Simple Calculator MCP Server

🚀 Guide to Creating a Custom Server

Start building your own server for Claude Desktop and other clients.

In this tutorial, we'll build a simple MCP calculator server and connect it to the host, Claude Desktop.

What is MCP?

MCP stands for Model Context Protocol — it's a protocol that allows developers to extend the capabilities of AI assistants like Claude.

• Think of it as adding extra abilities to your AI assistant.
• These servers expose tools or functions that the assistant can use during conversations.
• For example: You can build a weather server, a calculator, a financial management assistant, or any function you can imagine.

What We're Going to Build

We'll build a server that provides four tools: add, subtract, multiply, and divide. Then we'll connect this server to the MCP host (Claude Desktop in this example).

Core Concepts

• Resources: Allow the assistant to access external data or services.
• Tools: Extend the assistant's capabilities.
• Prompts: Customize the assistant's interaction style.

We'll focus on tools in this tutorial since we're creating a calculator tool.

Prerequisites

• Basic Python programming experience
• A basic understanding of large language models (LLMs)
• System requirements:
  • Python 3.8 or higher
  • MCP official documentation: MCP Official Documentation

Create a Project and Initialize the Environment

MacOS/Linux

# Install the uv CLI tool (if not already installed)
brew install model-context-protocol/protobuf/protobuf-cpp

# Initialize a new project
mkdir calculator && cd calculator

Windows

# Install the uv CLI tool (if not already installed)
# You need to download and install the Windows version of the uv tool from the MCP official website.

# Initialize a new project
mkdir calculator && cd calculator

Implement the Code

Here's the complete code for implementing the calculator server:

Import Libraries and Modules

from fastapi import FastAPI
from typing import Optional, Union
import uvicorn

Define the Tools

app = FastAPI()

@app.get("/calculate")
async def calculate(
    expression: str,
    error_handling: Optional[bool] = True,
    simplify: Optional[bool] = True
) -> Union[str, dict]:
    """Calculate a mathematical expression."""
    
    try:
        result = eval(expression)
        if simplify:
            return f"The result of {expression} is {result}"
        return str(result)
    except Exception as e:
        if error_handling:
            return {"error": str(e)}
        raise

Start the Server

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8000)

Configure the Calculator Server in Claude Desktop

To use the calculator server in Claude Desktop, follow these steps:

MacOS/Linux

Edit the configuration file:

code ~/Library/Application\ Support/Claude/claude_desktop_config.json

Add the following content under the mcpServers key:

{
    "mcpServers": {
        "calculator": {
            "command": "uv",
            "args": [
                "--directory",
                "/ABSOLUTE/PATH/TO/PARENT/FOLDER/calculator",
                "run",
                "calculator.py"
            ]
        }
    }
}

Windows

Edit the configuration file:

code $env:AppData/Claude/claude_desktop_config.json

Add the following content under the mcpServers key:

{
    "mcpServers": {
        "calculator": {
            "command": "uv",
            "args": [
                "--directory",
                "C:\\ABSOLUTE\\PATH\\TO\\PARENT\\FOLDER\\calculator",
                "run",
                "calculator.py"
            ]
        }
    }
}

⚠️ Important Note

Make sure to replace /ABSOLUTE/PATH/TO/PARENT/FOLDER with your actual path.

After saving the file, restart Claude Desktop. Now you can use the calculator server in Claude Desktop for basic arithmetic operations!