[Bugfix][Frontend] Normalize constrained Harmony recipients

[tarjan1]

Summary

Normalize standalone <|constrain|>... markers that older openai-harmony versions misparse as recipients at the shared HarmonyParser boundary.

This keeps constrained final output classified as regular content while exposing recipient=None through HarmonyParser.current_recipient and Segment.recipient. It covers non-streaming parsing, streaming parser state, and emitted segments.

Related to #45570. This is the separate HarmonyParser follow-up requested in #45570 (comment).

Duplicate check

• Searching open PRs for 45570 in:body and Harmony parser/constrained-recipient keywords only found [Bugfix][Frontend] Handle constrained Harmony output without recipient #45571.
• [Bugfix][Frontend] Handle constrained Harmony output without recipient #45571 is the existing Responses-specific guard. This PR instead applies the normalization to the shared HarmonyParser, covering Chat Completions and the future unified Responses path described by the maintainer.
• [Responses API] Sanitize leaked Harmony control tokens in tool names and recipients #35906 is a broader Responses API sanitization and malformed-control-token recovery proposal; this PR only normalizes the valid standalone constrained-output marker at the parser boundary.
• [Bugfix] Sanitize malformed tool call recipients in Harmony parser #31677 targets malformed tool-call recipients rather than constrained final output without a recipient.

Tests

Run remotely on Python 3.12.8 against vLLM main at 64833f8158236a7bddeeb89efc6a3bde5d16f468:

PYTHONPATH="$WT" .venv-codex/bin/python -m pytest tests/parser/test_harmony.py -q
# openai-harmony 0.0.4: 38 passed, 17 warnings

PYTHONPATH="$SITE08:$WT" .venv-codex/bin/python -m pytest tests/parser/test_harmony.py -q
# openai-harmony 0.0.8: 38 passed, 17 warnings

Also run on the changed files:

ruff check: passed
ruff format --check: passed
typos: passed
git diff --check: passed

AI assistance disclosure: OpenAI Codex assisted with investigation, implementation, test execution, and drafting this PR. I reviewed the changed lines and validated the behavior and tests.

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[yzong-rh]

Minor comments. Thanks for the work.

[yzong-rh]

Should we discard anything that starts with <|constrain|>? or should be prune the recipient that may contain <|constrain|> ...?

Not sure if a case like to=function.foo <|constrain|> ... is buggy and possible. If it is, stripping away the ending <|constrain|> ... might work better.

[yzong-rh]

Doesn't need the self here. A static method might be better.

[bbrowning]

@yzong-rh As a general note, if we were to swap over from openai-harmony to oss-harmony, these tokens would stop leaking into the vLLM layer at all (at least not in this way). Just an FYI, and not a reason to not fix this in this layer as well.

[tarjan1]

Hi @yzong-rh , thanks again for your previous guidance!

I’ve addressed all the requested changes in the latest push. Please feel free to let me know if there are any further suggestions or concerns.

[tarjan1]

Hi @yzong-rh , Sorry for missing this detail earlier. I looked into the recipient pruning question more closely.

I tested these representative parser-level cases against openai_harmony 0.0.4 and 0.0.8:

1. Standalone constrained final output:
   <|channel|>final <|constrain|>json<|message|>{"result":true}<|end|>

   Result: Harmony returns recipient='<|constrain|>json' and content_type=None.
   This is the reproduced bad case.

2. Legal tool call with a space before the constraint marker:
   <|channel|>commentary to=functions.get_weather <|constrain|>json<|message|>{"location":"San Francisco"}<|call|>

   Result: Harmony returns recipient='functions.get_weather' and
   content_type='<|constrain|>json'.

3. Official docs-style analysis + tool call sequence:
   <|channel|>analysis<|message|>Need to use function get_weather.<|end|><|start|>assistant<|channel|>commentary to=functions.get_weather <|constrain|>json<|message|>{"location":"San Francisco"}<|call|>

   Result: the tool-call message returns recipient='functions.get_weather' and
   content_type='<|constrain|>json'.

