It's a spec for how external info can get into a language model's context window. Part of that is tool calling, but there's also templated input, cancellation, progress tracking, recursive generation sampling, statefulness of tool servers, etc.

There's a MCP "server" spec for writing code that exposes tool definitions and the ability to execute them, and an MCP "client" spec for building systems that can make use of those servers.

it's more than just tools. there're prompts & resources too