[gpt-oss] use vLLM instead of openai types for streaming

[qandrew]

Purpose

• with changes such as [Frontend] Responses API messages out, just harmony for now #24985, vLLM types are diverging from the openai-python types. We'll need to send the extra payload of our vLLM types in streaming, this PR fixes such
• for the future? move streaming to a separate protocol.py file

Test Plan

Test Result

client

curl http://localhost:20001/v1/responses   -H "Content-Type: application/json"   -N   -d '{
    "model": "/data/users/axia/checkpoints/gpt-oss-120b",
    "input": [
        {
            "role": "user",
            "content": "Hello."
        }
    ],
    "temperature": 0.7,
    "max_output_tokens": 256,
    "stream": true, 
    "enable_response_messages": true
}'  

ResponseCompletedEvent output (see that the input/output messages are there)

{
    "response": {
        "id": "resp_aed05365be36434fb820a6eab52bfc39",
        "created_at": 1758328028,
        "incomplete_details": null,
        "instructions": null,
        "metadata": null,
        "model": "/data/users/axia/checkpoints/gpt-oss-120b",
        "object": "response",
        "output": [
            {
                "id": "rs_2ebdf86c76a54ddb85c5a867c31f9b77",
                "summary": [],
                "type": "reasoning",
                "content": [
                    {
                        "text": "The user says \"Hello.\" Probably a greeting. Should respond politely, maybe ask how can help. Keep brief friendly.",
                        "type": "reasoning_text"
                    }
                ],
                "encrypted_content": null,
                "status": null
            },
            {
                "id": "msg_30edf4e235b546078b529ebfaa6d7bee",
                "content": [
                    {
                        "annotations": [],
                        "text": "Hello! How can I assist you today?",
                        "type": "output_text",
                        "logprobs": null
                    }
                ],
                "role": "assistant",
                "status": "completed",
                "type": "message"
            }
        ],
        "input_messages": [
            {
                "author": {
                    "role": "system",
                    "name": null
                },
                "content": [
                    {}
                ],
                "channel": null,
                "recipient": null,
                "content_type": null
            },
            {
                "author": {
                    "role": "user",
                    "name": null
                },
                "content": [
                    {}
                ],
                "channel": null,
                "recipient": null,
                "content_type": null
            }
        ],
        "output_messages": [
            {
                "author": {
                    "role": "assistant",
                    "name": null
                },
                "content": [
                    {}
                ],
                "channel": "analysis",
                "recipient": null,
                "content_type": null
            },
            {
                "author": {
                    "role": "assistant",
                    "name": null
                },
                "content": [
                    {}
                ],
                "channel": "final",
                "recipient": null,
                "content_type": null
            }
        ],
        "parallel_tool_calls": true,
        "temperature": 0.7,
        "tool_choice": "auto",
        "tools": [],
        "top_p": 1.0,
        "background": false,
        "max_output_tokens": 256,
        "max_tool_calls": null,
        "previous_response_id": null,
        "prompt": null,
        "reasoning": null,
        "service_tier": "auto",
        "status": "completed",
        "text": null,
        "top_logprobs": null,
        "truncation": "disabled",
        "usage": {
            "input_tokens": 67,
            "input_tokens_details": {
                "cached_tokens": 64
            },
            "output_tokens": 43,
            "output_tokens_details": {
                "reasoning_tokens": 25,
                "tool_output_tokens": 0
            },
            "total_tokens": 110
        },
        "user": null
    },
    "sequence_number": 45,
    "type": "response.completed"
}

---

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.

[mergify]

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

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

[qandrew]

cc @alecsolder , @yeqcharlotte , @chaunceyjiang ready for review. i think the readthedocs CI failure is a flake

[lacora]

maybe we need to think about whether allowing input_messages / output_messages for sync responses only or for streaming as well, appending long messages kills the efficiency that streaming provides cc @yeqcharlotte

[qandrew]

maybe we need to think about whether allowing input_messages / output_messages for sync responses only or for streaming as well, appending long messages kills the efficiency that streaming provides cc @yeqcharlotte

that's a good point! this PR only modifies events in streaming right now (ResponseCreatedEvent, ResponseInProgressEvent, ResponseCompletedEvent), so none of the deltas / intermediate events will not get additional payload. Or i could remove from ResponseCreatedEvent, ResponseInProgressEvent but maybe it's fine to keep it for now for consistency?

[yeqcharlotte]

LGTM. could you add a basic skeleton tests that load the type. cc:
@chaunceyjiang @aarnphm PTAL too thanks!

[yeqcharlotte]

is the format change intentional

[qandrew]

yep, yapf/isort had a conflict, it seems like it happens pretty often...

[yeqcharlotte]

can we keep the original format?

[qandrew]

@yeqcharlotte we can't. The introduction of below makes the conflict, i tried to but there will be conflicts. You can see in serving_responses.py below # yapf conflicts with isort for this block happens often

from openai.types.responses import (
    ResponseInProgressEvent as OpenAIResponseInProgressEvent)

[yeqcharlotte]

LGTM! thanks for the change!