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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

Note: This page covers connecting to MCP servers as a client. To create your own MCP server, see Creating MCP Servers.

Quick Start

import { Agent } from "agents";

export class MyAgent extends Agent {
  async onRequest(request: 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");
  }
}

Adding MCP Servers

Use addMcpServer() to connect to an MCP server:

const result = await this.addMcpServer(name, url, options?);

Basic Usage

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

// With explicit callback host (rarely needed — auto-derived from request or WebSocket URI)
await this.addMcpServer("github", "https://mcp.github.com/mcp", {
  callbackHost: "https://my-worker.workers.dev"
});

Transport Options

MCP supports multiple transport types:

await this.addMcpServer("server", "https://mcp.example.com/mcp", {
  transport: {
    // Transport type: "streamable-http" (default), "sse", or "auto"
    type: "streamable-http"
  }
});
Transport Description
"streamable-http" HTTP with streaming - recommended default
"sse" Server-Sent Events - legacy / compatibility transport
"auto" Auto-detect based on server response

Custom Headers

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

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

Retry Options

Configure retry behavior for connection and reconnection attempts:

await this.addMcpServer("github", "https://mcp.github.com/mcp", {
  retry: {
    maxAttempts: 5,
    baseDelayMs: 1000,
    maxDelayMs: 10000
  }
});

These options are persisted and used when reconnecting after hibernation or after OAuth completion. Default: 3 attempts, 500ms base delay, 5s max delay. See Retries for more details.

URL Security

MCP server URLs are validated before connection to prevent Server-Side Request Forgery (SSRF). The following URL targets are blocked:

  • Private/internal IP ranges (RFC 1918: 10.x, 172.16-31.x, 192.168.x)
  • Unspecified addresses (0.0.0.0, ::)
  • Link-local addresses (169.254.x, fe80::)
  • Cloud metadata endpoints (169.254.169.254)
  • IPv6 unique-local addresses (fc00::/7)

Loopback development URLs such as localhost, 127.0.0.1, and ::1 are allowed.

If you need to connect to another internal MCP server, use the RPC transport with a Durable Object binding instead of HTTP.

Return Value

addMcpServer() returns the connection state:

type AddMcpServerResult =
  | { id: string; state: "ready" }
  | { id: string; state: "authenticating"; authUrl: string };
  • 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)
Loading

Handling OAuth in Your Agent

async onRequest(request: 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 Callback Handling

For custom OAuth completion behavior:

// In your agent constructor or 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) => {
    return new Response(
      JSON.stringify({
        success: result.authSuccess,
        serverId: result.serverId,
        error: result.authError
      }),
      {
        headers: { "Content-Type": "application/json" }
      }
    );
  }
});

Custom OAuth Provider

By default, agents use dynamic client registration to authenticate with MCP servers. If you need to use a different OAuth strategy — such as pre-registered client credentials, mTLS-based authentication, or other mechanisms — override the createMcpOAuthProvider method in your agent subclass:

import { Agent } from "agents";
import type { AgentMcpOAuthProvider } from "agents";

class MyAgent extends Agent {
  createMcpOAuthProvider(callbackUrl: string): AgentMcpOAuthProvider {
    return new MyCustomOAuthProvider(this.ctx.storage, this.name, callbackUrl);
  }
}

Your custom class must implement the AgentMcpOAuthProvider interface, which extends the MCP SDK's OAuthClientProvider with additional properties (authUrl, clientId, serverId) and methods (checkState, consumeState, deleteCodeVerifier) used by the agent's MCP connection lifecycle.

The override is used for both new connections (addMcpServer) and restored connections after a Durable Object restart, so your custom provider is always used consistently.

Custom storage backend

The most common customization is using a different storage backend while keeping the built-in OAuth logic (CSRF state, PKCE, nonce generation, token management). Import DurableObjectOAuthClientProvider and pass your own storage adapter:

import { Agent, DurableObjectOAuthClientProvider } from "agents";
import type { AgentMcpOAuthProvider } from "agents";

class MyAgent extends Agent {
  createMcpOAuthProvider(callbackUrl: string): AgentMcpOAuthProvider {
    return new DurableObjectOAuthClientProvider(
      myCustomStorage, // any DurableObjectStorage-compatible adapter
      this.name,
      callbackUrl
    );
  }
}

Using MCP Capabilities

Once connected, access the server's capabilities:

Getting Available Tools

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

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

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:

import { generateText } from "ai";

async function chat(prompt: string) {
  const response = await generateText({
    model: openai("gpt-4"),
    prompt,
    tools: this.mcp.getAITools() // Converts MCP tools to AI SDK format
  });

  return response;
}

Note: getMcpServers().tools returns raw MCP Tool objects for inspection. Use this.mcp.getAITools() when passing tools to the AI SDK.

