[Frontend] Raise VLLMValidationError for an unknown Harmony channel in Responses output parsing by longluubao · Pull Request #58385 · vllm-project/vllm

longluubao · 2026-09-23T15:26:23Z

Purpose

Fixes #58384 (partially, see "Not addressed" below).

vllm/entrypoints/openai/responses/harmony.py::_parse_message_no_recipient raises a bare ValueError("Unknown channel: <x>") when gpt-oss emits a Harmony message on a channel outside analysis / commentary / final (seen in production as comment; #22533 reported commentaryanalysis). Per review, this PR keeps that behaviour and raises VLLMValidationError instead, consistent with the other VLLMValidationError raises in the same module, so the error is classified as a vLLM client-side error rather than an unexpected exception.

Change

vllm/entrypoints/openai/responses/harmony.py (+1/-1): raise ValueError(...) -> raise VLLMValidationError(...) (the import already exists).
tests/entrypoints/openai/responses/test_harmony_utils.py (+8): test_unknown_channel_raises_validation_error, parametrized over comment / commentaryanalysis, through harmony_to_response_output (the entry point serving.py uses).

An earlier revision dropped such messages instead of raising; it was replaced following the review discussion.

Not addressed

For stream=true, the raise still happens when responses_stream_generator assembles the final response after the deltas were delivered, so the SSE stream still ends without response.completed. Happy to follow up separately (for example emitting a response.failed / error event there) if maintainers want that.

Duplicate-work check

gh searches of vllm-project/vllm issues and PRs (all states) for "Unknown channel", "harmony channel", "harmony_to_response_output", "_parse_message_no_recipient": #22533 and #22578 (closed stale, same symptom), no open PR changes this raise; #35906 / #35907 touch the same file but not this line.

Test Plan

.venv/bin/python -m pytest -q tests/entrypoints/openai/responses/test_harmony_utils.py
.venv/bin/python -m pytest -q tests/entrypoints/openai/responses/test_harmony_utils.py -k unknown_channel   # before / after
ruff check / ruff format --check on the two changed files

Test Result

On daddc24 (rebased on main 4e35590), macOS import-only build (VLLM_TARGET_DEVICE=empty, Python 3.12):

tests/entrypoints/openai/responses/test_harmony_utils.py: 89 passed in 13.20s
-k unknown_channel, with the old `raise ValueError`:  2 failed, 87 deselected
-k unknown_channel, with this change:                 2 passed, 87 deselected
ruff: All checks passed! / 2 files already formatted

AI assistance disclosure

This change was made with AI assistance (Claude, Anthropic). The submitter reviewed every changed line, ran the tests above and is responsible for the change.

claude

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

github-actions · 2026-09-23T15:28:56Z

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longluubao · 2026-09-28T04:01:27Z

Hi @njhill @yewentao256, this is a small [Bugfix][Frontend] for the Responses API Harmony parser (fixes #58384): _parse_message_no_recipient currently raises ValueError("Unknown channel: ...") when gpt-oss emits a Harmony message on a channel outside analysis/commentary/final, and because the raise happens while the final Response is assembled it escapes the streaming error handler and drops the SSE connection.

The change replaces the raise with logger.warning_once + return [], matching the three neighbouring code paths that already ignore unknown channels (streaming_events.py:599/:638, parser/harmony.py:79). +11/-1 in harmony.py, 7 unit tests (fail before / pass after on CPU and on an A100 80GB), pre-commit 14/14, and an end-to-end run serving gpt-oss-20b with 40 /v1/responses requests and 0 "Unknown channel" errors (details in the PR description).

The same error class was reported earlier in #22533 (auto-closed stale). If you would prefer the message to be rendered as user-visible output instead of dropped, I can change the PR that way. Happy to run anything else you want.

longluubao · 2026-09-28T04:07:55Z

@njhill @yewentao256 could one of you add the ready label so pre-commit and fastcheck run on this PR?
This is my first vLLM PR so the pre-run-check gate needs a maintainer to release it. Thanks.

longluubao · 2026-09-28T07:42:30Z

Tagging the code owners of vllm/entrypoints/openai per CODEOWNERS, since my earlier note went to the wrong group: @aarnphm @chaunceyjiang @DarkLight1337.

