MCP Server Calculator

How to build an MCP server - Calculator Example

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License: MIT

About

This is a very simple example of how to build an MCP server that provides basic math operations to an MCP client.

What is MCP?

Model Context Protocol (MCP) is a protocol that enables LLMs to access custom tools and services. An MCP client can make requests to MCP servers to access tools that they provide. This example represents a simple calculator server that provides basic math operations to a client.

"MCP can provide a single, standardized way for AI models to interact with external systems. You write code once and all AI systems can use it." 1

"Servers are the fundamental building block that enriches LLMs with external data and context." 2

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Tech Stack

Python3
Pytest
MCP SDK
MCP CLI
Nix (optional)

How to setup and use

Smithery

To install mcp-server-calculator for Claude Desktop automatically via Smithery:

npx -y @smithery/cli install @simonberner/mcp-server-calculator --client claude

Development Setup

As we are using Nix in this project for having a reproducible and isolated development environment, there is no need to install Python or any other dependencies. You will get everything you need out of the box. Also there is no need to create a virtual environment in Python using venv or poetry, but you have to be willing to install Nix on your system.

Install direnv (used to load the development environment)
Install Lix
Clone this repository
cd into the directory
Run direnv allow
Run pytest

Alternative

If you don't want to take the Smithery or the Nix route, you can of course also use uv to build the package/project. Here is some guidance:

# Install uv (see https://docs.astral.sh/uv/getting-started/installation/)
curl -LsSf https://astral.sh/uv/install.sh | sh

# Create a virtual environment named .venv
uv venv

# Activate the virtual environment (example for bash/zsh)
source .venv/bin/activate

# Now run the install or build commands
uv pip install -e .
# or
uv pip wheel . -w dist

Running & Testing the Server locally

MCP Inspector is a handy GUI tool that lets you test your custom MCP server via web interface at http://localhost:5173 without integrating it with LLM / AI agents.

mcp dev src/mcp_server_calculator/calculator.py

Host/Client integration

Add the following to your mcp config:

{
  "mcpServers": {
    "calculator": {
      "command": "python",
      "args": [
        "<YOUR-PATH>/mcp-server-calculator/src/mcp_server_calculator/calculator.py"
      ]
    }
  }
}

Notes

When using Windsurf IDE as MCP Host, you need to have a Pro Plan to be able to use Cascade (as MCP Client) with MCP Servers.