parser processes a boundary delta. Feedback suggests making the preservation logic more robust by ensuring other fields, such as role, are also carried over if set during the reasoning phase.

[gemini-code-assist]

The current implementation only preserves the reasoning field from the delta_message produced during the reasoning phase. If the reasoning parser were to set other fields, such as role, they would be lost when delta_message is overwritten by the tool parser at line 705. To make this more robust, consider preserving the entire delta_message or at least the role field, ensuring that any metadata set during the reasoning phase is carried over to the final delta.

[ashwing]

Good point — updated to preserve all fields (reasoning, content, role) from the boundary delta instead of just reasoning. This covers the case at basic_parsers.py:127 where a boundary delta returns both reasoning and content simultaneously.

[ashwing]

Addressed feedback and CI issues:

1. DCO: Added Signed-off-by trailer to all commits
2. Review feedback (gemini-code-assist): Updated fix to preserve all DeltaMessage fields (reasoning, content, role) from boundary deltas, not just reasoning. This covers the edge case in basic_parsers.py:127 where a boundary delta sets both reasoning and content simultaneously.

All tests still pass (9/9, re-validated on Docker).

Note: pre-run-check failure is expected — it requires the ready label or 4+ merged PRs from the author, which is added by reviewers when approving.

[ashwing]

Friendly ping @sfeng33 @chaunceyjiang — this is a 2-file fix for speculative decoding truncating reasoning output (Gemma 4 with MTP). The parser boundary logic was dropping reasoning/content fields when the tool parser re-processed a delta spanning the reasoning→content transition.

Happy to address any feedback!

[benchislett]

Seems reasonable at a glance

[ashwing]

@benchislett Any other information needed to push this through?

[sfeng33]

Thanks for the fix! This seems to be duplicate to #42691, so I added you as co-author.

[ashwing]

Thanks @sfeng33! Happy to have this covered in #42691 — closing this one in favor of yours since it's further along in review. Appreciate the co-author credit.