The Client SDK

Building Your Own MCP Client

Most developers interact with MCP through Host applications like Claude Desktop. But what if you want to build your own application that connects to MCP servers? You need the Client SDK.

When to Build a Custom Client

Client Connection Setup

import { Client } from "@modelcontextprotocol/sdk/client/index.js";
import { StdioClientTransport } from "@modelcontextprotocol/sdk/client/stdio.js";

// 1. Create client
const client = new Client({
  name: "my-app",
  version: "1.0.0"
}, {
  capabilities: {
    // Declare what your client supports
    roots: { listChanged: true },
    sampling: {}
  }
});

// 2. Create transport (launches server as child process)
const transport = new StdioClientTransport({
  command: "node",
  args: ["./path/to/server/dist/index.js"],
  env: { NOTES_DIR: "./notes" }
});

// 3. Connect (performs initialization handshake)
await client.connect(transport);
console.log("Connected! Server capabilities:", client.getServerCapabilities());

The Initialization Handshake

1. Client → Server: initialize — sends client name, version, capabilities
2. Server → Client: Response with server name, version, capabilities
3. Client → Server: initialized — confirms handshake complete