MCP Builder Skill

This skill guides you in building MCP (Model Context Protocol) servers using the official @modelcontextprotocol/sdk package with TypeScript.

When to Use This Skill

• User asks to "create an MCP server"
• User wants to "expose an API to AI agents"
• User needs "tools for Claude/LLM"
• User mentions "Model Context Protocol"
• User wants to build "AI agent integrations"

MCP Core Concepts

What is MCP?

Model Context Protocol (MCP) is an open standard that enables structured, secure interactions between Large Language Models (LLMs) and external systems. MCP acts as a bridge allowing AI agents to:

• Access external data sources safely
• Execute tools and functions
• Maintain consistent communication protocols
• Integrate with APIs and databases

Key Components

Tools - Interactive functions that perform actions or computations:

• Execute operations and computations
• Accept parameters from the LLM
• Return structured responses
• Can change system state (side effects allowed)
• Examples: calculations, database operations, API calls, sending emails

Resources - Read-only data endpoints:

• Passive data sources (no side effects)
• Loaded into AI context for reasoning
• URI-based identification
• Examples: documentation, config files, status data, user profiles

Prompts - Reusable prompt templates:

• Pre-defined conversation starters
• Can accept arguments for customization
• Return structured messages for the LLM

Transport Options

Recommendation: Use Streamable HTTP for most production servers.

Project Structure

my-mcp-server/
├── src/
│   ├── server.ts          # Express server with transport setup
│   ├── mcp-server.ts      # MCP server with tools/resources/prompts
│   └── minimal/           # Optional: organized components
│       ├── tool.ts
│       ├── resource.ts
│       └── prompt.ts
├── package.json
├── tsconfig.json
└── .env

Required Dependencies

{
  "dependencies": {
    "@modelcontextprotocol/sdk": "^1.12.0",
    "express": "^4.21.0",
    "cors": "^2.8.5",
    "dotenv": "^16.5.0"
  },
  "devDependencies": {
    "@types/node": "^20.0.0",
    "@types/express": "^4.17.21",
    "@types/cors": "^2.8.17",
    "typescript": "^5.6.3",
    "tsx": "^4.20.3"
  }
}

TypeScript Configuration

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "outDir": "./dist",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true
  },
  "include": ["src/**/*"]
}

Implementation Pattern

1. MCP Server Setup (mcp-server.ts)

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

export function createMCPServer() {
  const server = new McpServer({
    name: 'my-mcp-server',
    version: '1.0.0',
  });

  // Register tools
  server.tool('toolName', 'Tool description', {
    param1: { type: 'string', description: 'Parameter description' }
  }, async (args) => {
    // Implementation
    return {
      content: [{ type: 'text', text: JSON.stringify(result) }]
    };
  });

  // Register resources
  server.resource('resource://uri', 'Resource description', async () => ({
    contents: [{
      uri: 'resource://uri',
      mimeType: 'application/json',
      text: JSON.stringify(data)
    }]
  }));

  // Register prompts
  server.prompt('promptName', 'Prompt description', async (args) => ({
    messages: [{
      role: 'user',
      content: { type: 'text', text: `Prompt content with ${args.param}` }
    }]
  }));

  return server;
}

2. HTTP Server with Session Management (server.ts)

import express from 'express';
import cors from 'cors';
import { randomUUID } from 'node:crypto';
import { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/streamableHttp.js';
import { isInitializeRequest } from '@modelcontextprotocol/sdk/types.js';
import { createMCPServer } from './mcp-server.js';

const app = express();
const PORT = process.env.PORT || 3001;

// CORS for browser clients
app.use(cors({
  origin: '*',
  methods: ['GET', 'POST', 'DELETE', 'OPTIONS'],
  allowedHeaders: ['Content-Type', 'mcp-session-id', 'mcp-protocol-version', 'Authorization'],
  exposedHeaders: ['mcp-session-id'],
  credentials: true,
}));
app.use(express.json());

// Session storage
const transports: Record<string, StreamableHTTPServerTransport> = {};

