first-mcp-server

The smallest MCP server that actually does something — one tool, stdio transport, no build step. About 35 lines in server.ts.

I wrote this while figuring out how MCP works. If you've read Understanding MCP Servers, this is that post's weather example as code you can actually run.

What it does

Exposes a single tool, getCityWeather, over stdio. Ask Claude "what's the weather in Delhi?" and it discovers the tool, calls it, and answers from the result.

The weather itself is a hardcoded map of three cities. That's on purpose — the point here is the protocol, not the data. Swap the lookup for a real API call and nothing else about the server changes.

Related MCP server: Weather MCP Server

Run it

npm install
npm start

That starts the server on stdio. On its own it does nothing visible — it's waiting for a client to talk to it, which is the whole idea.

Connect it to Claude

Add it to your MCP config (claude_desktop_config.json for Claude Desktop, or .mcp.json for Claude Code):

{
  "mcpServers": {
    "first-server": {
      "command": "npx",
      "args": ["tsx", "/absolute/path/to/first-mcp-server/server.ts"]
    }
  }
}

Use an absolute path — the host spawns this as a subprocess and won't be in your project directory. Restart the host, then ask it about the weather in Delhi, Mumbai or Bangalore.

The three pieces

Everything in server.ts is one of these:

The description and schema matter more than they look. They're the only thing the model reads when deciding whether this tool is relevant to your question, so vague descriptions produce a tool that never gets called.

License

MIT