MCP Servers: External Connections

{
  "mcpServers": {
    "github": {
      "command": "npx",
      "args": ["@modelcontextprotocol/server-github"],
      "env": {
        "GITHUB_TOKEN": "${GITHUB_TOKEN}"
      }
    },
    "postgres": {
      "command": "npx",
      "args": [
        "@modelcontextprotocol/server-postgres",
        "--connection-string",
        "${DATABASE_URL}"
      ]
    },
    "slack": {
      "command": "npx",
      "args": ["@modelcontextprotocol/server-slack"],
      "env": {
        "SLACK_BOT_TOKEN": "${SLACK_BOT_TOKEN}"
      }
    }
  }
}

Place in your project root or ~/.claude/.mcp.json for global config. Environment variables are expanded at runtime.

import { Server } from "@modelcontextprotocol/sdk/server/index.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";

const server = new Server({
  name: "my-custom-server",
  version: "1.0.0"
}, {
  capabilities: { tools: {} }
});

// Define a tool
server.setRequestHandler("tools/list", async () => ({
  tools: [{
    name: "get_weather",
    description: "Get current weather for a city",
    inputSchema: {
      type: "object",
      properties: {
        city: { type: "string", description: "City name" }
      },
      required: ["city"]
    }
  }]
}));

// Handle tool calls
server.setRequestHandler("tools/call", async (request) => {
  if (request.params.name === "get_weather") {
    const { city } = request.params.arguments;
    return {
      content: [{ type: "text", text: `Weather in ${city}: 72F, sunny` }]
    };
  }
});

const transport = new StdioServerTransport();
await server.connect(transport);

Install with npm install @modelcontextprotocol/sdk. The SDK handles all the protocol details — you just define tools and implement handlers.