4. Tool call without a space before the constraint marker:
   <|channel|>commentary to=functions.get_weather<|constrain|>json<|message|>{"location":"San Francisco"}<|call|>

   Result: Harmony still returns recipient='functions.get_weather' and
   content_type='<|constrain|>json'.

5. Constraint marker before to=...:
   <|channel|>commentary <|constrain|>json to=functions.get_weather<|message|>{"location":"San Francisco"}<|call|>

   Result: Harmony rejects this with HarmonyError: unexpected tokens remaining in message header.
   It does not produce a recipient.

So I did not find a real parsed case where Harmony returns something like
recipient='functions.foo <|constrain|>...'.

However, I agree that pruning at the first <|constrain|> is more robust than only checking startswith.
The updated normalization prunes at the first <|constrain|>, so a hypothetical
functions.foo <|constrain|>json becomes functions.foo, while
<|constrain|>json ... still becomes None because there is no trusted recipient
before the marker.

I also normalized completed_message.recipient and kept the test coverage in
TestProcessChunk to assert both Segment.recipient and
completed_message.recipient.

[yzong-rh]

Sorry for the delay. This LGTM, thanks

cc maintainer @bbrowning

[tarjan1]

Sorry for the delay. This LGTM, thanks

cc maintainer @bbrowning

Hi @bbrowning, could you please take a look when you have a chance?

This PR has been approved by @yzong-rh. Please let me know if there’s anything else I should update before it can move forward.

Thanks!

cc @sfeng33

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @tarjan1.

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

[bbrowning]

Does this work in the real world? Here's a prior example I happened to have written down of how the constrain often leaks into the wrong place:

<|channel|>analysis<|message|>User asks: "What's the weather like in Paris today?" We need to get weather info. Use get_weather function. We need coordinates for Paris. Latitude 48.8566, longitude 2.3522.<|end|><|start|>assistant<|channel|>commentary <|constrain|>functions.get_weather<|message|>{"latitude":48.8566,"longitude":2.3522}<|call|>

I'm worried about the find and rstrip here in cases like this. It would potentially catch some cases, but I'm unsure how much it will hold up in the typical long context real-world failure scenarios we frequently see with these models. Do we have evidence this helps those broadly?

[tarjan1]

Does this work in the real world? Here's a prior example I happened to have written down of how the constrain often leaks into the wrong place:

<|channel|>analysis<|message|>User asks: "What's the weather like in Paris today?" We need to get weather info. Use get_weather function. We need coordinates for Paris. Latitude 48.8566, longitude 2.3522.<|end|><|start|>assistant<|channel|>commentary <|constrain|>functions.get_weather<|message|>{"latitude":48.8566,"longitude":2.3522}<|call|>

I'm worried about the find and rstrip here in cases like this. It would potentially catch some cases, but I'm unsure how much it will hold up in the typical long context real-world failure scenarios we frequently see with these models. Do we have evidence this helps those broadly?

Hi @bbrowning ，

Thanks for confirming, this works for the specific case addressed here. This PR is only a narrow fix for #45570.

I’ll continue looking into the broader <|constrain|> leakage issue and follow up with a more robust solution in a separate PR.

[bbrowning]

I'm approving this as it does help the cases where constrains leak into the final channel as recipients, which is what #45570 was about.

I noted an example where this does not solve the more generic problem of constrain tokens leaking into recipients for tool calling scenarios, but realize that was out of scope for the original intent of this PR so am ok with the narrower fix for the misidentified as MCP issue.

[tarjan1]

I'm approving this as it does help the cases where constrains leak into the final channel as recipients, which is what #45570 was about.

I noted an example where this does not solve the more generic problem of constrain tokens leaking into recipients for tool calling scenarios, but realize that was out of scope for the original intent of this PR so am ok with the narrower fix for the misidentified as MCP issue.

Hi @bbrowning, Thank you for the approval and clarification.

The current CI failures appear unrelated to the changes in this PR. Would it be reasonable to proceed with the merge if you agree, or would you prefer another CI rerun?

[chaunceyjiang]

ci error fix: #47126