Requests to fastapi-mcp hangs if Sentry is enabled

[emirsavran]

How do you use Sentry?

Sentry Saas (sentry.io)

Version

2.36.0

Steps to Reproduce

1. Clone the example repo
2. Run uv sync
3. Add DSN to with_sentry.py
4. Run uv run with_sentry.py
5. On another terminal, run MCP inspector with npx @modelcontextprotocol/inspector
6. Select "Streamable HTTP" as transport type
7. Enter "http://localhost:8000/mcp" as URL
8. 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

[linear]

PY-1831 Requests to fastapi-mcp hangs if Sentry is enabled

[szokeasaurusrex]

Thanks for the reproduction @emirsavran. I was able to reproduce, also on 2.37.0.

We will investigate this issue and release a fix as soon as we can!

[alexander-alderman-webb]

Hi @emirsavran,

A temporary solution to prevent the inspector hanging is setting the max_request_body_size as below. Setting the option to never prevents Sentry from reading the body of the inspector's HTTP request.

sentry_sdk.init(
    ...
+   max_request_body_size="never",
)

We will keep working on a fix since the SDK should not cause hangs.

[alexander-alderman-webb]

So fastapi_mcp is already overriding a method from a class in the mcp repository, but this only helps with POST requests using the SseServerTransport. There are two transport options in fastapi_mcp: SseServerTransport and StreamableHTTPTransport.

https://github.com/tadata-org/fastapi_mcp/blob/e5cad13cabfc725bbcb047e526816d887d96da62/fastapi_mcp/transport/sse.py#L19-L35

The mcp library only works on the low-level ASGI interface. The interface is comprised of scope, receive() and send(). Reading the request body through the high-level FastAPI accessors deprives the receive() callable, leading to the MCP client hanging. The FastAPI and Starlette integrations use the high-level API here:

sentry-python/sentry_sdk/integrations/starlette.py

Lines 626 to 649 in 5a122b5

	# Add JSON body, if it is a JSON request
	json = await self.json()
	if json:
	request_info["data"] = json
	return request_info
	# Add form as key/value pairs, if request has form data
	form = await self.form()
	if form:
	form_data = {}
	for key, val in form.items():
	is_file = isinstance(val, UploadFile)
	form_data[key] = (
	val
	if not is_file
	else AnnotatedValue.removed_because_raw_data()
	)
	request_info["data"] = form_data
	return request_info
	# Raw data, do not add body just an annotation
	request_info["data"] = AnnotatedValue.removed_because_raw_data()
	return request_info