Express MCP Server

Express.js MCP OAuth Authentication with Scalekit

Overview

This skill documents the pattern for building production-ready MCP (Model Context Protocol) servers using Express.js, TypeScript, and OAuth 2.1 Bearer token authentication via Scalekit. This approach provides fine-grained control over HTTP request handling, middleware chains, and server behavior for Node.js-based MCP implementations.

When to Use This Pattern

Use this Express.js MCP integration when you need:

• Node.js ecosystem: Leverage existing npm packages, TypeScript tooling, and JavaScript libraries
• Custom middleware chains: Implement rate limiting, request logging, or complex authorization logic with Express middleware
• Existing Express applications: Add MCP capabilities to established Express.js codebases without rewriting
• Fine-grained HTTP control: Manage routing, CORS policies, health checks, and multiple endpoints
• Production flexibility: Deploy on serverless platforms (AWS Lambda, Vercel), containers, or traditional Node.js hosts

Don't use this pattern if you prefer Python's ecosystem or if a simpler MCP server setup (without Express) meets your requirements.

Core Architecture

Token Validation Flow

MCP Client → Express Server (401 + WWW-Authenticate)
MCP Client → Scalekit (Exchange code for token)
Scalekit → MCP Client (Bearer token)
MCP Client → Express Server (POST /mcp + Bearer token)
Express Middleware → Scalekit SDK (Validate token)
McpServer → Tool Handler → Response

Key Components

1. Express Middleware: Custom authentication middleware that intercepts requests and validates Bearer tokens before routing to MCP handlers
2. Scalekit Node SDK: TypeScript SDK validates JWT signatures, expiration, issuer, and audience claims
3. McpServer: Official MCP SDK server that handles protocol details (JSON-RPC, tool registration)
4. StreamableHTTPServerTransport: MCP transport layer that bridges Express HTTP requests to MCP protocol
5. Zod Schema Validation: Type-safe input validation for MCP tool parameters
6. OAuth Resource Metadata Endpoint: Well-known endpoint (/.well-known/oauth-protected-resource) for client discovery

Implementation Patterns

1. Environment Configuration

Required variables:

• SK_ENV_URL: Scalekit environment URL (issuer)
• SK_CLIENT_ID + SK_CLIENT_SECRET: SDK authentication credentials
• EXPECTED_AUDIENCE: The resource identifier that tokens must target
• PROTECTED_RESOURCE_METADATA: Complete OAuth discovery metadata JSON
• PORT: Server listening port (must match registered server URL)

Security:

• Never commit .env files to version control
• Add .env to .gitignore immediately
• Use secret managers in production (AWS Secrets Manager, Doppler, HashiCorp Vault)
• Rotate SK_CLIENT_SECRET regularly
• Validate EXPECTED_AUDIENCE matches your server's public URL exactly (including trailing slash)

2. Scalekit Client Initialization

import { Scalekit } from '@scalekit-sdk/node';

const scalekit = new Scalekit(
  SK_ENV_URL,
  SK_CLIENT_ID,
  SK_CLIENT_SECRET
);

Best practices:

• Initialize once at module level for connection pooling
• SDK handles token caching and JWKS key rotation automatically
• All validation methods are async—always use await

3. MCP Server Setup

import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { z } from 'zod';

const server = new McpServer({
  name: 'Greeting MCP',
  version: '1.0.0'
});

server.tool(
  'greet_user',
  'Greets the user with a personalized message.',
  {
    name: z.string().min(1, 'Name is required'),
  },
  async ({ name }: { name: string }) => ({
    content: [
      {
        type: 'text',
        text: `Hi ${name}, welcome to Scalekit!`
      }
    ]
  })
);

Tool registration:

• First parameter: Tool name (snake_case recommended for consistency)
• Second parameter: Human-readable description for AI discoverability
• Third parameter: Zod schema for input validation
• Fourth parameter: Async handler function

Zod validation benefits:

• Type-safe parameters with TypeScript inference
• Runtime validation prevents malformed inputs
• Automatic error messages for invalid data
• Composable schemas for complex validation rules

