The Client SDK — Client Development

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 (@modelcontextprotocol/client in TypeScript SDK v2, succeeding v1's @modelcontextprotocol/sdk/client).

When to Build a Custom Client

Client Connection Setup

// TypeScript SDK v2 (@modelcontextprotocol/client)
// v1 legacy path: "@modelcontextprotocol/sdk/client/index.js" & "stdio.js"
import { Client, StdioClientTransport } from "@modelcontextprotocol/client";

// 1. Create client
const client = new Client({
  name: "my-app",
  version: "1.0.0"
}, {
  capabilities: {
    // Declare client capabilities (e.g., elicitation; roots/sampling are deprecated in 2026-07-28)
    elicitation: { form: {}, url: {} }
  }
});

// 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 (discovers server capabilities automatically)
await client.connect(transport);
console.log("Connected! Server capabilities:", client.getServerCapabilities());

Capability Discovery & Protocol Negotiation

When you call client.connect(transport), the Client SDK negotiates protocol versions and capabilities automatically:

• Stateless 2026-07-28 (Current): Calls server/discover to query supported protocol versions, capabilities, and server identity, then sends protocolVersion and clientCapabilities inline inside _meta on each request - no stateful handshake required.
• Legacy 2024-11-05 – 2025-11-25 Fallback: Performs the 3-step stateful handshake (initialize → server capabilities response → notifications/initialized).