[Bugfix] Actually enable serialize_messages for harmony Responses (related to #26185)

[jacobthebanana]

Purpose

For the OpenAI-compatible v1/responses route, enable raw messages to be sent when enable_response_messages is set to True in extra_body.

Previously, the responses are empty because of an issue in openai/harmony. (openai/harmony#78)

#26185 implements most of the fix, but these aren't actually invoked, at least not when serving the model through the vllm serve. The reason is that the said PR specifies when_used="json". Thus, this serialization method is ignored because of the use of model_dump() in vllm/entrypoints/openai/api_server.py#L527-L529.

The fix is to trigger the serializers by setting mode="json" when invoking model_dump.

Test Plan

Start vLLM server vllm serve openai/gpt-oss-20b

Send a Response request with enable_response_messages set to True in extra_body

resp = client.responses.create(
        model=model,
        input=prompt,
        extra_body={"enable_response_messages": True,}
)

print(resp.model_dump_json(indent=2))

Repeat the above for the streaming case.

Test Result

Original:

Details

``` "input_messages": [ { "author": { "role": "system", "name": null }, "content": [ {} ], "channel": null, "recipient": null, "content_type": null }, ... ], "output_messages": [ ... { "author": { "role": "assistant", "name": null }, "content": [ {} ], "channel": "final", "recipient": null, "content_type": null } ] } ```

After adding mode="json"

Details

``` "input_messages": [ { "role": "system", "name": null, "content": [ { "model_identity": "You are ChatGPT, a large language model trained by OpenAI.", "reasoning_effort": "Medium", "conversation_start_date": "2025-10-22", "knowledge_cutoff": "2024-06", "channel_config": { "valid_channels": [ "analysis", "final" ], "channel_required": true }, "type": "system_content" } ] }, { "role": "user", "name": null, "content": [ { "type": "text", "text": "Write a haiku about autumn leaves." } ] } ], "output_messages": [ { "role": "assistant", "name": null, "content": [ { "type": "text", "text": "User wants a haiku about autumn leaves. Simple. Use 5-7-5 syllable structure. Let's produce one. Ensure it's about autumn leaves. Provide in one paragraph." } ], "channel": "analysis" }, { "role": "assistant", "name": null, "content": [ { "type": "text", "text": "Leaves whisper, fall— \ncrimson and amber drift down, \nautumn sighs in wind." } ], "channel": "final" } ] } ```

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request correctly enables message serialization for Harmony Responses by calling model_dump(mode="json"). The change is a necessary fix for an upstream issue in openai/harmony, and the problem is well-described in the pull request. The code modification is simple, targeted, and correctly applied in both the create_responses and retrieve_responses functions. The inclusion of a TODO comment with a link to the upstream issue is good practice for maintainability. The change appears correct and complete, and I have no further suggestions.

[jacobthebanana]

(force-pushing to add sign-off)

[mergify]

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

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