// MCP endpoint handler
const handleMCPRequest = async (req, res) => {
  const sessionId = req.headers['mcp-session-id'] as string | undefined;
  let transport: StreamableHTTPServerTransport;

  if (sessionId && transports[sessionId]) {
    transport = transports[sessionId];
  } else if (!sessionId && isInitializeRequest(req.body)) {
    transport = new StreamableHTTPServerTransport({
      sessionIdGenerator: () => randomUUID(),
      onsessioninitialized: (newSessionId) => {
        transports[newSessionId] = transport;
      },
    });

    transport.onclose = () => {
      if (transport.sessionId) delete transports[transport.sessionId];
    };

    const server = createMCPServer();
    await server.connect(transport);
  } else {
    res.status(400).json({
      jsonrpc: '2.0',
      error: { code: -32000, message: 'No valid session ID or not an init request' },
      id: null,
    });
    return;
  }

  await transport.handleRequest(req, res, req.body);
};

app.post('/', handleMCPRequest);
app.post('/mcp', handleMCPRequest);

app.get('/health', (req, res) => {
  res.json({ status: 'ok', server: 'my-mcp-server', version: '1.0.0' });
});

app.listen(PORT, () => console.log(`MCP Server running on port ${PORT}`));

Tool Implementation Examples

Simple Tool

server.tool('echo', 'Echo a message back', {
  message: { type: 'string', description: 'Message to echo' }
}, async ({ message }) => ({
  content: [{ type: 'text', text: `Echo: ${message}` }]
}));

Tool with Multiple Parameters

server.tool('calculate', 'Perform arithmetic operations', {
  a: { type: 'number', description: 'First number' },
  b: { type: 'number', description: 'Second number' },
  operation: { type: 'string', description: 'Operation: add, subtract, multiply, divide' }
}, async ({ a, b, operation }) => {
  let result: number;
  switch (operation) {
    case 'add': result = a + b; break;
    case 'subtract': result = a - b; break;
    case 'multiply': result = a * b; break;
    case 'divide': 
      if (b === 0) throw new Error('Division by zero');
      result = a / b; 
      break;
    default: throw new Error('Invalid operation');
  }
  return { content: [{ type: 'text', text: JSON.stringify({ result }) }] };
});

External API Tool

server.tool('getWeather', 'Get current weather for a city', {
  city: { type: 'string', description: 'City name' }
}, async ({ city }) => {
  const response = await fetch(`https://api.weather.com/v1/current?city=${encodeURIComponent(city)}&key=${process.env.WEATHER_API_KEY}`);
  const data = await response.json();
  return {
    content: [{ type: 'text', text: JSON.stringify({
      city,
      temperature: data.main.temp,
      conditions: data.weather[0].description
    }) }]
  };
});

Resource Implementation Examples

// Static resource
server.resource('config://settings', 'Application settings', async () => ({
  contents: [{
    uri: 'config://settings',
    mimeType: 'application/json',
    text: JSON.stringify({ theme: 'dark', language: 'en' })
  }]
}));

// Dynamic resource
server.resource('server://info', 'Server information', async () => ({
  contents: [{
    uri: 'server://info',
    mimeType: 'application/json',
    text: JSON.stringify({
      name: 'my-server',
      version: '1.0.0',
      uptime: process.uptime(),
      timestamp: new Date().toISOString()
    })
  }]
}));

Editing Guidelines

Core Principles

1. Edit existing files, don't recreate - Template files are provided; modify them
2. Preserve existing structure - Don't change fundamental architecture
3. Minimal necessary changes - Make smallest changes to achieve requirements
4. Maintain backward compatibility - Don't break existing functionality

DO

• Add new tools/resources/prompts to existing service files
• Extend existing functionality
• Follow existing code patterns
• Use TypeScript for type safety

DON'T

• Completely rewrite template files
• Remove essential boilerplate code
• Create unnecessary new files
• Add terminal/command instructions (user will run commands themselves)

Running the Server

The user will handle running commands. Your job is to provide the code files only.

Standard commands the user might run:

• npm install - Install dependencies
• npm run dev or npx tsx src/server.ts - Development mode
• npm run build - Build for production
• npm start - Run production build