Streaming works in backend, but Agent Chat UI does not display messages during stream

[he-or-h]

Description

I’m using a Python LangGraph backend with the Agent Chat UI (Next.js).
The backend correctly streams messages via text/event-stream, and I can see event: message and event: done in the browser Network tab.

However, messages never appear in the chat UI.

What works:

• Streaming backend is returning:

Transfer-Encoding: chunked
x-accel-buffering: no
Access-Control-Allow-Origin: *

• Events look correct:

event: message
data: {"id": "0", "role": "ai", "content": [{"type": "text", "text": "Hello"}]}
id: 0

• /threads/{id}/history contains the correct snapshots

What’s broken:

• The chat UI does not show any streamed messages
• No errors in console

Minimal backend example

from sse_starlette.sse import EventSourceResponse
from uuid import uuid4

@app.post("/threads/{thread_id}/runs/stream")
def run_stream(thread_id: str, payload: dict, request: Request):
    agent_id = payload.get("assistant_id")
    messages = payload.get("input")
    stream_mode = payload.get("stream_mode")

    if not agent_id or agent_id not in AVAILABLE_GRAPHS:
        raise HTTPException(status_code=400, detail="Invalid agent_id")

    graph = AVAILABLE_GRAPHS[agent_id]

    def event_stream():
        for i, (message, _) in enumerate(graph.stream(messages, config=RunnableConfig(configurable={"thread_id": thread_id}), stream_mode="messages")):
            yield {
                "id": str(i),
                "event": "message",
                "data": json.dumps({
                    "id": message.id or str(uuid4()),
                    "role": "ai",
                    "content": [{"type": "text", "text": message.content}]
                })
            }
        yield {"event": "done", "data": "{}"}

    return EventSourceResponse(event_stream())