calculator-mcp-server

A mini Java Model Context Protocol (MCP) server that exposes two tools:

• calculate — performs arithmetic (add, subtract, multiply, divide)
• get_skill — returns usage guidance from SKILL.md, helping AI clients understand how to best use the server

Built with the official MCP Java SDK, using stdio transport so it can be connected directly to Claude Desktop.

Project structure

src/
  main/
    java/mortengf/ai/mcp/
      CalculatorMcpServer.java   # MCP server with two tools: calculate and get_skill
    resources/
      SKILL.md                   # Usage guidance returned by the get_skill tool

Motivation

mini-agent implements a mini Java agent loop that calls Claude API directly, with CalculatorTool defined using Claude-specific JSON (input_schema).

This server solves the portability problem identified in mini-agent: tools are now defined in the MCP-standard format and can be used by any MCP-compatible AI client — not just Claude.

What is a skill?

A skill is an MCP tool that returns guidance instead of performing an action. When an AI client connects to this server, it can call get_skill to understand how to best use the calculate tool.

The guidance lives in SKILL.md (packed into the JAR as a classpath resource) — the same pattern used by Claude Desktop's built-in skill system.

Prerequisites

• Java 17+
• Maven 3.6+
• Claude Desktop (to connect the server as an MCP tool)

Building

mvn package -q

Connecting to Claude Desktop

Add this to ~/Library/Application Support/Claude/claude_desktop_config.json:

"calculator": {
  "command": "/path/to/java",
  "args": ["-jar", "/path/to/calculator-mcp-server/target/calculator-mcp-server-1.0-SNAPSHOT.jar"]
}

Replace /path/to/java with the output of:

sdk home java 17.0.1-tem
# → /Users/mgf/.sdkman/candidates/java/17.0.1-tem/bin/java

Restart Claude Desktop — you can now ask Claude to calculate something and it will use your tool.

Example

You: What is 1 / 0?

Claude: Division by zero is not allowed.

You: What is (123 * 456) + (789 / 3)?

Claude: 56,351