MCP

Build MCP servers that give Claude access to external tools and data.

What is MCP?

Model Context Protocol lets you create servers that Claude can call:

•Query databases
•Call APIs
•Access file systems
•Run custom tools

When to Build an MCP Server

•You want Claude to access live data (not just files)
•You're repeating the same API calls manually
•You want Claude to take actions (create records, trigger workflows)

Project Setup

bash

mkdir my-mcp-server && cd my-mcp-server
pnpm init
pnpm add @modelcontextprotocol/sdk zod
pnpm add -D typescript @types/node tsx

json

// package.json
{
  "type": "module",
  "scripts": {
    "dev": "tsx watch src/index.ts",
    "build": "tsc",
    "start": "node dist/index.js"
  }
}

json

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

Basic Server Template

typescript

// src/index.ts
import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js'
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js'
import { z } from 'zod'

const server = new McpServer({
  name: 'my-server',
  version: '1.0.0',
})

// Define a tool
server.tool(
  'hello',
  'Say hello to someone',
  { name: z.string().describe('Name to greet') },
  async ({ name }) => ({
    content: [{ type: 'text', text: `Hello, ${name}!` }],
  })
)

// Start server
const transport = new StdioServerTransport()
await server.connect(transport)

Common Patterns

Database Access (Postgres)

typescript

import pg from 'pg'

const pool = new pg.Pool({
  connectionString: process.env.DATABASE_URL,
})

server.tool(
  'query',
  'Run a read-only SQL query',
  { sql: z.string().describe('SELECT query to run') },
  async ({ sql }) => {
    if (!sql.trim().toLowerCase().startsWith('select')) {
      return { content: [{ type: 'text', text: 'Only SELECT queries allowed' }] }
    }
    const result = await pool.query(sql)
    return { content: [{ type: 'text', text: JSON.stringify(result.rows, null, 2) }] }
  }
)

API Wrapper

typescript

server.tool(
  'get_weather',
  'Get current weather for a city',
  { city: z.string() },
  async ({ city }) => {
    const res = await fetch(`https://api.weather.com/current?city=${city}`)
    const data = await res.json()
    return { content: [{ type: 'text', text: JSON.stringify(data, null, 2) }] }
  }
)

File System Access

typescript

import { readdir, readFile } from 'fs/promises'

server.tool(
  'list_projects',
  'List projects in studio-manager',
  {},
  async () => {
    const dirs = await readdir('/Users/brew/studio-manager', { withFileTypes: true })
    const projects = dirs.filter(d => d.isDirectory()).map(d => d.name)
    return { content: [{ type: 'text', text: projects.join('\n') }] }
  }
)

Claude Desktop Config

Add to ~/Library/Application Support/Claude/claude_desktop_config.json:

json

{
  "mcpServers": {
    "my-server": {
      "command": "node",
      "args": ["/path/to/my-mcp-server/dist/index.js"],
      "env": {
        "DATABASE_URL": "postgres://..."
      }
    }
  }
}

Restart Claude Desktop after changes.

Testing

bash

# Run server directly to test
echo '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' | pnpm start

Security Notes

•Validate all inputs with Zod
•Limit database access to SELECT only (unless intentional)
•Don't expose secrets in tool responses
•Consider rate limiting for external APIs

Reference

•MCP Docs: https://modelcontextprotocol.io/docs
•MCP SDK: https://github.com/modelcontextprotocol/sdk
•Anthropic's mcp-builder: https://github.com/anthropics/skills/tree/main/skills/mcp-builder