I Tried to Deploy My MCP Server to Vercel. Here's What Actually Happened.

I built a working MCP server. It connected to my database, returned tool results, and worked flawlessly in Claude Desktop locally. Then I pushed to Vercel. TypeError: Cannot read properties of undefined (reading 'addEventListener') 500 errors everywhere. The MCP adapter was trying to use persistent SSE connections inside ephemeral serverless functions. Everything broke — and it wasn't obvious why or how to fix it. I wasn't alone. This is a known, documented problem across the community. Why MCP and Serverless Don't Get Along MCP was designed for long-lived processes. The original spec only supported two transports: stdio (local-only) and SSE (persistent server-sent events over HTTP). Both assume the server stays alive between calls. Vercel Functions don't work that way. Each request can land on a different function instance. Memory is ephemeral. There's no persistent filesystem. And SSE connections stored in memory — poof, gone on the next cold start. The result is a mess developers ac