A stateful MCP server using McpAgent backed by a Durable Object. State persists across requests — the built-in UI lets you call tools and read resources to see it in action.

• McpAgent — the Agents SDK class for building MCP servers with persistent state
• Tools — registering an add tool that modifies the counter
• Resources — exposing the counter value as an MCP resource
• State management — setState and onStateChanged for durable state
• Streamable HTTP transport — the default transport for McpAgent

npm install
npm run dev

Open the browser to see the built-in tool tester. You can also connect with the MCP Inspector — set the transport to Streamable HTTP and URL to http://localhost:5173/mcp.

The McpAgent class extends Agent with MCP protocol support. Define your tools and resources in the init() method:

export class MyMCP extends McpAgent<Env, State, {}> {
  server = new McpServer({ name: "Demo", version: "1.0.0" });
  initialState: State = { counter: 1 };

  async init() {
    this.server.resource("counter", "mcp://resource/counter", (uri) => ({
      contents: [{ text: String(this.state.counter), uri: uri.href }]
    }));

    this.server.registerTool(
      "add",
      { description: "Add to the counter", inputSchema: { a: z.number() } },
      async ({ a }) => {
        this.setState({ ...this.state, counter: this.state.counter + a });
        return {
          content: [{ type: "text", text: `Total: ${this.state.counter}` }]
        };
      }
    );
  }
}

export default MyMCP.serve("/mcp", { binding: "MyMCP" });

• mcp-worker — simplest stateless MCP server using createMcpHandler
• mcp-worker-authenticated — adding OAuth to an MCP server
• mcp-client — connecting to MCP servers as a client