Appetizer: Calculator Tool Server

A simple MCP server that exposes basic calculator operations as tools. Perfect for learning how mcp-framework tools work — add, subtract, multiply, divide.

title: "Appetizer: Calculator Tool Server" description: "A simple MCP server that exposes basic calculator operations as tools. Perfect for learning how mcp-framework tools work — add, subtract, multiply, divide." order: 1 category: "appetizer" level: "beginner" duration: "10 min" date: "2026-04-01" tags:

• tools
• math
• beginner keywords:
• mcp calculator server
• mcp-framework tools
• beginner mcp server
• calculator mcp tool

What You Get

A clean, minimal MCP server with a CalculatorTool that handles basic arithmetic. This is the best starting point if you have never built an MCP server before.

Tools included:

• calculate — accepts an operation (add, subtract, multiply, divide) and two numbers, returns the result

Quick Start

npx mcp-framework create calculator-server
cd calculator-server

The Code

Create src/tools/CalculatorTool.ts:

import { MCPTool } from "mcp-framework";
import { z } from "zod";

const CalculatorInput = z.object({
  operation: z.enum(["add", "subtract", "multiply", "divide"]),
  a: z.number().describe("First number"),
  b: z.number().describe("Second number"),
});

class CalculatorTool extends MCPTool<typeof CalculatorInput> {
  name = "calculate";
  description = "Perform basic arithmetic operations";
  schema = { input: CalculatorInput };

  async execute(input: z.infer<typeof CalculatorInput>) {
    const { operation, a, b } = input;

    switch (operation) {
      case "add":
        return { result: a + b };
      case "subtract":
        return { result: a - b };
      case "multiply":
        return { result: a * b };
      case "divide":
        if (b === 0) throw new Error("Division by zero");
        return { result: a / b };
    }
  }
}

export default CalculatorTool;

Build & Run

npm run build

Then add the server to your Claude Desktop config:

{
  "mcpServers": {
    "calculator": {
      "command": "node",
      "args": ["./dist/index.js"]
    }
  }
}

What You Learn

• How to define an MCP tool with MCPTool
• Input validation with Zod schemas
• Returning structured results from tools
• Connecting a server to Claude Desktop

Next Up

Ready for something a bit more involved? Try the Hello World Server or jump straight to the Weather API Server entree.

Built with mcp-framework (3.3M+ downloads) by @QuantGeekDev. Validated by Anthropic.