A Python based MCP server calculator tool

Summary
Leverage MCP server python SDK to build a local MCP server.

Requirements
Install python version 3.14+
vscode or any IDE
Claude desktop as MCP Client

Steps
Create project directory and configure python virtual env

mkdir calculator
python -m venv venv
source .venv/Scripts/activate

Install and initialize uv

pip install uv
uv init

Add MCP CLI for the project dependencies

pip install mcp[cli]

Edit the created main.py file with the calculator tool

from mcp.server.fastmcp import FastMCP

# Initialize the MCP server
mcp = FastMCP("Simple Calculator")

@mcp.tool()
def add(a: float, b: float) -> float:
    """Adds two numbers together."""
    return a + b

@mcp.tool()
def subtract(a: float, b: float) -> float:
    """Subtracts b from a."""
    return a - b

@mcp.tool()
def multiply(a: float, b: float) -> float:
    """Multiplies two numbers."""
    return a * b

@mcp.tool()
def divide(a: float, b: float) -> str:
    """Divides a by b. Handles division by zero errors."""
    if b == 0:
        return "Error: Cannot divide by zero."
    return str(a / b)

if __name__ == "__main__":
    mcp.run(transport="stdio")

Install claude code and run the MCP server

uv run mcp install main.py
[01/18/26 13:34:04] INFO     Added server 'Simple Calculator' to Claude config                                                                      claude.py:136
                    INFO     Successfully installed Simple Calculator in Claude app                                                                    cli.py:485
(.venv) 

Check File -> Settings -> Developer on claude code if the recently created MCP server is running.

Click Edit config to see the MCP servers configured on the Claude desktop

{
    "mcpServers": {
        "Simple Calculator": {
            "command": "C:\\Users\\MCP\\calculator\\.venv\\Scripts\\uv.EXE",
            "args": [
                "run",
                "--frozen",
                "--with",
                "mcp[cli]",
                "mcp",
                "run",
                "C:\\Users\\MCP\\calculator\\main.py"
            ]
        }
    }
}

Test the MCP server

Start the conversation and provide an arithmetic calculation. Notice that claude leverages the calculator MCP server.
Claude asks for tool access and performs subsequent calculation.