Host, Client and Server

Three roles

The host is the AI application the user talks to (such as Claude Desktop, an IDE or your own agent). The host creates one client connection per server. The server is a program exposing tools, resources and prompts. They exchange JSON-RPC 2.0 messages, starting with an initialize handshake where both sides declare their capabilities.

A tool call on the wire

Simplified messages: the client asks the server to run a tool, and the server responds with content. Host applications hide this plumbing from you.

// client -> server
{ "jsonrpc": "2.0", "id": 7, "method": "tools/call",
  "params": { "name": "add", "arguments": { "a": 2, "b": 3 } } }

// server -> client
{ "jsonrpc": "2.0", "id": 7,
  "result": { "content": [ { "type": "text", "text": "5" } ] } }

One client per server

A host with three servers runs three client connections. Each connection is isolated, so one server cannot read another server's data directly; the host decides what the model sees from each.

Quick check: Which role runs the tools and data access in MCP?

• The host
• The user
• The server
• The model provider's billing system