MCP — Calculation Server

A stateless Model Context Protocol (MCP) server that exposes arithmetic tools over HTTP. MCP clients (Claude Desktop, VS Code Copilot, Cursor, etc.) can connect to it and invoke the tools conversationally.

Architecture

MCP Client (Claude, VS Code, etc.)
        │
        │  HTTP POST /mcp  (JSON-RPC 2.0)
        ▼
  Express.js server  :3000
        │
        │  per-request lifecycle
        ▼
  McpServer  ──  StreamableHTTPServerTransport
        │
  4 calculation tools

Each incoming request gets a fresh McpServer instance that is torn down when the response closes (stateless mode).

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Tools

Tool	Description	Inputs	Output
add_two_numbers	Add two numbers	a: number, b: number	result: number
subtract_two_numbers	Subtract the second number from the first	a: number, b: number	result: number
multiply_two_numbers	Multiply two numbers	a: number, b: number	result: number
divide_two_numbers	Divide the first number by the second	a: number, b: number	result: number

divide_two_numbers returns an error response when b is 0 rather than throwing.

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Prerequisites

• Node.js 18+
• npm

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Getting Started

1. Install dependencies

npm install

2. Run in development mode (hot reload)

npm run dev

The server starts at http://localhost:3000/mcp.

3. Build for production

Note: tsconfig.json compiles to ./build/, but package.json's start script points to dist/index.js. Fix one of them before running production mode.

# Option A — update start script to match tsconfig outDir
npm run build
node build/index.js

# Option B — update tsconfig outDir to "dist" and then run
npm run build
npm start

4. Run with Docker

docker build -t mcp-calculation .
docker run -p 3000:3000 mcp-calculation

Custom port:

docker run -p 8080:8080 -e PORT=8080 mcp-calculation

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Connecting an MCP Client

VS Code (Copilot / Claude extension)

A .vscode/mcp.json is already included:

{
  "servers": {
    "my-mcp-server-de2625e8": {
      "url": "http://localhost:3000/mcp",
      "type": "http"
    }
  }
}

Start the server (npm run dev), then open the MCP panel in VS Code — the Calculation Server will appear automatically.

Claude Desktop

Add the following to your Claude Desktop config (claude_desktop_config.json):

{
  "mcpServers": {
    "calculation-server": {
      "url": "http://localhost:3000/mcp",
      "type": "http"
    }
  }
}

Any HTTP MCP client

Setting	Value
URL	http://localhost:3000/mcp
Transport	HTTP (Streamable HTTP)
Method	POST
Protocol	JSON-RPC 2.0

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Using the Tools

Once connected, ask your MCP client naturally:

What is 42 plus 58?
What is 100 divided by 4?
Multiply 7 by 8.
Subtract 15 from 100.

Direct HTTP example (curl)

# List available tools
curl -X POST http://localhost:3000/mcp \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "tools/list"
  }'

# Call add_two_numbers
curl -X POST http://localhost:3000/mcp \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 2,
    "method": "tools/call",
    "params": {
      "name": "add_two_numbers",
      "arguments": { "a": 12, "b": 30 }
    }
  }'

Expected response:

{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "content": [{ "type": "text", "text": "The sum of 12 and 30 is 42" }],
    "structuredContent": { "result": 42 }
  }
}

# Division by zero — returns an error response (not an HTTP error)
curl -X POST http://localhost:3000/mcp \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 3,
    "method": "tools/call",
    "params": {
      "name": "divide_two_numbers",
      "arguments": { "a": 10, "b": 0 }
    }
  }'

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Environment Variables

Variable	Default	Description
PORT	3000	Port the HTTP server listens on

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Project Structure

mcp-calculation/
├── src/
│   └── index.ts        # Server entry point — tools + Express setup
├── .vscode/
│   └── mcp.json        # VS Code MCP client config
├── Dockerfile
├── package.json
└── tsconfig.json

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Tech Stack

Package	Purpose
@modelcontextprotocol/sdk	MCP server + transport primitives
express	HTTP server (v5)
zod	Input/output schema validation
tsx	TypeScript execution for dev mode
typescript	Type checking + build