Clerk

Installation

Install the Clerk plugin:

npm install @xmcp-dev/clerk

Clerk Setup

Before integrating the plugin, configure your Clerk application:

1. Navigate to your Clerk Dashboard
2. Create a new application (or use an existing one)
3. Go to Configure and access to API Keys to get the following values:
   • Secret Key (sk_...)
   • Frontend API URL (your-app.clerk.accounts.dev)
4. Click on Development, enter to OAuth Applications and enable Dynamic Client Registration

Environment Variables

Create a .env file in the root of your project and configure the following environment variables:

CLERK_SECRET_KEY=sk_...
CLERK_DOMAIN=your-app.clerk.accounts.dev

BASE_URL=http://127.0.0.1:3001 

Set up the Provider

Create a middleware.ts file in your xmcp app's src directory and import the provider from the package:

import { clerkProvider } from "@xmcp-dev/clerk";

export default clerkProvider({
  secretKey: process.env.CLERK_SECRET_KEY!,
  clerkDomain: process.env.CLERK_DOMAIN!,
  baseURL: process.env.BASE_URL!
});

Configuration Options

• secretKey: Clerk Secret Key
• clerkDomain: Clerk Frontend domain
• baseURL: Base URL of your MCP server
• scopes: (Optional) OAuth scopes to request (default: ['profile', 'email'])
• docsURL: (Optional) URL to your MCP server's API documentation

Get a user session

Access the authenticated user in your xmcp tools using getSession:

Example: Greet the user with their Clerk identity

import { z } from "zod";
import type { InferSchema, ToolMetadata } from "xmcp";
import { getSession } from "@xmcp-dev/clerk";

// Define the schema for tool parameters
export const schema = {
  name: z.string().describe("The name of the user to greet"),
};

// Define tool metadata
export const metadata: ToolMetadata = {
  name: "greet",
  description: "Greet the user with their Clerk identity",
};

// Tool implementation
export default function greet({ name }: InferSchema<typeof schema>): string {
  const session = getSession();

  return `Hello, ${name}! Your Clerk user ID is ${session.userId}`;
}

Get user details

Access the authenticated user in your xmcp tools using getUser:

Example: Get user details

import type { ToolMetadata } from "xmcp";
import { getUser } from "@xmcp-dev/clerk";

// Define tool metadata
export const metadata: ToolMetadata = {
  name: "get-user-info",
  description: "Get user details",
};

// Tool implementation
export default async function getUserInfo(): Promise<string>  {
  const user = await getUser();

  return JSON.stringify(user, null, 2);
}

Access the client

The getClient() function gives you access to the full Clerk Backend SDK, allowing you to leverage all Clerk features in your MCP tools.

Example: Retrieve an Organization

import { getSession, getClient } from "@xmcp-dev/clerk";

export default async function getOrganization() {
  const session = getSession();
  const clerk = getClient();

  if (!session.organizationId) {
    return "User is not in an organization";
  }

  const org = await clerk.organizations.getOrganization({
    organizationId: session.organizationId,
  });

  return JSON.stringify(org, null, 2);
}

Troubleshooting

Missing Configuration Errors

If you see "[Clerk] Missing required config: ..." at startup:

• Ensure all required environment variables are set (CLERK_SECRET_KEY, CLERK_DOMAIN, BASE_URL)
• Check your .env file is being loaded correctly

Session or Client Not Initialized

If getSession(), getUser(), or getClient() throws "... not initialized":

• Ensure clerkProvider is exported as default from middleware.ts
• Ensure the tool is called on a route under /mcp/*
• Don't call these functions at module load time only inside tool handlers

Token Expired Errors

Access tokens are short-lived. If you see token_expired errors:

• MCP clients should automatically refresh tokens
• Users can disconnect and reconnect to get fresh tokens

Invalid Token Errors

If tokens are consistently invalid:

• Verify your CLERK_SECRET_KEY matches your Clerk application
• Verify your CLERK_DOMAIN matches your Clerk Frontend API URL
• Check that you're using the correct environment (development vs production)

Authentication Service Misconfigured

If you see a 500 error with "Authentication service misconfigured":

• Your CLERK_SECRET_KEY is invalid or doesn't match your Clerk application
• Double-check you're using the correct key for your environment (development vs production)