Managing Servers

Removing a Server

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

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:

import { useAgent } from "agents/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>
  );
}

Advanced: MCPClientManager

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

Step-by-Step Connection

// 1. Register the server (saves to storage and creates in-memory connection)
const id = "my-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 (initializes transport, handles OAuth if needed)
const connectResult = await this.mcp.connectToServer(id);

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

if (connectResult.state === "authenticating") {
  console.log("OAuth required:", connectResult.authUrl);
  return;
}

// 3. Discover capabilities (transitions from "connected" to "ready")
if (connectResult.state === "connected") {
  const discoverResult = await this.mcp.discoverIfConnected(id);

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

Event Subscription

// Listen for state changes (onServerStateChanged is an Event<void>)
const disposable = this.mcp.onServerStateChanged(() => {
  console.log("MCP server state changed");
  this.broadcastMcpServers(); // Notify connected clients
});

// Clean up the subscription when no longer needed
// disposable.dispose();

Waiting for Connections

After hibernation or when connections are being restored in the background, MCP tools may not be immediately available. Use waitForConnections() to wait until all in-flight connection and discovery operations have settled:

// Wait indefinitely for all connections to be ready
await this.mcp.waitForConnections();

// Wait with a timeout (in milliseconds)
await this.mcp.waitForConnections({ timeout: 10_000 });

This is useful when you need to call this.mcp.getAITools() immediately after the agent wakes from hibernation. Without waiting, tools from servers that are still reconnecting will be missing.

Note: AIChatAgent handles this automatically via the waitForMcpConnections property (defaults to { timeout: 10_000 }). You only need waitForConnections() directly when using Agent with MCP, or when you want finer control inside onChatMessage.

Error Recovery

async retryConnection(serverId: string) {
  const result = await this.mcp.connectToServer(serverId);

  if (result.state === "connected") {
    await this.mcp.discoverIfConnected(serverId);
  } else if (result.state === "failed") {
    console.error("Reconnection failed:", result.error);
  }
}

Examples

MCP Client Demo

The examples/mcp-client example demonstrates:

  • Adding and removing MCP servers dynamically
  • Custom OAuth callback handling (popup-closing behavior)
  • Listing tools from connected servers
  • Real-time state updates to the frontend
// From examples/mcp-client/src/server.ts
export class MyAgent extends Agent {
  onStart() {
    // Custom OAuth callback that closes the popup window
    this.mcp.configureOAuthCallback({
      customHandler: (result) => {
        if (result.authSuccess) {
          return new Response("<script>window.close();</script>", {
            headers: { "content-type": "text/html" }
          });
        }
        // Handle error...
      }
    });
  }

  async onRequest(request: Request) {
    const url = new URL(request.url);

    if (url.pathname.endsWith("add-mcp")) {
      const { name, url } = await request.json();
      await this.addMcpServer(name, url);
      return new Response("Ok");
    }
    // ...
  }
}

API Reference

addMcpServer()

// HTTP transport (Streamable HTTP, SSE)
async addMcpServer(
  name: string,
  url: string,
  options?: {
    callbackHost?: string;  // auto-derived from request or WebSocket connection URI; only set to override
    callbackPath?: string;  // custom callback URL path (bypasses default /agents/{class}/{name}/callback)
    agentsPrefix?: string;
    client?: ClientOptions;
    transport?: {
      headers?: HeadersInit;
      type?: "sse" | "streamable-http" | "auto"; // default: "auto"
    };
    retry?: RetryOptions; // retry options for connection/reconnection
  }
): Promise<
  | { id: string; state: "ready" }
  | { id: string; state: "authenticating"; authUrl: string }
>

// RPC transport (Durable Object binding — no HTTP overhead)
async addMcpServer(
  name: string,
  binding: DurableObjectNamespace,
  options?: {
    props?: Record<string, unknown>; // passed to the McpAgent's onStart(props)
    client?: ClientOptions;
    retry?: RetryOptions;
  }
): Promise<{ id: string; state: "ready" }>

// Legacy signature (still supported)
async addMcpServer(
  name: string,
  url: string,
  callbackHost?: string,
  agentsPrefix?: string,
  options?: { ... }
): Promise<...>

Add and connect to an MCP server. Throws if connection or discovery fails.

callbackHost is automatically derived from the incoming HTTP request or WebSocket connection URI — you almost never need to set it explicitly. It is only needed when the auto-detected host does not match your desired OAuth callback origin (for example, behind a reverse proxy). For RPC transport, pass a DurableObjectNamespace binding instead of a URL. See MCP Transports for details.

Calling addMcpServer is idempotent when both the server name and URL match an existing active connection — the existing connection is returned without creating a duplicate. This makes it safe to call in onStart() without worrying about duplicate connections on restart.

If you call addMcpServer with the same name but a different URL, a new connection is created. Both connections remain active and their tools are merged in getAITools(). To replace a server, call removeMcpServer(oldId) first.

Note: URLs are normalized before comparison (trailing slashes, default ports, and hostname case are handled), so https://MCP.Example.com and https://mcp.example.com/ are treated as the same URL.

removeMcpServer()

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

Disconnect from and remove an MCP server.

getMcpServers()

getMcpServers(): MCPServersState

Get the current state of all MCP servers and their capabilities.

MCPServersState

type MCPServersState = {
  servers: {
    [id: string]: MCPServer;
  };
  tools: (Tool & { serverId: string })[];
  prompts: (Prompt & { serverId: string })[];
  resources: (Resource & { serverId: string })[];
};

MCPServer

type MCPServer = {
  name: string;
  server_url: string;
  auth_url: string | null;
  state:
    | "ready"
    | "authenticating"
    | "connecting"
    | "connected"
    | "discovering"
    | "failed";
  error: string | null;
  instructions: string | null;
  capabilities: ServerCapabilities | null;
};