[Bugfix] Handle HarmonyError in process_chunk to fix gpt-oss streaming 500s - #52055
[Bugfix] Handle HarmonyError in process_chunk to fix gpt-oss streaming 500s#52055rajathpi wants to merge 3 commits into
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…g 500s process_chunk() called the Harmony streamable parser with no exception handling, so a malformed message header (for example a duplicated `to=` clause) raised HarmonyError all the way out to the API server and failed the request with a 500. flush() already catches HarmonyError and recovers by returning the raw decoded text. Give process_chunk() the same contract: decode the buffered tokens plus the unconsumed tail of the chunk as raw final-channel text, emit it as a delta segment plus a completed message so streaming, non-streaming parse(), and the Responses message loop all surface it, reset parser state, and stop processing the chunk. Fixes vllm-project#51977. Assisted-by: AI coding assistant Signed-off-by: rajathpi <rajathpai2000@gmail.com>
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FIX #51977
Purpose
gpt-oss requests fail with HTTP 500 (
openai_harmony.HarmonyError: unexpected tokens remaining in message header) when the model samples a malformed Harmonymessage header.
HarmonyParser.flush()already catchesHarmonyErrorandrecovers by returning the raw decoded text, but
process_chunk()callsself._harmony_parser.process(token_id)with no handling, so a mid-stream parsefailure escapes to the API server and kills the request.
This gives
process_chunk()the same recovery contract asflush(): onHarmonyError, decode the buffered message tokens plus the unconsumed tail ofthe chunk as raw final-channel text, emit it as a delta segment plus a completed
message, reset parser state, and stop processing the chunk. The dual-segment
shape matches what
flush()already emits, so streaming, non-streamingparse(), and the Responses message loop all surface the text.parse()andHarmonyContext.append_outputread onlycompleted_message, so a delta-onlysegment would have turned the 500 into an empty 200 instead.
Nothing the model generated is dropped, and the request completes.
Note on the trigger: the issue points at a hyphen in the tool name, but
openai_harmony 0.0.8 accepts hyphenated recipients. Reproducing directly against
the library shows the real trigger is leftover tokens in the header. A duplicated
to=clause reproduces the reporter's exact error string, which also explains whythe failure was intermittent rather than reliable.
Scope: vLLM cannot stop a model sampling a malformed header, so it has to tolerate
HarmonyErrorfrom the library.flush()established that defensive contractalready; this completes it for the mid-stream path. A leniency fix upstream in
openai_harmony would be complementary, not a substitute.
Not duplicating existing work: #51977 is unclaimed, and no open PR references it or
touches
vllm/parser/harmony.py.Test Plan
New regression test
tests/parser/test_harmony.py::test_process_chunk_recovers_from_malformed_headerdrives a duplicated
to=header throughprocess_chunk, asserts the generatedtext comes back as a final-channel delta segment plus a completed message instead
of an exception, then asserts a normal message parses afterwards to prove the
state reset. It fails without the source change.
Test Result
With
vllm/parser/harmony.pyreverted, the new test fails:ruff checkandruff format --checkclean on both files; mypy (CIconfiguration) reports no issues on either file.
No model evaluation run: only the error-recovery path changes, and behaviour on
well-formed output is untouched, covered by the 65 pre-existing tests in this file.
This change was developed with AI assistance; I reviewed and tested every line.