Calculator MCP Server

A minimal MCP server exposing four tools — add, subtract, multiply, divide — in two versions:

• calculator_server.py — built with the Python mcp SDK (FastMCP). @mcp.tool() derives the JSON schema from type hints/docstrings and handles the JSON-RPC protocol for you.

• calculator_server_raw.py — the same server implemented from scratch with only the standard library (json + sys), no mcp dependency. It hand-rolls the JSON-RPC 2.0 message handling to show what FastMCP is doing underneath: read a newline-delimited JSON request from stdin, dispatch on method (initialize, tools/list, tools/call, ping), write a JSON-RPC response to stdout. See the file's top-of-file docstring for the framing details (no LSP-style headers, just one JSON object per line).

Setup

python -m venv .venv
.venv\Scripts\pip install -r requirements.txt

Related MCP server: Calculator MCP Server

Run it directly (stdio)

MCP servers talk over stdio (or HTTP), not a normal CLI — you don't "run and see output," a client connects to it. Two ways to try it:

1. MCP Inspector (visual, interactive) — requires Node.js/npx:

.venv\Scripts\mcp dev calculator_server.py

Opens a browser UI where you can call add, multiply, etc. by hand.

If the page loads but says "Connect to an MCP server to start inspecting" and there's no Tools tab, the sidebar didn't pick up the command automatically (can happen if a stale/previous session config is cached). Fill in the sidebar manually and click Connect:

• Transport Type: STDIO

• Command: C:\Users\simon\mcpserversample\.venv\Scripts\python.exe

• Arguments: C:\Users\simon\mcpserversample\calculator_server.py

Once connected, a Tools tab appears in the top nav where you can click "List Tools" and run each one.

2. test_client.py — a scripted client included in this repo that connects, lists tools, and calls each one. Defaults to calculator_server.py, or pass a path to test the other version:

.venv\Scripts\python test_client.py
.venv\Scripts\python test_client.py calculator_server_raw.py

Wiring it into Claude Desktop

Add to claude_desktop_config.json:

{
  "mcpServers": {
    "calculator": {
      "command": "C:\\Users\\simon\\mcpserversample\\.venv\\Scripts\\python.exe",
      "args": ["C:\\Users\\simon\\mcpserversample\\calculator_server.py"]
    }
  }
}

How it works

• calculator_server.py creates a FastMCP server and registers each Python function as a tool via the @mcp.tool() decorator. The function's type hints and docstring become the tool's JSON schema and description, which is what the calling client (Claude, the Inspector, or test_client.py) sees.

• mcp.run() starts the server listening on stdin/stdout using the MCP stdio transport — the standard way a client launches and talks to a local MCP server as a subprocess.

• A client connects, calls list_tools() to discover what's available, then call_tool(name, args) to invoke one and get the result back.