What message format does MCP use for communication between clients and servers?

MCP uses JSON messages for all communication between clients and servers. Each message type serves a specific purpose, whether it is calling a tool, listing available resources, reading a prompt, or sending notifications about system events. The complete set of message types is defined in the official MCP specification repository on GitHub, which is separate from the SDK repositories. The specification uses TypeScript notation to describe data structures clearly, but the actual messages are plain JSON objects transmitted over the chosen transport.