4. Express Middleware Authentication

app.use(async (req: Request, res: Response, next: NextFunction) => {
  // Exempt public endpoints
  if (req.path === '/.well-known/oauth-protected-resource' || req.path === '/health') {
    next();
    return;
  }

  // Extract Bearer token
  const header = req.headers.authorization;
  const token = header?.startsWith('Bearer ')
    ? header.slice('Bearer '.length).trim()
    : undefined;

  if (!token) {
    res.status(401)
      .set('WWW-Authenticate', WWW_HEADER_VALUE)
      .json({ error: 'Missing Bearer token' });
    return;
  }

  try {
    // Validate with Scalekit SDK
    await scalekit.validateToken(token, {
      audience: [EXPECTED_AUDIENCE]
    });
    next();
  } catch (error) {
    res.status(401)
      .set('WWW-Authenticate', WWW_HEADER_VALUE)
      .json({ error: 'Token validation failed' });
  }
});

Key principles:

• Use Express app.use() for middleware that runs on every request
• Explicitly exempt public endpoints before token extraction
• Return early with return after sending 401 responses (prevents "headers already sent" errors)
• Always set WWW-Authenticate header on 401 responses
• Use next() to pass control to subsequent middleware/routes

Common mistake: Forgetting to return after sending a response leads to "Cannot set headers after they are sent" errors.

5. MCP Transport Layer

import { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/streamableHttp.js';

app.post('/', async (req: Request, res: Response) => {
  const transport = new StreamableHTTPServerTransport({
    sessionIdGenerator: undefined
  });
  await server.connect(transport);

  try {
    await transport.handleRequest(req, res, req.body);
  } catch (error) {
    res.status(500).json({ error: 'MCP transport error' });
  }
});

Transport responsibilities:

• Converts HTTP requests to MCP JSON-RPC format
• Handles streaming responses for long-running operations
• Manages session state (stateless when sessionIdGenerator: undefined)
• Bridges Express request/response objects to MCP protocol

Stateless design: Setting sessionIdGenerator: undefined ensures each request is independent—suitable for serverless deployments.

6. Resource Metadata Endpoint

app.get('/.well-known/oauth-protected-resource', (_req: Request, res: Response) => {
  if (!PROTECTED_RESOURCE_METADATA) {
    res.status(500).json({ error: 'PROTECTED_RESOURCE_METADATA config missing' });
    return;
  }

  const metadata = JSON.parse(PROTECTED_RESOURCE_METADATA);
  res.type('application/json').send(JSON.stringify(metadata, null, 2));
});

Purpose:

• Enables MCP client discovery of authorization requirements
• Clients fetch this when they receive a 401 response with WWW-Authenticate header
• Contains authorization server endpoints, supported grant types, and token types
• Must be publicly accessible (no authentication required)

Error handling: Return 500 if metadata is missing to signal misconfiguration (deployment should fail fast).

7. CORS Configuration

import cors from 'cors';

app.use(cors({
  origin: true,      // Allow all origins
  credentials: false // No credentials needed for Bearer tokens
}));

Configuration options:

• origin: true: Reflects request origin (development convenience)
• origin: ['https://app.example.com']: Whitelist specific origins (production)
• credentials: false: Bearer tokens don't require cookies/credentials
• methods: ['GET', 'POST', 'OPTIONS']: Limit allowed HTTP methods

Production recommendation: Use explicit origin whitelist instead of origin: true.

Security Considerations

Token Validation Requirements

• Always validate issuer: Prevents tokens from other OAuth servers being accepted
• Always validate audience: Ensures token was issued for your specific resource
• Check expiration: Scalekit SDK automatically validates exp claim
• Verify signature: SDK checks JWT signature against Scalekit's public keys (JWKS)

Common Vulnerabilities to Avoid

1. Skipping audience validation: Tokens from other Scalekit resources could be used
2. Custom JWT parsing: Use SDK validation—don't implement manual jwt.verify()
3. Logging tokens: Never log Bearer tokens in middleware, error handlers, or debug output
4. Missing CORS configuration: Can enable cross-origin attacks or block legitimate clients
5. Hardcoded secrets: Use environment variables and secret managers
6. Not returning after response: Causes "headers already sent" errors and potential security issues

