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McpAgent — API Reference

When you build MCP Servers on Cloudflare, you extend the McpAgent class ↗ from the Agents SDK:

JavaScript
import { McpAgent } from "agents/mcp";
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";


export class MyMCP extends McpAgent {
  server = new McpServer({ name: "Demo", version: "1.0.0" });


  async init() {
    this.server.tool(
      "add",
      { a: z.number(), b: z.number() },
      async ({ a, b }) => ({
        content: [{ type: "text", text: String(a + b) }],
      }),
    );
  }
}

This means that each instance of your MCP server has its own durable state, backed by a Durable Object, with its own SQL database.

Your MCP server doesn't necessarily have to be an Agent. You can build MCP servers that are stateless, and just add tools to your MCP server using the @modelcontextprotocol/sdk package.

But if you want your MCP server to:

  • remember previous tool calls, and responses it provided
  • provide a game to the MCP client, remembering the state of the game board, previous moves, and the score
  • cache the state of a previous external API call, so that subsequent tool calls can reuse it
  • do anything that an Agent can do, but allow MCP clients to communicate with it

You can use the APIs below in order to do so.

API overview

Property/MethodDescription
stateCurrent state object (persisted)
initialStateDefault state when instance starts
setState(state)Update and persist state
onStateUpdate(state)Called when state changes
sqlExecute SQL queries on embedded database
serverThe McpServer instance for registering tools
propsUser identity and tokens from OAuth authentication
elicitInput(options, context)Request structured input from user
McpAgent.serve(path, options)Static method to create a Worker handler

Deploying with McpAgent.serve()

The McpAgent.serve() static method creates a Worker handler that routes requests to your MCP server:

JavaScript
import { McpAgent } from "agents/mcp";
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";


export class MyMCP extends McpAgent {
  server = new McpServer({ name: "my-server", version: "1.0.0" });


  async init() {
    this.server.tool("square", { n: z.number() }, async ({ n }) => ({
      content: [{ type: "text", text: String(n * n) }],
    }));
  }
}


// Export the Worker handler
export default MyMCP.serve("/mcp");

This is the simplest way to deploy an MCP server — about 15 lines of code. The serve() method handles Streamable HTTP transport automatically.

With OAuth authentication

When using the OAuth Provider Library ↗, pass your MCP server to apiHandlers:

JavaScript
import { OAuthProvider } from "@cloudflare/workers-oauth-provider";


export default new OAuthProvider({
  apiHandlers: { "/mcp": MyMCP.serve("/mcp") },
  authorizeEndpoint: "/authorize",
  tokenEndpoint: "/token",
  clientRegistrationEndpoint: "/register",
  defaultHandler: AuthHandler,
});

Data jurisdiction

For GDPR and data residency compliance, specify a jurisdiction to ensure your MCP server instances run in specific regions:

JavaScript
// EU jurisdiction for GDPR compliance
export default MyMCP.serve("/mcp", { jurisdiction: "eu" });

With OAuth:

JavaScript
export default new OAuthProvider({
  apiHandlers: {
    "/mcp": MyMCP.serve("/mcp", { jurisdiction: "eu" }),
  },
  // ... other OAuth config
});

When you specify jurisdiction: "eu":

  • All MCP session data stays within the EU
  • User data processed by your tools remains in the EU
  • State stored in the Durable Object stays in the EU

Available jurisdictions include "eu" (European Union) and "fedramp" (FedRAMP compliant locations). Refer to Durable Objects data location for more options.

Hibernation support

McpAgent instances automatically support WebSockets Hibernation, allowing stateful MCP servers to sleep during inactive periods while preserving their state. This means your agents only consume compute resources when actively processing requests, optimizing costs while maintaining the full context and conversation history.

Hibernation is enabled by default and requires no additional configuration.

Authentication and authorization

The McpAgent class provides seamless integration with the OAuth Provider Library ↗ for authentication and authorization.

