tl;dr: MCP (Model Context Protocol) is an open standard that lets AI apps (hosts like Claude, Cursor, or ChatGPT) connect to external tools and data via small servers that expose capabilities. The model decides what to invoke, but the host app mediates all calls—models never talk to servers directly. It uses JSON-RPC as the envelope over stdin/stdout or a single HTTP endpoint, with MCP defining the methods, capability negotiation, transports, and auth on top.