@halfords-pro/calculator

An MCP (Model Context Protocol) server providing calculator tools with precise decimal arithmetic, suitable for accounting and tax calculations.

Features

• Precise decimal arithmetic — uses decimal.js to avoid floating-point errors
• Excel-style rounding — round half away from zero (ROUND_HALF_UP), the standard for accounting and tax
• MCP protocol — integrates with any MCP-compatible client (Claude Desktop, Claude Code, etc.)

Installation

npm install -g @halfords-pro/calculator

Usage

Claude Desktop

Add to your Claude Desktop configuration (claude_desktop_config.json):

{
  "mcpServers": {
    "calculator": {
      "command": "npx",
      "args": ["-y", "@halfords-pro/calculator"]
    }
  }
}

Claude Code

claude mcp add calculator -- npx -y @halfords-pro/calculator

Tools

sum

Sum an array of signed decimal numbers and round the result to a specified number of decimal places.

Parameters:

| Name | Type | Required | Description | |------|------|----------|-------------| | values | number[] | Yes | Array of signed decimal numbers to sum | | decimalPlaces | number | Yes | Number of decimal places to round the result to |

Examples:

sum({ values: [1.00, -2.50, 3.89], decimalPlaces: 2 })
→ "2.39"

sum({ values: [1.00, -2.50, 3.89283829, -539.38390293], decimalPlaces: 4 })
→ "-536.9911"

sum({ values: [0.1, 0.2], decimalPlaces: 2 })
→ "0.30"  (not 0.30000000000000004)

Rounding behaviour:

Uses "round half away from zero" (equivalent to Excel ROUND()):

| Value | Rounded to 2dp | |-------|---------------| | 2.125 | 2.13 | | -2.125 | -2.13 | | 2.124 | 2.12 |

Development

# Install dependencies
npm install

# Build
npm run build

# Run tests
npm test

# Run tests with coverage
npm run test:coverage

# Launch MCP Inspector
npm run inspector

License

ISC