When a user authenticates to your MCP server, their identity information and tokens are made available through the props parameter, allowing you to:

  • access user-specific data
  • check user permissions before performing operations
  • customize responses based on user attributes
  • use authentication tokens to make requests to external services on behalf of the user

State synchronization APIs

The McpAgent class provides full access to the Agent state APIs:

  • state — Current persisted state
  • initialState — Default state when instance starts
  • setState — Update and persist state
  • onStateUpdate — React to state changes
  • sql — Execute SQL queries on embedded database

For example, the following code implements an MCP server that remembers a counter value, and updates the counter when the add tool is called:

JavaScript
import { McpAgent } from "agents/mcp";
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";


export class MyMCP extends McpAgent {
  server = new McpServer({
    name: "Demo",
    version: "1.0.0",
  });


  initialState = {
    counter: 1,
  };


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


    this.server.tool(
      "add",
      "Add to the counter, stored in the MCP",
      { a: z.number() },
      async ({ a }) => {
        this.setState({ ...this.state, counter: this.state.counter + a });


        return {
          content: [
            {
              type: "text",
              text: String(`Added ${a}, total is now ${this.state.counter}`),
            },
          ],
        };
      },
    );
  }


  onStateUpdate(state) {
    console.log({ stateUpdate: state });
  }
}

Elicitation (human-in-the-loop)

MCP servers can request additional user input during tool execution using elicitation. The MCP client (like Claude Desktop) renders a form based on your JSON Schema and returns the user's response.

When to use elicitation

  • Request structured input that was not part of the original tool call
  • Confirm high-stakes operations before proceeding
  • Gather additional context or preferences mid-execution

elicitInput(options, context)

Request structured input from the user during tool execution.

Parameters:

ParameterTypeDescription
options.messagestringMessage explaining what input is needed
options.requestedSchemaJSON SchemaSchema defining the expected input structure
context.relatedRequestIdstringThe extra.requestId from the tool handler

Returns: Promise<{ action: "accept" | "decline", content?: object }>

JavaScript
import { McpAgent } from "agents/mcp";
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";


export class CounterMCP extends McpAgent {
  server = new McpServer({
    name: "counter-server",
    version: "1.0.0",
  });


  initialState = { counter: 0 };


  async init() {
    this.server.tool(
      "increase-counter",
      "Increase the counter by a user-specified amount",
      { confirm: z.boolean().describe("Do you want to increase the counter?") },
      async ({ confirm }, extra) => {
        if (!confirm) {
          return { content: [{ type: "text", text: "Cancelled." }] };
        }


        // Request additional input from the user
        const userInput = await this.server.server.elicitInput(
          {
            message: "By how much do you want to increase the counter?",
            requestedSchema: {
              type: "object",
              properties: {
                amount: {
                  type: "number",
                  title: "Amount",
                  description: "The amount to increase the counter by",
                },
              },
              required: ["amount"],
            },
          },
          { relatedRequestId: extra.requestId },
        );


        // Check if user accepted or cancelled
        if (userInput.action !== "accept" || !userInput.content) {
          return { content: [{ type: "text", text: "Cancelled." }] };
        }


        // Use the input
        const amount = Number(userInput.content.amount);
        this.setState({
          ...this.state,
          counter: this.state.counter + amount,
        });


        return {
          content: [
            {
              type: "text",
              text: `Counter increased by ${amount}, now at ${this.state.counter}`,
            },
          ],
        };
      },
    );
  }
}

JSON Schema for forms

The requestedSchema defines the form structure shown to the user:

TypeScript
const schema = {
  type: "object",
  properties: {
    // Text input
    name: {
      type: "string",
      title: "Name",
      description: "Enter your name",
    },
    // Number input
    amount: {
      type: "number",
      title: "Amount",
      minimum: 1,
      maximum: 100,
    },
    // Boolean (checkbox)
    confirm: {
      type: "boolean",
      title: "I confirm this action",
    },
    // Enum (dropdown)
    priority: {
      type: "string",
      enum: ["low", "medium", "high"],
      title: "Priority",
    },
  },
  required: ["name", "amount"],
};

Handling responses

JavaScript
const result = await this.server.server.elicitInput(
  { message: "Confirm action", requestedSchema: schema },
  { relatedRequestId: extra.requestId },
);


switch (result.action) {
  case "accept":
    // User submitted the form
    const { name, amount } = result.content;
    // Process the input...
    break;


  case "decline":
    // User cancelled
    return { content: [{ type: "text", text: "Operation cancelled." }] };
}

For more human-in-the-loop patterns including workflow-based approval, refer to Human-in-the-loop patterns.

Next steps

Build a Remote MCP server Get started with MCP servers on Cloudflare.
MCP Tools Design and add tools to your MCP server.
Authorization Set up OAuth authentication.
Securing MCP servers Security best practices for production.
createMcpHandler Build stateless MCP servers.