This is a small Responses-API parser fix in vllm/entrypoints/openai/responses/harmony.py (fixes #58384): _parse_message_no_recipient raises ValueError("Unknown channel: ...") for a Harmony message on a channel outside analysis/commentary/final, and the raise escapes the streaming error handler and drops the SSE connection. The patch drops the message with logger.warning_once, matching the neighbouring paths that already ignore unknown channels; +11/-1 plus 7 unit tests, fail-before/pass-after on CPU and A100, gpt-oss-20b end-to-end 40 requests clean. Details and the trade-off (drop vs render) are in the description and the comment above. Happy to adjust or run anything else you would like to see.

longluubao · 2026-09-30T04:18:51Z

Follow-up @aarnphm @chaunceyjiang @DarkLight1337: still looking for a first look at this parser fix (fixes #58384; +11/-1 in vllm/entrypoints/openai/responses/harmony.py plus 7 tests). If it would make review easier I can (a) add a regression test that replays the exact Harmony token sequence from the field failure (<|channel|>comment<|message|>...), or (b) switch from drop-with-warning to rendering the text as a message item, whichever you prefer. Thanks.

longluubao · 2026-09-30T07:31:04Z

Adding the remaining reviewers GitHub assigned to this PR, plus a recent committer to this file: @robertgshaw2-redhat @NickLucche @AndreasKaratzas @hmellor. Scope is one function in vllm/entrypoints/openai/responses/harmony.py (_parse_message_no_recipient: drop a Harmony message on an unrecognised channel with logger.warning_once instead of raising and dropping the SSE connection), +11/-1 with 7 tests; the trade-off and the end-to-end evidence are in the description. Any of the two shapes (drop vs render) is fine by me, I just need a decision. Thanks.

longluubao · 2026-10-01T03:33:56Z

@DarkLight1337 quick check: is a fix in this area wanted at all? If the preference is to keep the current raise and handle it differently (for example on the serving side), tell me and I will close this PR and leave #58384 open with the reproduction. If a fix is wanted, I will take whatever shape you prefer (drop with warning as here, or render the text as a message item).

DarkLight1337 · 2026-10-01T04:38:20Z

@chaunceyjiang PTAL

mergify · 2026-10-01T20:50:46Z

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

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

longluubao · 2026-10-02T03:59:25Z

Rebased onto current main (f1c24c835). The only conflict was the import block in tests/entrypoints/openai/responses/test_streaming_events.py after #59652; resolved by keeping both sets of imports. No change to the fix itself (+11/-1 in harmony.py, 7 tests). On the new base: ruff clean, test_harmony_utils.py + test_streaming_events.py 103 passed.

longluubao · 2026-10-02T04:06:39Z

@chaunceyjiang following DarkLight1337's hand-off: the branch is rebased on current main (f1c24c835, no conflicts, ruff clean, 103 tests passing in the two touched test files), so it can be reviewed as-is. The one design choice to confirm is drop-with-warning (current) versus rendering the stray text as a message item; I will switch if you prefer the latter.

longluubao · 2026-10-04T06:29:59Z

@chaunceyjiang @DarkLight1337 checking in after the hand-off on Oct 1: the branch has been on current main since Oct 2 (f1c24c835, no conflicts, 103 tests passing in the two touched files). If chaunceyjiang is busy this week, is there someone else from the frontend group who could take the first pass? The change is one function in harmony.py.

longluubao · 2026-10-06T04:01:15Z

@chaunceyjiang @DarkLight1337 gentle follow-up: this one-function parser fix has been rebased and green since Oct 2 (f1c24c835). If the frontend group is short on time, I am also fine with a maintainer deciding the one open question (drop the stray message with a warning, as implemented, or render it) and I will adjust the same day.

longluubao · 2026-10-07T07:03:58Z

@chaunceyjiang @DarkLight1337 @aarnphm one more follow-up on this parser fix: still rebased and green (f1c24c835), no reviewer comments yet. If nobody in the frontend group has bandwidth, is there anyone else you would like me to ask?

chaunceyjiang · 2026-10-08T08:04:19Z

+    # final} (e.g. "comment"). Drop it, as the streaming dispatchers and the
+    # chat parser (_SegmentType.IGNORE) already do; raising here aborts the
+    # whole /v1/responses request and, when streaming, the SSE connection.
+    logger.warning_once(


I don't quite understand why we need to remove this restriction. We should always handle the three channels: analysis, commentary, and final. If any content falls outside these three channels, we should find a way to map it to one of them.

