@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