# Calculator MCP Server

A very basic MCP (Model Context Protocol) server built for training purposes. It exposes five simple tools over stdio so you can see the core MCP mechanics — tool listing, input schemas, and structured responses — without any extra complexity like auth, persistence, or external APIs.

## Tools

| Tool | Input | Description |
|---|---|---|
| `add` | `{a: number, b: number}` | Returns `a + b` |
| `subtract` | `{a: number, b: number}` | Returns `a - b` |
| `multiply` | `{a: number, b: number}` | Returns `a * b` |
| `divide` | `{a: number, b: number}` | Returns `a / b` (errors on `b = 0`) |
| `reverse_string` | `{text: string}` | Returns the reversed string |

## Setup

```bash
npm install
npm run build
```

## Testing locally

Use the [MCP Inspector](https://github.com/modelcontextprotocol/inspector) to interactively call each tool from a browser UI:

```bash
npm run inspect
```

This opens a local web page where you can select a tool, fill in arguments, and see the raw JSON-RPC request/response.

## Using it in Claude Desktop / Claude Code

Add this to your MCP client's config (e.g. `claude_desktop_config.json`):

```json
{
  "mcpServers": {
    "calculator": {
      "command": "node",
      "args": ["/Users/douglasbailey/my-dev/claude-mcp/dist/index.js"]
    }
  }
}
```

Restart the client, and the five calculator tools will be available for Claude to call.

## How it works

- `src/index.ts` creates an MCP `Server`, declares the `tools` capability, and connects over `StdioServerTransport` (the standard transport for local MCP servers).
- `ListToolsRequestSchema` handler returns the tool definitions (name, description, JSON-schema input).
- `CallToolRequestSchema` handler dispatches on `request.params.name` and returns `{content: [{type: "text", text: ...}]}`.
- Throwing an `Error` inside a handler (e.g. divide-by-zero) is automatically surfaced to the client as a tool error — no special handling needed.