Thanks @chaunceyjiang. Agreed that the model should only ever use analysis, commentary and final; the question is what to do when a generated header is outside that set, and today the answer is "fail the finished response". Two reasons this PR stops raising, and I am open to mapping instead of dropping:

Where the raise fires. _parse_message_no_recipient runs inside harmony_to_response_output, i.e. when the final Response is assembled after generation has completed. For stream=true, responses_stream_generator calls responses_full_generator (serving.py:1408) after its except GenerationError block (serving.py:1395), so one unexpected header closes the SSE connection with no response.completed although every delta was already delivered to the client. For stream=false the whole request returns an error. So the restriction is not protecting the client; it discards a complete generation at the last step.

The rest of the pipeline already tolerates it. streaming_events.emit_content_delta_events and emit_previous_item_done_events return [] for a segment on an unknown channel with no recipient (streaming_events.py:597), and the chat-completions parser maps it to _SegmentType.IGNORE (vllm/parser/harmony.py:76). The completed-message parser is the only place that raises, which is why the failure only shows up at assembly and only on /v1/responses.

What we have actually seen are malformed headers rather than a fourth channel: comment (from an openai/gpt-oss-120b deployment; most likely a truncated commentary) and commentaryanalysis (two headers run together, #22533).

If you would rather map than drop, I can change the branch to resolve the header by prefix against the three known names (comment -> commentary, commentaryanalysis -> commentary, fin -> final, and so on) and keep the warning_once + drop only for a header that matches nothing. One caveat to decide on: in streaming mode the deltas of such a message were never emitted (the dispatchers dropped them), so a mapped item would appear in response.completed without earlier output_item.added / delta events, unless the dispatchers are given the same mapping. Which behaviour do you prefer: (a) drop as in this PR, (b) prefix-map at assembly only, or (c) prefix-map in both the dispatchers and the assembly parser? I will update the PR accordingly.

longluubao · 2026-10-11T09:14:40Z

@chaunceyjiang @DarkLight1337 following up on the thread question (#58385 (comment)): which direction do you want for a header outside analysis/commentary/final, (a) drop it as in this PR, (b) prefix-map it to the nearest of the three at assembly only, or (c) prefix-map it in both the streaming dispatchers and the assembly parser? Any of the three is a small change and I will update the PR the same day; the current behaviour (ValueError at assembly, SSE closed before response.completed) is what we want to get rid of.

mergify · 2026-10-12T10:21:33Z

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

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

longluubao · 2026-10-13T08:15:07Z

Rebased onto current main (e66713e); the only conflict was again the import block and the new test classes appended at the end of tests/entrypoints/openai/responses/test_streaming_events.py (both sides kept, ruff clean). No change to the fix itself. @chaunceyjiang the question on drop vs. prefix-map (#58385 (comment)) is still open; I will implement whichever you prefer.

longluubao · 2026-10-14T17:28:00Z

@robertgshaw2-redhat @NickLucche you are on the review list for this one; a decision on direction is all that is blocking it. The thread with @chaunceyjiang (#58385 (comment)) has three options for a Harmony header outside analysis/commentary/final: (a) drop it as in this PR, (b) prefix-map it to the nearest of the three at assembly, or (c) prefix-map in both the streaming dispatchers and the assembly parser. Any of them is a small change on this branch (rebased on main, e66713e, ruff clean); the current behaviour (ValueError at response assembly, SSE closed before response.completed) is what the fix removes. If you have a preference, I will update the PR the same day.

longluubao · 2026-10-16T06:00:33Z

@chaunceyjiang @DarkLight1337 one more nudge on the open question in #58385 (comment): (a) keep the drop as in this PR, (b) prefix-map an out-of-set Harmony header to the nearest of analysis/commentary/final at assembly, or (c) map in both the streaming dispatchers and the assembly parser. The branch is on current main (e66713e) and I will push whichever you pick the same day; without a decision this fix for #58384 (SSE closed before response.completed) stays stuck.

chaunceyjiang · 2026-10-16T06:34:02Z

        # See: https://cookbook.openai.com/articles/openai-harmony
        return [_parse_final_message(message, incomplete=incomplete)]

-    raise ValueError(f"Unknown channel: {message.channel}")


I suggest keeping the current behavior and simply replacing ValueError with VLLMValidationError.