Production Hardening

• HTTPS termination: Run behind reverse proxy (Nginx, Caddy, AWS ALB) with TLS
• Process management: Use PM2, systemd, or container orchestration for auto-restart
• Multiple workers: Use Node.js cluster module or container scaling
• Rate limiting: Implement per-client/token rate limits using express-rate-limit
• Request logging: Add structured logging middleware (Winston, Pino) without token values
• Health checks: Separate health endpoint for load balancers and orchestrators
• Error monitoring: Integrate Sentry, Datadog, or similar for production error tracking

Testing Strategy

Local Testing with MCP Inspector

npx @modelcontextprotocol/inspector@latest

Testing workflow:

1. Start your Express server: npm run dev
2. Launch MCP Inspector
3. Connect to http://localhost:3002/
4. Inspector automatically handles OAuth flow
5. Test each tool with various inputs
6. Verify middleware logs show successful validation

Manual Token Testing with cURL

# Get token from Scalekit (via OAuth flow or test endpoint)
export TOKEN="<your-access-token>"

# Test authenticated MCP request
curl -X POST http://localhost:3002/ \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "tools/call",
    "params": {
      "name": "greet_user",
      "arguments": {
        "name": "Saif"
      }
    }
  }'

# Test missing token (should return 401)
curl -v -X POST http://localhost:3002/

# Test invalid token
curl -X POST http://localhost:3002/ \
  -H "Authorization: Bearer invalid-token" \
  -H "Content-Type: application/json"

Integration Tests with Jest/Vitest

import request from 'supertest';
import { app } from './server';

describe('MCP Authentication', () => {
  test('returns 401 without token', async () => {
    const response = await request(app).post('/');
    expect(response.status).toBe(401);
    expect(response.headers['www-authenticate']).toContain('Bearer');
  });

  test('returns 401 with invalid token', async () => {
    const response = await request(app)
      .post('/')
      .set('Authorization', 'Bearer invalid-token');
    expect(response.status).toBe(401);
  });

  test('health check is public', async () => {
    const response = await request(app).get('/health');
    expect(response.status).toBe(200);
    expect(response.body).toEqual({ status: 'healthy' });
  });

  test('metadata endpoint is public', async () => {
    const response = await request(app).get('/.well-known/oauth-protected-resource');
    expect(response.status).toBe(200);
    expect(response.headers['content-type']).toContain('application/json');
  });
});

Test dependencies:

{
  "devDependencies": {
    "@types/jest": "^29.5.12",
    "jest": "^29.7.0",
    "supertest": "^6.3.4",
    "ts-jest": "^29.1.2"
  }
}

Load Testing

# Install autocannon for HTTP load testing
npm install -g autocannon

# Test authenticated endpoint throughput
autocannon -c 10 -d 30 \
  -m POST \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -b '{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}' \
  http://localhost:3002/

Common Pitfalls

1. Mismatched Audience

Symptom: Tokens fail validation with "invalid audience" error Cause: EXPECTED_AUDIENCE doesn't match the Server URL registered in Scalekit Fix: Ensure both values are identical including protocol, host, port, and trailing slash

Example:

// ❌ Wrong - missing trailing slash
EXPECTED_AUDIENCE=http://localhost:3002

// ✅ Correct - matches Scalekit registration
EXPECTED_AUDIENCE=http://localhost:3002/

2. Headers Already Sent Error

Symptom: Error: Cannot set headers after they are sent to the client Cause: Forgetting to return after sending a response in middleware Fix: Always return immediately after res.json() or res.send()

Example:

// ❌ Wrong - continues to next middleware
if (!token) {
  res.status(401).json({ error: 'Missing token' });
}
next(); // This runs even after sending 401

// ✅ Correct - returns after response
if (!token) {
  res.status(401).json({ error: 'Missing token' });
  return; // Prevents calling next()
}

3. Middleware Order Issues

Symptom: CORS errors, authentication bypassed, or parsing failures Cause: Middleware execution order matters in Express Fix: Correct order is: CORS → body parsing → authentication → routes

