The Model Context Protocol (MCP) is an open standard connecting AI apps to external tools and data. A server declares what it can do, the host app shows that to the model, and when the model wants something the app calls the server and returns the result. The model never talks to a server directly. MCP runs as JSON-RPC over stdin/stdout or a single HTTP endpoint, with servers able to talk back. A worked example traces 11 messages across 5 steps: the app lists tools, adds 'create_issue' to context, the user asks to file a bug, the model requests the tool, and the server returns 'created #482'. The spec is dated 2026-07-28 and is stateless with no handshake but multiple round-trips.