Three roles: host, client, Server

MCP has three distinct roles, and understanding them is key to everything else. Let's walk through each one using our DevTools AI Assistant example.

The Host is the AI application itself: Claude Desktop, Cursor, your DevTools Assistant. It's the user-facing app that contains one or more MCP clients. Think of it as the building that houses everything. The host orchestrates the conversation: it receives user queries, passes them to the LLM, and coordinates tool calls through its clients.

The MCP Client lives inside the host. Each client maintains a 1:1 connection with exactly one MCP server. Its job is protocol management: connecting, discovering what the server can do, and routing tool calls. If your host talks to 3 servers, it has 3 MCP clients.

Exactly. Each MCP client has a strict 1:1 relationship with one server. If you need to connect to three different servers, your host creates three separate clients, one per server. This keeps connections isolated so a problem with one server does not affect the others.

The MCP Server is a lightweight program that wraps one data source or service. This is where the actual work happens: searching arXiv, querying Wikipedia, reading files. Each server exposes its capabilities through the MCP protocol, and any MCP client can use them.

Good question. The separation matters because a single host can manage multiple clients simultaneously. Claude Desktop might connect to a filesystem server, a database server, and a web search server all at once. Each client handles its own server connection independently. The host coordinates them all, but each client-server pair is isolated.