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Connecting to MCP servers

Connect your agent to external MCP (Model Context Protocol) servers to use their tools, resources, and prompts. This enables your agent to interact with GitHub, Slack, databases, and other services through a standardized protocol.

Overview

The MCP client capability lets your agent:

  • Connect to external MCP servers - GitHub, Slack, databases, AI services
  • Use their tools - Call functions exposed by MCP servers
  • Access resources - Read data from MCP servers
  • Use prompts - Leverage pre-built prompt templates

Quick start

JavaScript
import { Agent } from "agents";


export class MyAgent extends Agent {
  async onRequest(request) {
    // Add an MCP server
    const result = await this.addMcpServer(
      "github",
      "https://mcp.github.com/mcp",
    );


    if (result.state === "authenticating") {
      // Server requires OAuth - redirect user to authorize
      return Response.redirect(result.authUrl);
    }


    // Server is ready - tools are now available
    const state = this.getMcpServers();
    console.log(`Connected! ${state.tools.length} tools available`);


    return new Response("MCP server connected");
  }
}

Connections persist in the agent's SQL storage, and when an agent connects to an MCP server, all tools from that server become available automatically.

Adding MCP servers

Use addMcpServer() to connect to an MCP server:

JavaScript
// Simple connection
await this.addMcpServer("notion", "https://mcp.notion.so/mcp");


// With explicit callback host
await this.addMcpServer("github", "https://mcp.github.com/mcp", {
  callbackHost: "https://my-worker.workers.dev",
});

Transport options

MCP supports multiple transport types:

JavaScript
await this.addMcpServer("server", "https://mcp.example.com/mcp", {
  transport: {
    type: "streamable-http",
  },
});
TransportDescription
streamable-httpHTTP with streaming - recommended default
sseServer-Sent Events - legacy/compatibility transport
autoAuto-detect based on server response

Custom headers

For servers behind authentication (like Cloudflare Access) or using bearer tokens:

JavaScript
await this.addMcpServer("internal", "https://internal-mcp.example.com/mcp", {
  transport: {
    headers: {
      Authorization: "Bearer my-token",
      "CF-Access-Client-Id": "...",
      "CF-Access-Client-Secret": "...",
    },
  },
});

Return value

addMcpServer() returns the connection state:

  • ready - Server connected and tools discovered
  • authenticating - Server requires OAuth; redirect user to authUrl

OAuth authentication

Many MCP servers require OAuth authentication. The agent handles the OAuth flow automatically.

How it works

sequenceDiagram
    participant Client
    participant Agent
    participant MCPServer

    Client->>Agent: addMcpServer(name, url)
    Agent->>MCPServer: Connect
    MCPServer-->>Agent: Requires OAuth
    Agent-->>Client: state: authenticating, authUrl
    Client->>MCPServer: User authorizes
    MCPServer->>Agent: Callback with code
    Agent->>MCPServer: Exchange for token
    Agent-->>Client: onMcpUpdate (ready)

Handling OAuth in your agent

JavaScript
class MyAgent extends Agent {
  async onRequest(request) {
    const result = await this.addMcpServer(
      "github",
      "https://mcp.github.com/mcp",
    );


    if (result.state === "authenticating") {
      // Option 1: Redirect the user
      return Response.redirect(result.authUrl);


      // Option 2: Return the URL for client-side redirect
      return Response.json({
        status: "needs_auth",
        authUrl: result.authUrl,
      });
    }


    return Response.json({ status: "connected", id: result.id });
  }
}

OAuth callback

The callback URL is automatically constructed:

https://{host}/{agentsPrefix}/{agent-name}/{instance-name}/callback

For example: https://my-worker.workers.dev/agents/my-agent/default/callback

OAuth tokens are securely stored in SQLite, and persist across agent restarts.

