Calculator MCP Server

A simple Model Context Protocol (MCP) server that provides basic calculator operations as tools for Claude Desktop and other MCP clients.

Features

• Add: Sum two numbers
• Subtract: Find the difference between two numbers
• Multiply: Calculate the product of two numbers
• Divide: Divide numbers with proper zero-division handling

All operations support integers, floating-point numbers, and negative numbers.

Installation

Prerequisites

• Python 3.10 or higher
• pip (Python package manager)

Steps

1. Clone or download this repository

git clone https://github.com/snedea/calc-mcp-server.git
cd calc-mcp-server

2. Install dependencies

pip install -r requirements.txt

This installs:

• fastmcp>=2.0.0 - MCP framework
• pytest>=7.0.0 - Testing framework

Usage

Testing the Server

Run the server standalone to verify it works:

python3 calc_server.py

You should see:

🧮 Calculator MCP Server Starting

📋 Available tools:
   - add(a, b): Add two numbers
   - subtract(a, b): Subtract b from a
   - multiply(a, b): Multiply two numbers
   - divide(a, b): Divide a by b (handles zero)

💡 Configure in Claude Desktop to use these tools!

Press Ctrl+C to stop the server.

Configuring in Claude Desktop

To use these calculator tools in Claude Desktop:

1. Locate your Claude Desktop config file:

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

2. Edit the config file (create it if it doesn't exist):

{
  "mcpServers": {
    "calculator": {
      "command": "python3",
      "args": ["/absolute/path/to/calc-mcp-server/calc_server.py"]
    }
  }
}

Important: Replace /absolute/path/to/calc-mcp-server/ with the actual full path to this directory.

3. Restart Claude Desktop

The calculator tools will now be available! Claude can use them automatically when you ask questions involving calculations.

Available Tools

add(a, b)

Add two numbers together.

Parameters:

• a (float): First number
• b (float): Second number

Returns: JSON with operation details and result

Example:

Input: add(5, 3)
Output: {
  "operation": "add",
  "a": 5,
  "b": 3,
  "result": 8
}

subtract(a, b)

Subtract the second number from the first.

Parameters:

• a (float): Number to subtract from
• b (float): Number to subtract

Returns: JSON with operation details and result

Example:

Input: subtract(10, 3)
Output: {
  "operation": "subtract",
  "a": 10,
  "b": 3,
  "result": 7
}

multiply(a, b)

Multiply two numbers together.

Parameters:

• a (float): First number
• b (float): Second number

Returns: JSON with operation details and result

Example:

Input: multiply(4, 5)
Output: {
  "operation": "multiply",
  "a": 4,
  "b": 5,
  "result": 20
}

divide(a, b)

Divide the first number by the second.

Parameters:

• a (float): Numerator
• b (float): Denominator

Returns: JSON with operation details and result, or error if dividing by zero

Example:

Input: divide(10, 2)
Output: {
  "operation": "divide",
  "a": 10,
  "b": 2,
  "result": 5
}

Input: divide(10, 0)
Output: {
  "error": "Cannot divide by zero"
}

Testing

Run the test suite to verify all operations work correctly:

pytest tests/test_calculator.py -v

Expected output:

tests/test_calculator.py::TestAddition::test_add_positive_numbers PASSED
tests/test_calculator.py::TestAddition::test_add_negative_numbers PASSED
... (30+ tests)
================================ 30 passed in 0.XX s ================================

All tests should pass, covering:

• Basic arithmetic operations
• Edge cases (zero, negatives, decimals)
• Error handling (division by zero)
• JSON response format validation

Project Structure

calc-mcp-server/
├── calc_server.py          # Main MCP server with calculator tools
├── requirements.txt        # Python dependencies
├── tests/
│   └── test_calculator.py  # Comprehensive unit tests
├── README.md               # This file
└── .gitignore              # Git exclusions

Troubleshooting

Tools don't appear in Claude Desktop

• ✅ Check that the config file path is correct for your OS
• ✅ Verify the args path is absolute (not relative)
• ✅ Ensure Python 3.10+ is installed: python3 --version
• ✅ Confirm FastMCP is installed: pip show fastmcp
• ✅ Restart Claude Desktop completely

Import errors when running server

• ✅ Install dependencies: pip install -r requirements.txt
• ✅ Use Python 3.10 or higher
• ✅ Consider using a virtual environment:

  python3 -m venv venv
  source venv/bin/activate  # On Windows: venv\Scripts\activate
  pip install -r requirements.txt
  

Tests failing

• ✅ Ensure pytest is installed: pip install pytest
• ✅ Run from the project root directory
• ✅ Check Python version is 3.10+

Technologies Used

• Python 3.10+: Modern Python with type hints
• FastMCP 2.0+: Official MCP framework for Python
• pytest: Testing framework

How It Works

MCP (Model Context Protocol) is a standard for connecting AI assistants like Claude to external tools and data sources. This server:

1. Defines calculator operations as Python functions
2. Decorates them with @mcp.tool() to expose them via MCP
3. Communicates with Claude Desktop using standard input/output (stdio transport)
4. Returns structured JSON responses for reliable parsing

Claude can then call these tools automatically when needed for calculations!

License

MIT

Contributing

Contributions welcome! Feel free to:

• Report bugs
• Suggest new calculator features
• Improve documentation
• Add more tests

Created By

🤖 Generated with Context Foundry - Autonomous Development Platform