Example:

// ✅ Correct order
app.use(cors());
app.use(express.json());
app.use(authMiddleware);
app.get('/public', publicRoute);
app.post('/', protectedRoute);

4. Missing Resource Metadata

Symptom: Clients can't discover how to authenticate Cause: PROTECTED_RESOURCE_METADATA not set or malformed JSON Fix: Copy exact JSON from Scalekit dashboard, verify with JSON.parse()

Debugging:

# Test metadata endpoint
curl http://localhost:3002/.well-known/oauth-protected-resource

# Should return valid JSON with authorization_endpoint

5. TypeScript Module Resolution

Symptom: Cannot find module '@modelcontextprotocol/sdk/server/mcp.js' Cause: Missing .js extension in ES module imports Fix: Always include .js extension when importing from MCP SDK

Example:

// ❌ Wrong - missing .js
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp';

// ✅ Correct - includes .js
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';

6. Token Expiration During Testing

Symptom: Tests pass initially then fail after 1 hour Cause: Access tokens expire (default 3600 seconds) Fix: Implement token refresh before each test run or use shorter test cycles

Extension Patterns

Adding Scope-Based Authorization

import jwt from 'jsonwebtoken';

// Extend Express Request type
declare global {
  namespace Express {
    interface Request {
      tokenPayload?: {
        sub: string;
        scope: string[];
      };
    }
  }
}

app.use(async (req: Request, res: Response, next: NextFunction) => {
  // ... existing token validation ...

  // Decode token to access claims (after validation)
  const decoded = jwt.decode(token) as any;
  req.tokenPayload = {
    sub: decoded.sub,
    scope: decoded.scope?.split(' ') || []
  };

  next();
});

// Tool with scope requirement
server.tool(
  'admin_action',
  'Performs an admin action',
  { action: z.string() },
  async ({ action }, { req }) => {
    if (!req.tokenPayload?.scope.includes('admin')) {
      throw new Error('Requires admin scope');
    }
    // ... admin logic ...
  }
);

Multi-Tenancy Support

app.use(async (req: Request, res: Response, next: NextFunction) => {
  // ... validate token ...

  const decoded = jwt.decode(token) as any;
  req.orgId = decoded.org_id;

  next();
});

server.tool(
  'get_org_data',
  'Retrieves organization-specific data',
  {},
  async (_params, { req }) => {
    const orgId = req.orgId;
    const data = await fetchDataForOrg(orgId);

    return {
      content: [{ type: 'text', text: JSON.stringify(data) }]
    };
  }
);

Rate Limiting

import rateLimit from 'express-rate-limit';

const limiter = rateLimit({
  windowMs: 60 * 1000, // 1 minute
  max: 100, // 100 requests per window
  message: 'Too many requests, please try again later',
  standardHeaders: true,
  legacyHeaders: false,
  // Rate limit by token subject (user ID)
  keyGenerator: (req: Request) => {
    const token = req.headers.authorization?.slice('Bearer '.length);
    if (!token) return req.ip;

    const decoded = jwt.decode(token) as any;
    return decoded.sub || req.ip;
  }
});

// Apply to MCP endpoint
app.post('/', limiter, async (req: Request, res: Response) => {
  // ... MCP transport handling ...
});

Structured Logging

import pino from 'pino';

const logger = pino({
  level: process.env.LOG_LEVEL || 'info',
  redact: ['req.headers.authorization'], // Never log tokens
});

app.use((req: Request, res: Response, next: NextFunction) => {
  const start = Date.now();

  res.on('finish', () => {
    logger.info({
      method: req.method,
      path: req.path,
      status: res.statusCode,
      duration: Date.now() - start,
    });
  });

  next();
});

Error Handling Middleware

app.use((err: Error, req: Request, res: Response, next: NextFunction) => {
  logger.error({ err, path: req.path }, 'Unhandled error');

  res.status(500).json({
    error: process.env.NODE_ENV === 'production'
      ? 'Internal server error'
      : err.message
  });
});