Custom OAuth callback handling

For custom OAuth completion behavior:

JavaScript
export class MyAgent extends Agent {
  onStart() {
    this.mcp.configureOAuthCallback({
      // Redirect after successful auth
      successRedirect: "https://myapp.com/success",


      // Redirect on error
      errorRedirect: "https://myapp.com/error",


      // Or use a custom handler
      customHandler: (result) => {
        if (result.authSuccess) {
          // Close popup window after auth
          return new Response("<script>window.close();</script>", {
            headers: { "content-type": "text/html" },
          });
        }
        return new Response("Auth failed", { status: 400 });
      },
    });
  }
}

Using MCP capabilities

Once connected, access the server's capabilities:

Getting available tools

JavaScript
const state = this.getMcpServers();


// All tools from all connected servers
for (const tool of state.tools) {
  console.log(`Tool: ${tool.name}`);
  console.log(`  From server: ${tool.serverId}`);
  console.log(`  Description: ${tool.description}`);
}

Resources and prompts

JavaScript
const state = this.getMcpServers();


// Available resources
for (const resource of state.resources) {
  console.log(`Resource: ${resource.name} (${resource.uri})`);
}


// Available prompts
for (const prompt of state.prompts) {
  console.log(`Prompt: ${prompt.name}`);
}

Server status

JavaScript
const state = this.getMcpServers();


for (const [id, server] of Object.entries(state.servers)) {
  console.log(`${server.name}: ${server.state}`);
  // state: "ready" | "authenticating" | "connecting" | "connected" | "discovering" | "failed"
}

Integration with AI SDK

To use MCP tools with the Vercel AI SDK, use this.mcp.getAITools() which converts MCP tools to AI SDK format:

JavaScript
import { generateText } from "ai";
import { openai } from "@ai-sdk/openai";


export class MyAgent extends Agent {
  async onRequest(request) {
    const response = await generateText({
      model: openai("gpt-4"),
      prompt: "What's the weather in San Francisco?",
      tools: this.mcp.getAITools(), // Converts MCP tools to AI SDK format
    });


    return new Response(response.text);
  }
}

Managing servers

Removing a server

JavaScript
await this.removeMcpServer(serverId);

This disconnects from the server and removes it from storage.

Persistence

MCP servers persist across agent restarts:

  • Server configuration stored in SQLite
  • OAuth tokens stored securely
  • Connections restored automatically when agent wakes

Listing all servers

JavaScript
const state = this.getMcpServers();


for (const [id, server] of Object.entries(state.servers)) {
  console.log(`${id}: ${server.name} (${server.server_url})`);
}

Client-side integration

Connected clients receive real-time MCP updates via WebSocket:

JavaScript
import { useAgent } from "agents/react";
import { useState } from "react";


function Dashboard() {
  const [tools, setTools] = useState([]);
  const [servers, setServers] = useState({});


  const agent = useAgent({
    agent: "MyAgent",
    onMcpUpdate: (mcpState) => {
      setTools(mcpState.tools);
      setServers(mcpState.servers);
    },
  });


  return (
    <div>
      <h2>Connected Servers</h2>
      {Object.entries(servers).map(([id, server]) => (
        <div key={id}>
          {server.name}: {server.connectionState}
        </div>
      ))}


      <h2>Available Tools ({tools.length})</h2>
      {tools.map((tool) => (
        <div key={`${tool.serverId}-${tool.name}`}>{tool.name}</div>
      ))}
    </div>
  );
}

API reference

addMcpServer()

Add a connection to an MCP server and make its tools available to your agent.

TypeScript
async addMcpServer(
  serverName: string,
  url: string,
  options?: {
    callbackHost?: string;
    agentsPrefix?: string;
    client?: ClientOptions;
    transport?: {
      headers?: HeadersInit;
      type?: "sse" | "streamable-http" | "auto";
    };
  }
): Promise<
  | { id: string; state: "authenticating"; authUrl: string }
  | { id: string; state: "ready" }
>

Parameters

  • serverName (string, required) — Display name for the MCP server
  • url (string, required) — URL of the MCP server endpoint
  • options (object, optional) — Connection configuration:
    • callbackHost — Host for OAuth callback URL. If omitted, automatically derived from the incoming request
    • agentsPrefix — URL prefix for OAuth callback path. Default: "agents"
    • client — MCP client configuration options (passed to @modelcontextprotocol/sdk Client constructor). By default, includes CfWorkerJsonSchemaValidator for validating tool parameters against JSON schemas
    • transport — Transport layer configuration:
      • headers — Custom HTTP headers for authentication
      • type — Transport type: "streamable-http" (default), "sse", or "auto"

Returns

A Promise that resolves to a discriminated union based on connection state:

  • When state is "authenticating":

    • id (string) — Unique identifier for this server connection
    • state ("authenticating") — Server is waiting for OAuth authorization
    • authUrl (string) — OAuth authorization URL for user authentication

  • When state is "ready":

    • id (string) — Unique identifier for this server connection
    • state ("ready") — Server is fully connected and operational

removeMcpServer()

Disconnect from an MCP server and clean up its resources.

