[BUG] Uvicorn reload hangs when FastAPI-MCP has active SSE connections

[jagerzhang]

Describe the bug
When using FastAPI-MCP with SSE connections active, Uvicorn's reload functionality gets stuck during shutdown phase, requiring manual process termination. This severely impacts development workflow.

To Reproduce

1. Create minimal FastAPI app main.py with MCP mounted:

from fastapi import FastAPI
from fastapi_mcp import FastApiMCP

app = FastAPI()

mcp = FastApiMCP(app)

# Mount the MCP server directly to your FastAPI app
mcp.mount()

2. Start server with reload:

uvicorn --reload --port 9000 main:app

Open browser to SSE endpoint: http://localhost:9000/mcp and hold on:

3. Modify any file (e.g. main.py) to trigger reload

Expected Behavior:

Server should:

1. Detect file changes
2. Gracefully close existing connections
3. Restart with new code

Actual Behavior:

Server hangs at shutdown phase:
INFO:     Shutting down
INFO:     Waiting for connections to close. (CTRL+C to force quit)

System Information:
fastapi==0.115.12
fastapi-mcp==0.3.6
uvicorn==0.34.2
Python==3.12.10
OS: Linux

Proposed Solutions:

1. Add proper SSE connection tracking in FastAPI-MCP
2. Implement graceful shutdown handler for SSE
3. Provide API to manually close connections before reload

[sqrt676]

Hi,
I replicated and didn't face any issue, can you pls elaborate ?

[jagerzhang]

@sqrt676

Docker Reproduction Environment

1. ​File Structure​

fastapi-mcp-reload-issue/  
├── main.py              # FastAPI + MCP entrypoint  
└── Dockerfile           # Image build  

2. ​File Contents
​

• main.py

from fastapi import FastAPI
from fastapi_mcp import FastApiMCP

app = FastAPI()

mcp = FastApiMCP(app)

# Mount the MCP server directly to your FastAPI app
mcp.mount()

• Dockerfile

FROM python:3.12
WORKDIR /app
RUN pip install --no-cache-dir fastapi fastapi-mcp
EXPOSE 9000
CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "9000", "--reload"]

​3. Build Docker image

cd fastapi-mcp-reload-issue
docker build -t test-fastapi-mcp-reload

4. Start Docker Container

docker run --rm -ti -v "$(pwd):/app" -p 9000:9000 test-fastapi-mcp-reload  

5. Reproduction Steps​

Initiate SSE connection​ (new terminal):

curl -v -N http://localhost:9000/mcp

****Modify main.py​ (new terminal; e.g., add a comment):

cd fastapi-mcp-reload-issue
echo "# Trigger reload" >> main.py  

Active SSE connection:

Hanging reload process and can not stop with ctrl+c:

Reload proceeds normally ONLY AFTER terminating the curl request​ - stopping the SSE client connection allows the shutdown sequence to complete.