[gpt-oss][1a] create_responses stream outputs BaseModel type, api server is SSE still (#24759)

Signed-off-by: Andrew Xia <axia@meta.com>

vllm/entrypoints/openai/api_server.py

@@ -15,7 +15,7 @@ import socket
 import tempfile
 import uuid
 from argparse import Namespace
-from collections.abc import AsyncIterator, Awaitable
+from collections.abc import AsyncGenerator, AsyncIterator, Awaitable
 from contextlib import asynccontextmanager
 from functools import partial
 from http import HTTPStatus
@@ -29,6 +29,7 @@ from fastapi import APIRouter, Depends, FastAPI, Form, HTTPException, Request
 from fastapi.exceptions import RequestValidationError
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse, Response, StreamingResponse
+from openai import BaseModel
 from prometheus_client import make_asgi_app
 from prometheus_fastapi_instrumentator import Instrumentator
 from starlette.concurrency import iterate_in_threadpool
@@ -577,6 +578,18 @@ async def show_version():
     return JSONResponse(content=ver)
 
 
+async def _convert_stream_to_sse_events(
+        generator: AsyncGenerator[BaseModel,
+                                  None]) -> AsyncGenerator[str, None]:
+    """Convert the generator to a stream of events in SSE format"""
+    async for event in generator:
+        event_type = getattr(event, 'type', 'unknown')
+        # https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#event_stream_format
+        event_data = (f"event: {event_type}\n"
+                      f"data: {event.model_dump_json(indent=None)}\n\n")
+        yield event_data
+
+
 @router.post("/v1/responses",
              dependencies=[Depends(validate_json_request)],
              responses={
@@ -612,7 +625,9 @@ async def create_responses(request: ResponsesRequest, raw_request: Request):
                             status_code=generator.error.code)
     elif isinstance(generator, ResponsesResponse):
         return JSONResponse(content=generator.model_dump())
-    return StreamingResponse(content=generator, media_type="text/event-stream")
+
+    return StreamingResponse(content=_convert_stream_to_sse_events(generator),
+                             media_type="text/event-stream")
 
 
 @router.get("/v1/responses/{response_id}")
@@ -640,10 +655,10 @@ async def retrieve_responses(
     if isinstance(response, ErrorResponse):
         return JSONResponse(content=response.model_dump(),
                             status_code=response.error.code)
-    elif stream:
-        return StreamingResponse(content=response,
-                                 media_type="text/event-stream")
-    return JSONResponse(content=response.model_dump())
+    elif isinstance(response, ResponsesResponse):
+        return JSONResponse(content=response.model_dump())
+    return StreamingResponse(content=_convert_stream_to_sse_events(response),
+                             media_type="text/event-stream")
 
 
 @router.post("/v1/responses/{response_id}/cancel")