Done in daddc24: kept the current behaviour and replaced ValueError with VLLMValidationError (the import already exists), with a test asserting the raise for comment / commentaryanalysis; the drop logic and its tests are gone, so the PR is now +9/-1. Title and description are updated. One note for later, not part of this PR: with stream=true the raise still happens at final assembly after the deltas were sent, so the SSE stream ends without response.completed; I can follow up separately if you want an error event emitted there. @chaunceyjiang could you take another look?

…armony channel `_parse_message_no_recipient` in vllm/entrypoints/openai/responses/harmony.py raises `ValueError: Unknown channel: <name>` for any Harmony message whose channel is not analysis, commentary or final. In the streaming /v1/responses path that exception propagates out of the SSE generator and the client sees a truncated chunked read. gpt-oss models occasionally emit other channels (the production case was `comment`), so a single sampled token aborts the whole response. Treat an unrecognized channel the way the streaming dispatchers and the chat parser already do: log it once with logger.warning_once and drop the message, instead of raising. Unit tests cover `comment`-channel messages through harmony_to_response_output for both the streaming and non-streaming entry points; they fail on the previous code and pass with this change. Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Luu Bao Long <longbaoluu68@gmail.com>

Per review (chaunceyjiang): keep the current behaviour for a Harmony message on a channel outside analysis/commentary/final, but raise VLLMValidationError instead of a bare ValueError, consistent with the other VLLMValidationError raises in this module. Replace the drop tests with one test asserting the raise, and drop the streaming-dispatcher tests that only pinned the previous approach. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Luu Bao Long <longbaoluu68@gmail.com>

chaunceyjiang

Thanks~

github-actions · 2026-10-19T03:53:34Z

✅ @longluubao, CI is now available for this PR.

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chaunceyjiang · 2026-10-19T03:54:33Z

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github-actions · 2026-10-19T03:54:52Z

✅ Triggered Buildkite CI #95804 for commit daddc2470ad8.

⚠️ This PR is 1 commit behind upstream main. Running CI at your own risk because --allow-stale was requested; outdated CI configuration may cause failures. Before merging, merge or rebase onto the latest main, then rerun /ci run on the latest PR commit.

…n Responses output parsing (vllm-project#58385) Signed-off-by: Luu Bao Long <longbaoluu68@gmail.com> Co-authored-by: Claude <noreply@anthropic.com>

longluubao requested review from AndreasKaratzas, DarkLight1337, NickLucche, aarnphm, chaunceyjiang and robertgshaw2-redhat as code owners September 23, 2026 15:26

claude Bot reviewed Sep 23, 2026

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longluubao mentioned this pull request Sep 23, 2026

[Bug]: Responses API Harmony output parser raises ValueError: Unknown channel: comment at response assembly and closes the /v1/responses SSE stream before response.completed #58384

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mergify Bot added frontend gpt-oss Related to GPT-OSS models bug Something isn't working labels Sep 23, 2026

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github-project-automation Bot moved this to To Triage in gpt-oss Issues & Enhancements Sep 23, 2026

mergify Bot added the needs-rebase label Oct 1, 2026

longluubao force-pushed the fix/responses-harmony-unknown-channel branch from 0f9357a to f1c24c8 Compare October 2, 2026 03:58

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longluubao force-pushed the fix/responses-harmony-unknown-channel branch from f1c24c8 to e66713e Compare October 13, 2026 08:14

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longluubao and others added 2 commits October 19, 2026 10:30

longluubao force-pushed the fix/responses-harmony-unknown-channel branch from e66713e to daddc24 Compare October 19, 2026 03:49

longluubao changed the title ~~[Bugfix][Frontend] Drop Harmony messages on unrecognized channels in Responses output parsing instead of raising~~ [Frontend] Raise VLLMValidationError for an unknown Harmony channel in Responses output parsing Oct 19, 2026

chaunceyjiang approved these changes Oct 19, 2026

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github-project-automation Bot moved this from To Triage to Ready in gpt-oss Issues & Enhancements Oct 19, 2026

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chaunceyjiang merged commit 573f2f9 into vllm-project:main Oct 19, 2026
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