TypeScript
async removeMcpServer(id: string): Promise<void>

Parameters

  • id (string, required) — Server connection ID returned from addMcpServer()

getMcpServers()

Get the current state of all MCP server connections.

TypeScript
getMcpServers(): MCPServersState

Returns

TypeScript
type MCPServersState = {
  servers: Record<
    string,
    {
      name: string;
      server_url: string;
      auth_url: string | null;
      state:
        | "authenticating"
        | "connecting"
        | "connected"
        | "discovering"
        | "ready"
        | "failed";
      capabilities: ServerCapabilities | null;
      instructions: string | null;
    }
  >;
  tools: Array<Tool & { serverId: string }>;
  prompts: Array<Prompt & { serverId: string }>;
  resources: Array<Resource & { serverId: string }>;
  resourceTemplates: Array<ResourceTemplate & { serverId: string }>;
};

The state field indicates the connection lifecycle:

  • authenticating — Waiting for OAuth authorization to complete
  • connecting — Establishing transport connection
  • connected — Transport connection established
  • discovering — Discovering server capabilities (tools, resources, prompts)
  • ready — Fully connected and operational
  • failed — Connection failed

Advanced: MCPClientManager

For fine-grained control, use this.mcp directly:

Step-by-step connection

JavaScript
// 1. Register the server
await this.mcp.registerServer(id, {
  url: "https://mcp.example.com/mcp",
  name: "My Server",
  callbackUrl: "https://my-worker.workers.dev/agents/my-agent/default/callback",
  transport: { type: "auto" },
});


// 2. Connect
const connectResult = await this.mcp.connectToServer(id);


if (connectResult.state === "failed") {
  console.error("Connection failed:", connectResult.error);
  return;
}


if (connectResult.state === "authenticating") {
  // Handle OAuth...
  return;
}


// 3. Discover capabilities
const discoverResult = await this.mcp.discoverIfConnected(id);


if (!discoverResult?.success) {
  console.error("Discovery failed:", discoverResult?.error);
}

Event subscription

JavaScript
// Listen for state changes
this.mcp.onServerStateChanged(() => {
  console.log("MCP server state changed");
  this.broadcastMcpServers(); // Notify connected clients
});

Lifecycle methods

this.mcp.registerServer()

Register a server without immediately connecting.

TypeScript
async registerServer(
  id: string,
  options: {
    url: string;
    name: string;
    callbackUrl: string;
    clientOptions?: ClientOptions;
    transportOptions?: TransportOptions;
  }
): Promise<string>

this.mcp.connectToServer()

Establish a connection to a previously registered server.

TypeScript
async connectToServer(id: string): Promise<MCPConnectionResult>


type MCPConnectionResult =
  | { state: "failed"; error: string }
  | { state: "authenticating"; authUrl: string }
  | { state: "connected" }

this.mcp.discoverIfConnected()

Check server capabilities if a connection is active.

TypeScript
async discoverIfConnected(
  serverId: string,
  options?: { timeoutMs?: number }
): Promise<MCPDiscoverResult | undefined>


type MCPDiscoverResult = {
  success: boolean;
  state: MCPConnectionState;
  error?: string;
}

this.mcp.closeConnection()

Close the connection to a specific server while keeping it registered.

TypeScript
async closeConnection(id: string): Promise<void>

this.mcp.closeAllConnections()

Close all active server connections while preserving registrations.

TypeScript
async closeAllConnections(): Promise<void>

this.mcp.getAITools()

Get all discovered MCP tools in a format compatible with the AI SDK.

TypeScript
getAITools(): ToolSet

Tools are automatically namespaced by server ID to prevent conflicts when multiple MCP servers expose tools with the same name.

Error handling

Use error detection utilities to handle connection errors:

JavaScript
import { isUnauthorized, isTransportNotImplemented } from "agents/mcp";


export class MyAgent extends Agent {
  async onRequest(request) {
    try {
      await this.addMcpServer("Server", "https://mcp.example.com/mcp");
    } catch (error) {
      if (isUnauthorized(error)) {
        return new Response("Authentication required", { status: 401 });
      } else if (isTransportNotImplemented(error)) {
        return new Response("Transport not supported", { status: 400 });
      }
      throw error;
    }
  }
}

Next steps

Creating MCP servers Build your own MCP server.
Client SDK Connect from browsers with onMcpUpdate.
Store and sync state Learn about agent persistence.