Requests to fastapi-mcp hangs if Sentry is enabled #4764
Description
How do you use Sentry?
Sentry Saas (sentry.io)
Version
2.36.0
Steps to Reproduce
- Clone the example repo
- Run
uv sync - Add DSN to
with_sentry.py - Run
uv run with_sentry.py - On another terminal, run MCP inspector with
npx @modelcontextprotocol/inspector - Select "Streamable HTTP" as transport type
- Enter "http://localhost:8000/mcp" as URL
- See unsuccessful request.
Expected Result
MCP inspector should connect and show the tools like this:
You can also find the working version in main.py. Just follow the repro steps, but instead of number (4), run uv run main.py.
Actual Result
Requests are somehow hanging. Here are the logs:
DEBUG: 2025-09-05 01:00:14 [mcp.server.lowlevel.server] Initializing server 'FastAPI'
DEBUG: 2025-09-05 01:00:14 [mcp.server.lowlevel.server] Registering handler for ListToolsRequest
DEBUG: 2025-09-05 01:00:14 [mcp.server.lowlevel.server] Registering handler for CallToolRequest
INFO: 2025-09-05 01:00:14 [fastapi_mcp.server] No auth config provided, skipping auth setup
INFO: 2025-09-05 01:00:14 [fastapi_mcp.server] No auth config provided, skipping auth setup
INFO: 2025-09-05 01:00:14 [fastapi_mcp.server] MCP HTTP server listening at /mcp
INFO: 2025-09-05 01:00:14 [fastapi_mcp.server] MCP HTTP server listening at /mcp
DEBUG: 2025-09-05 01:00:14 [asyncio] Using selector: KqueueSelector
INFO: Started server process [71787]
INFO: Waiting for application startup.
INFO: Application startup complete.
INFO: Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit)
DEBUG: 2025-09-05 01:00:19 [fastapi_mcp.transport.http] Starting StreamableHTTP session manager
DEBUG: 2025-09-05 01:00:19 [fastapi_mcp.transport.http] Starting StreamableHTTP session manager
INFO: 2025-09-05 01:00:19 [mcp.server.streamable_http_manager] StreamableHTTP session manager started
INFO: 2025-09-05 01:00:19 [fastapi_mcp.transport.http] StreamableHTTP session manager is running
INFO: 2025-09-05 01:00:19 [fastapi_mcp.transport.http] StreamableHTTP session manager is running
DEBUG: 2025-09-05 01:00:19 [fastapi_mcp.transport.http] Handling FastAPI request: POST /mcp
DEBUG: 2025-09-05 01:00:19 [fastapi_mcp.transport.http] Handling FastAPI request: POST /mcp
DEBUG: 2025-09-05 01:00:19 [mcp.server.streamable_http_manager] Creating new transport
INFO: 2025-09-05 01:00:19 [mcp.server.streamable_http_manager] Created new transport with session ID: cc8a639a06b94dc89bc82aac77620897
DEBUG: 2025-09-05 01:00:19 [urllib3.connectionpool] Starting new HTTPS connection (1): o4506478296367104.ingest.us.sentry.io:443
DEBUG: 2025-09-05 01:00:20 [urllib3.connectionpool] https://o4506478296367104.ingest.us.sentry.io:443 "POST /api/4509963382161408/envelope/ HTTP/1.1" 200 0
Metadata
Metadata
Assignees
Labels
Projects
Status