MCP Demo

This is a demo of the MCP (Model Context Protocol) for testing and demonstration purposes. It includes a simple implementation of the MCP protocol and a sample calculator MCP server that can be used to test the protocol.

Running the Demo

npm install @modelcontextprotocol/sdk zod
node demo-client.mjs

And output should be:

✅ Calculator MCP server running on stdio
🔢 Calculator MCP Demo
───────────────────────────────────

📋 Available tools: multiply, add, subtract, divide

  7 × 6 = 42
  123 × 456 = 56088
  42 + 58 = 100
  100 − 37 = 63
  144 ÷ 12 = 12
  Error: Division by zero is undefined.

✅ Demo complete!

How It Works

There are two files:

server.mjs — the MCP Server

• Registers 4 tools: multiply, add, subtract, divide

• Each tool has a name, description, and typed inputs (using Zod)

• Listens for calls over stdio (stdin/stdout)

demo-client.mjs — the MCP Client

• Spawns server.mjs as a subprocess

• Calls listTools() to discover what the server offers

• Calls each tool with arguments, gets results back

The communication between them looks like this:

demo-client.mjs  ──── JSON-RPC over stdio ────▶  server.mjs
   (Client)             { tool: "multiply",         (Server)
                          args: { a:7, b:6 } }
                ◀────────────────────────────────
                         { result: "7 × 6 = 42" }

Connect to Claude Desktop

This lets Claude itself call your calculator tools in conversation.

Find your Claude Desktop config file:

Mac: ~/Library/Application Support/Claude/claude_desktop_config.json Windows: %APPDATA%\Claude\claude_desktop_config.json

See https://modelcontextprotocol.io/docs/develop/connect-local-servers for details.

Add this (replace the path with your actual folder path):

{
  "mcpServers": {
    "calculator": {
      "command": "node",
      "args": ["/Users/yourname/calculator-mcp/server.mjs"]
    }
  }
}

Restart Claude Desktop. Then try asking Claude: "What is 123 multiplied by 456?" — it will use your multiply tool to answer.