Hosts, Clients and Servers

Architecture

MCP splits the work into three roles: the host connects to the model, a client handles each connection, and a server provides the tools and data.

MCP Hoste.g. Claude Code

stdiostdioHTTP

LLMoften a cloud API

Host appchat, approvals

Client 1

Client 2

Client 3

Filesystem serverlocal process

Database serverlocal process

GitHub serverremote, over HTTPS

GitHub API

Host

The AI application that talks to the LLM, asks users before tools run, and runs one client per server. Examples: Claude Code, Claude Desktop, ChatGPT, Cursor and VS Code.

Client

A connector inside the host that keeps a stateful connection to exactly one server. It negotiates capabilities and relays messages.

Server

A program that exposes tools, resources and prompts over MCP. It runs locally as a subprocess or remotely as a web service.