How to build your first MCP server

1mkdir weather-mcp-server
2cd weather-mcp-server
3npm init -y
4npm install @modelcontextprotocol/sdk zod
5npm install -D typescript tsx @types/node
6npx tsc --init

1// package.json — add/update these fields:
2{
3  "type": "module",
4  "scripts": {
5    "dev": "tsx src/index.ts",
6    "build": "tsc",
7    "start": "node dist/index.js",
8    "inspect": "npx -y @modelcontextprotocol/inspector tsx src/index.ts"
9  }
10}
11
12// tsconfig.json — update compilerOptions:
13{
14  "compilerOptions": {
15    "target": "ES2022",
16    "module": "NodeNext",
17    "moduleResolution": "NodeNext",
18    "outDir": "./dist",
19    "rootDir": "./src",
20    "strict": true,
21    "esModuleInterop": true,
22    "skipLibCheck": true
23  },
24  "include": ["src/**/*"]
25}

1// src/index.ts
2import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
3import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
4import { z } from "zod";
5
6const server = new McpServer({
7  name: "weather-server",
8  version: "1.0.0",
9});
10
11// Tools and resources will be added here
12
13const transport = new StdioServerTransport();
14await server.connect(transport);
15console.error("Weather MCP server started");

1// Add before the transport connection in src/index.ts
2
3interface WeatherData {
4  temperature: number;
5  condition: string;
6  humidity: number;
7  wind_speed: number;
8}
9
10const weatherDb: Record<string, WeatherData> = {
11  "new york": { temperature: 18, condition: "Partly cloudy", humidity: 65, wind_speed: 12 },
12  "london": { temperature: 12, condition: "Overcast", humidity: 80, wind_speed: 20 },
13  "tokyo": { temperature: 22, condition: "Clear sky", humidity: 55, wind_speed: 8 },
14  "paris": { temperature: 15, condition: "Light rain", humidity: 75, wind_speed: 15 },
15  "sydney": { temperature: 25, condition: "Sunny", humidity: 45, wind_speed: 10 },
16};
17
18server.tool(
19  "get_weather",
20  "Get current weather conditions for a city",
21  {
22    city: z.string().describe("City name (e.g., 'New York', 'London', 'Tokyo')"),
23  },
24  async ({ city }) => {
25    const data = weatherDb[city.toLowerCase()];
26    if (!data) {
27      return {
28        content: [{
29          type: "text",
30          text: `Weather data not available for "${city}". Supported cities: ${Object.keys(weatherDb).join(", ")}`,
31        }],
32        isError: true,
33      };
34    }
35    return {
36      content: [{
37        type: "text",
38        text: [
39          `Weather in ${city}:`,
40          `  Temperature: ${data.temperature}°C`,
41          `  Condition: ${data.condition}`,
42          `  Humidity: ${data.humidity}%`,
43          `  Wind speed: ${data.wind_speed} km/h`,
44        ].join("\n"),
45      }],
46    };
47  }
48);

1server.tool(
2  "compare_weather",
3  "Compare weather between two cities",
4  {
5    city1: z.string().describe("First city name"),
6    city2: z.string().describe("Second city name"),
7  },
8  async ({ city1, city2 }) => {
9    const data1 = weatherDb[city1.toLowerCase()];
10    const data2 = weatherDb[city2.toLowerCase()];
11
12    if (!data1 || !data2) {
13      const missing = [!data1 && city1, !data2 && city2].filter(Boolean);
14      return {
15        content: [{ type: "text", text: `Weather data not available for: ${missing.join(", ")}` }],
16        isError: true,
17      };
18    }
19
20    const warmer = data1.temperature > data2.temperature ? city1 : city2;
21    const diff = Math.abs(data1.temperature - data2.temperature);
22
23    return {
24      content: [{
25        type: "text",
26        text: [
27          `${city1}: ${data1.temperature}°C, ${data1.condition}`,
28          `${city2}: ${data2.temperature}°C, ${data2.condition}`,
29          ``,
30          `${warmer} is ${diff}°C warmer.`,
31        ].join("\n"),
32      }],
33    };
34  }
35);

1server.resource(
2  "supported-cities",
3  "weather://cities",
4  { description: "List of all supported cities with current conditions" },
5  async (uri) => ({
6    contents: [{
7      uri: uri.href,
8      mimeType: "application/json",
9      text: JSON.stringify(
10        Object.entries(weatherDb).map(([city, data]) => ({
11          city: city.charAt(0).toUpperCase() + city.slice(1),
12          temperature: `${data.temperature}°C`,
13          condition: data.condition,
14        })),
15        null,
16        2
17      ),
18    }],
19  })
20);

1npx -y @modelcontextprotocol/inspector tsx src/index.ts

1// ~/Library/Application Support/Claude/claude_desktop_config.json
2{
3  "mcpServers": {
4    "weather": {
5      "command": "npx",
6      "args": ["-y", "tsx", "/absolute/path/to/weather-mcp-server/src/index.ts"]
7    }
8  }
9}