Race condition in StreamableHTTP: zero-buffer memory streams cause deadlock with concurrent SSE responses

[Ctariy]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

Bug Description

SSE connections hang indefinitely when using StreamableHTTPServerTransport in stateless mode with responses containing 3+ items. The issue is caused by zero-buffer memory streams that block send() until receive() is called, creating a race condition between the response writer and the SSE stream iterator.

Related issues: #262 describes similar symptoms (client hangs on call_tool()) but root cause wasn't identified. This issue provides the specific cause and fix.

Expected Behavior

All tool responses should complete regardless of response size.

Actual Behavior

• 1-2 items: Response returns immediately (~150ms)
• 3+ items: Request hangs indefinitely (deadlock)

Root Cause Analysis

The issue is in mcp/server/streamable_http.py:

Line 412 - zero-buffer request stream
self._request_streams[request_id] = anyio.create_memory_object_streamEventMessage

Line 460 - zero-buffer SSE stream
sse_stream_writer, sse_stream_reader = anyio.create_memory_object_streamdict[str, str]

Race condition flow:

1. tg.start_soon(response, ...) starts SSE response task (non-blocking)
2. await writer.send(session_message) sends request to MCP server
3. MCP server processes quickly and calls message_router
4. message_router tries await request_streams[id][0].send(EventMessage(...))
5. DEADLOCK: If SSE writer hasn't started iterating yet, send() blocks forever

With zero-buffer streams, send() blocks until the receiver calls receive(). When the MCP server processes faster than the SSE writer task starts, deadlock occurs.

Why timing matters:

• Small responses (1-2 items): SSE writer task starts before MCP response arrives → works
• Larger responses (3+ items): MCP processes faster → response arrives before SSE iterator starts → blocked forever

Proposed Fix

Increase buffer size from 0 to a reasonable value (e.g., 10 or 100):

Line 412
self._request_streams[request_id] = anyio.create_memory_object_streamEventMessage

Line 460
sse_stream_writer, sse_stream_reader = anyio.create_memory_object_streamdict[str, str]

Alternative fix: Use await tg.start() instead of tg.start_soon() to ensure SSE writer is ready before sending requests (requires EventSourceResponse to support task status protocol).

Workaround

We've applied this fix via sed patch in our Dockerfile:

RUN sed -i 's/create_memory_object_stream[EventMessage](0)/create_memory_object_streamEventMessage/g'
/usr/local/lib/python3.11/site-packages/mcp/server/streamable_http.py &&
sed -i 's/create_memory_object_stream[dict[str, str]](0)/create_memory_object_streamdict[str, str]/g'
/usr/local/lib/python3.11/site-packages/mcp/server/streamable_http.py
This resolves the issue in our production environment.

Example Code

from fastmcp import FastMCP
import json

mcp = FastMCP("test-server")

@mcp.tool()
async def test_tool() -> str:
    # Returns JSON with 3+ items - will hang
    return json.dumps({
        "results": [{"n": "a"}, {"n": "b"}, {"n": "c"}]
    })

app = mcp.http_app(path="/mcp", stateless_http=True)

1. Call the tool via HTTP POST to /mcp
2. Response hangs indefinitely for tools returning 3+ items in arrays
3. Tools returning 1-2 items work correctly

Python & MCP Python SDK

- MCP SDK version: 1.23.3
- Python version: 3.11
- FastMCP version: 2.13.1
- Transport: StreamableHTTP with stateless_http=True

[maxisbey]

I wasn't able to reproduce this issue with using your example code. Would you be able to provide both server and client code which reliably causes this deadlock? Please also make sure it doesn't use FastMCP 2, which is a third party package separate to the official MCP Python SDK (this repo).

[Ctariy]

@maxisbey You're right that I use FastMCP 2.13.1, but the patch I apply is specifically to mcp/server/streamable_http.py - which is from this repository (official MCP SDK), not FastMCP. FastMCP just uses StreamableHTTPSessionManager internally.

We experienced frequent hangs (~50% of requests) in production with Claude Code connecting to our MCP server over the internet. After patching all create_memory_object_stream(0) to create_memory_object_stream(10) in streamable_http.py, the hangs stopped completely.

However, I tried again and I cannot provide a minimal reproduction that reliably triggers the issue.

My hypothesis: Network latency - Claude Code connects over internet, not localhost. HTTP/SSE setup takes longer, creating a window where writer tries to send before reader is ready.

[maxisbey]

Hmm well that makes it quite hard to investigate to be honest. Without a reliable way of reproducing it, I can't really confirm any fix works or not :/

If you end up finding a reliable way to reproduce it, I'd really appreciate it!