Streamable HTTP: ASGI application returns before the full SSE body is sent

[jonpspri]

• I confirm that I'm using the newest release of my line (mcp 2.2.0, and mcp 1.30.0 for the 1.x comparison)
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Release line: 2.x (current stable) — also reproducible on the 1.x maintenance line, see below.

Description

On mcp 1.x and 2.x, a streamable-HTTP POST response can stop before the server sends the full SSE body. uvicorn writes ASGI callable returned without completing response and closes the connection. The client receives httpx.RemoteProtocolError: peer closed connection without sending complete message body. The failure occurs on the first initialize POST of a session. Session setup fails.

We observed this behavior in GitHub Actions and in controlled reproduction environments.

The failure rate grows with two factors: CPU contention, and repeated uvicorn server start/stop cycles in one process.

Verified results, each with a fresh server per attempt:

Experiment	mcp 2.2.0	mcp 1.30.0
One server per process, moderate CPU load	24 of 25 runs failed	0 of 25 runs failed
20 servers in one process, 1 CPU limit	17 of 20 failed	17 of 20 failed
One server, 200 sequential POSTs	0 failed	0 failed

In the 20-server test, the first two or three servers pass. All later servers fail. One reused server never fails. This pattern points at state that accumulates across server restarts on one event loop.

Expected behavior

The server must always send the last body chunk of the response. The client must always receive the complete stream for a request that has an answer.

Suspected cause (inference)

This section is an inference from code reading.

• The POST handler sends the response through an EventSourceResponse object.
• The response runs several helper tasks in one shared task group.
• The first task that ends cancels all other tasks in the group.
• The stream task sets active = False before it sends the last body chunk.
• If the last send operation waits, the disconnect task can end first and cancel the send.
• The h11 receive() function in uvicorn returns immediately after it consumes the request body. This lets the disconnect task run in a loop.

Example Code

"""Repro: streamable-HTTP SSE response terminates mid-body."""
import anyio, httpx, uvicorn
from fastmcp import FastMCP

mcp = FastMCP("repro")

@mcp.tool()
def echo(message: str) -> str:
    return message

INIT = {
    "jsonrpc": "2.0", "id": 1, "method": "initialize",
    "params": {"protocolVersion": "2025-06-18", "capabilities": {},
               "clientInfo": {"name": "repro", "version": "0"}},
}
HEADERS = {"Accept": "application/json, text/event-stream"}

async def one_attempt() -> str | None:
    config = uvicorn.Config(mcp.http_app(transport="streamable-http"),
                            host="127.0.0.1", port=0, log_level="error")
    server = uvicorn.Server(config)
    async with anyio.create_task_group() as tg:
        tg.start_soon(server.serve)
        while not server.started:
            await anyio.sleep(0.05)
        port = server.servers[0].sockets[0].getsockname()[1]
        try:
            async with httpx.AsyncClient() as client:
                r = await client.post(f"http://127.0.0.1:{port}/mcp", json=INIT, headers=HEADERS)
                r.raise_for_status()
        except (httpx.HTTPError, RuntimeError) as error:
            return f"{type(error).__name__}: {error}"
        finally:
            server.should_exit = True
    return None

async def main() -> None:
    failures = 0
    for attempt in range(20):
        if (error := await one_attempt()) is not None:
            failures += 1
            print(f"attempt {attempt}: {error}")
    print(f"RESULT: {failures}/20 initialize POSTs failed")

anyio.run(main)

Run it in a CPU-limited container:

1. Start a container: docker run --rm --cpus=1 -it python:3.13-slim bash.
2. Install the dependencies: pip install fastmcp==4.0.3 uvicorn==0.52.4 httpx anyio.
3. Run the script.

Expected result: 20 of 20 attempts pass. Actual result: approximately 17 of 20 attempts fail from the third or fourth server onwards.

Python & MCP Python SDK

Python 3.13; mcp 2.2.0 (newest 2.x) and mcp 1.30.0 (newest 1.x); fastmcp 4.0.3 / 3.4.7; uvicorn 0.52.4 (h11); starlette 1.6.0; anyio 4.14.2; httpx 0.28.1; Linux (Ubuntu 24.04, Debian slim; x86_64 and arm64). The closest existing issues (#2150, #3441) describe different defects.