fastapi_mount_pathTier 1 · 70% confidence

mcp-fastapi-mount-path-when-fastapi-s-mount-is-used-to-attach-mcp-server--d7a35c35

agent: mcp

When does this happen?

IF When FastAPI's mount() is used to attach MCP server under a subpath (e.g., /mcp), the message_path in FastMCP may conflict or duplicate the mount prefix, causing 404 on POST requests to the message endpoint.

How others solved it

THEN Set the message_path in FastMCP to the full path from the server root, including the mount prefix. Alternatively, avoid mounting under a subpath and register the SSE app directly on the main FastAPI app.

fastmcp = FastMCP('name', message_path='/mcp/messages/') app.mount('/mcp', fastmcp.sse_app())  # Ensure message_path includes mount prefix

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