Python: [Bug]: AG-UI client skips SSE events written as "data:" with no space #10248

Observed Behavior

AGUIHttpService only reads lines that start with data: (with a space). In SSE the space after data: is optional, and some servers send data:{...}. All of those events are skipped, so AGUIChatClient returns an empty response and no error.

Expected Behavior

Both data: {...} and data:{...} are parsed, as the SSE spec says: one leading space after the colon is removed if present.

Steps to Reproduce

1. Point AGUIChatClient at a server that writes SSE lines as data:{...}
2. Call get_response
3. The response text is empty

Minimal Reproduction

import asyncio
import json

import httpx

from agent_framework_ag_ui import AGUIChatClient

EVENTS = [
    {"type": "RUN_STARTED", "threadId": "t1", "runId": "r1"},
    {"type": "TEXT_MESSAGE_START", "messageId": "m1", "role": "assistant"},
    {"type": "TEXT_MESSAGE_CONTENT", "messageId": "m1", "delta": "Hello from the server"},
    {"type": "TEXT_MESSAGE_END", "messageId": "m1"},
    {"type": "RUN_FINISHED", "threadId": "t1", "runId": "r1"},
]


async def run(prefix: str) -> None:
    def handler(request: httpx.Request) -> httpx.Response:
        body = "".join(f"{prefix}{json.dumps(e)}\n\n" for e in EVENTS)
        return httpx.Response(200, headers={"content-type": "text/event-stream"}, text=body)

    http = httpx.AsyncClient(transport=httpx.MockTransport(handler))
    client = AGUIChatClient(endpoint="http://server/", http_client=http)
    response = await client.get_response("hi")
    print(f"{prefix!r:9} -> {response.text!r}")


asyncio.run(run("data: "))
asyncio.run(run("data:"))

Error Messages and Stack Traces

'data: '  -> 'Hello from the server'
'data:'   -> ''

Package Versions

agent-framework-core 1.25.0, agent-framework-ag-ui 1.8.0 (main at fe87fdd), httpx 0.28.1

Python Version

Python 3.12

Operating System

macOS

Regression

No

Additional Context

The check is line.startswith("data: ") in post_run in _http_service.py. A fix could be to match data: and drop one leading space if there is one. I have a fix with a test and will open a PR.

AI assistance: I used an AI assistant to help find the cause and write this up. I ran the repro myself.

Acknowledgements

• I searched existing issues and did not find a duplicate.
• I personally verified this behavior and the reproduction details are authentic.