FastMCP’s SSE endpoint is alive (that’s why you see session IDs in the browser) But Vapi expects a very specific MCP handshake + response format If anything is slightly off (headers, JSON shape, event naming, tool schema), Vapi just throws a 500 with no useful error If you want, you can just send: Your FastMCP server code How you’re starting it What URL you pasted into Vapi and let me help from there