Dependencies

Required Packages

{
  "dependencies": {
    "@modelcontextprotocol/sdk": "^1.13.0",
    "@scalekit-sdk/node": "^2.0.1",
    "cors": "^2.8.5",
    "dotenv": "^16.4.5",
    "express": "^5.1.0",
    "zod": "^3.25.57"
  },
  "devDependencies": {
    "@types/cors": "^2.8.19",
    "@types/express": "^4.17.21",
    "@types/node": "^20.11.19",
    "tsx": "^4.7.0",
    "typescript": "^5.4.5"
  }
}

Dependency purposes:

• @modelcontextprotocol/sdk: Official MCP protocol implementation
• @scalekit-sdk/node: Scalekit authentication SDK for token validation
• cors: Cross-Origin Resource Sharing middleware
• dotenv: Environment variable loading from .env files
• express: Fast, unopinionated web framework
• zod: TypeScript-first schema validation
• tsx: TypeScript execution for development (faster than ts-node)
• typescript: TypeScript compiler

Optional Production Dependencies

{
  "dependencies": {
    "express-rate-limit": "^7.1.5",
    "helmet": "^7.1.0",
    "pino": "^8.17.2",
    "pino-http": "^9.0.0"
  }
}

Production enhancements:

• express-rate-limit: Rate limiting middleware
• helmet: Security headers middleware
• pino: High-performance JSON logger
• pino-http: HTTP request logging middleware

Version Pinning Strategy

• Pin exact versions in production package.json (use "express": "5.1.0" not "^5.1.0")
• Use ^ for development flexibility
• Run npm audit regularly for security vulnerabilities
• Test version upgrades in staging before production
• Use npm ci in production for reproducible builds

Deployment Patterns

Docker

FROM node:20-alpine

WORKDIR /app

# Copy package files
COPY package*.json ./

# Install dependencies
RUN npm ci --only=production

# Copy source
COPY . .

# Build TypeScript
RUN npm run build

# Expose port
EXPOSE 3002

# Start server
CMD ["npm", "start"]

PM2 Process Manager

{
  "apps": [{
    "name": "mcp-server",
    "script": "dist/server.js",
    "instances": 4,
    "exec_mode": "cluster",
    "env": {
      "NODE_ENV": "production"
    }
  }]
}

Start with PM2:

npm run build
pm2 start ecosystem.config.json

AWS Lambda (Serverless)

import serverless from 'serverless-http';

// ... existing Express app setup ...

export const handler = serverless(app);

Note: Ensure stateless transport configuration for serverless environments.

Environment-Specific Configuration

const config = {
  development: {
    port: 3002,
    corsOrigin: true,
    logLevel: 'debug'
  },
  production: {
    port: parseInt(process.env.PORT || '3002'),
    corsOrigin: process.env.ALLOWED_ORIGINS?.split(',') || [],
    logLevel: 'info'
  }
};

const env = process.env.NODE_ENV || 'development';
const appConfig = config[env];

Complete Working Example

A full production-ready Express.js MCP server is available in the Scalekit MCP Auth Demos repository:

GitHub Repository: scalekit-inc/mcp-auth-demos/tree/main/greeting-mcp-node

This example includes:

• Complete server implementation with modular architecture
• OAuth 2.1 authentication middleware
• Tool registration with Zod validation
• CORS configuration and error handling
• Production-ready logging and monitoring

Key Files

• src/main.ts - Main server entry point
• src/lib/auth.ts - OAuth discovery endpoint handler
• src/lib/middleware.ts - Token validation middleware
• src/lib/transport.ts - MCP transport layer setup
• src/tools/ - Tool implementations

Getting Started

cd greeting-mcp-node
npm install
npm run build
npm start

See README.md for complete setup instructions.

Related Resources

• MCP SDK Documentation
• Express.js Guide
• Scalekit Node SDK
• Zod Documentation
• OAuth 2.1 Specification
• Scalekit MCP Authentication Docs
• MCP Protocol Specification
• Scalekit MCP Auth Demos

Changelog

• 2026-02-13: Initial skill documentation based on Express.js MCP quickstart guide