Powder Compass MCP ❄️

A small local MCP server that gives Claude Desktop a set of honest, tested tools for scoring a ski or snowboard day — plus the unit converters an imperial-units skier needs to feed it.

The scoring is a faithful port of the Rust CLI powder-compass: same bands, same verdicts, same practical tips, same safety override — ported, not redesigned.

Fun planning only — always check the resort's official conditions and avalanche information before you go.

The tools

Out-of-range or unknown-unit inputs are rejected with a clear error — never silently clamped or guessed. That is a design rule, not an accident (see Why this exists below).

Related MCP server: OpenWeather MCP Server

Quick start

Requires Python 3.10+ and uv.

uv run pytest -q          # run the test suite (13 tests)
uv run mcp dev server.py  # poke the tools by hand in the MCP Inspector

Use it in Claude Desktop

Add an entry like this to Claude Desktop's claude_desktop_config.json (Settings → Desktop app → Local MCP servers → Edit config shows you the exact file your install actually reads), then restart Claude:

{
  "mcpServers": {
    "PowderCompass": {
      "command": "uv",
      "args": [
        "run",
        "--directory",
        "C:\\path\\to\\powder-compass-mcp",
        "python",
        "server.py"
      ]
    }
  }
}

Windows note (learned the hard way): if mcp install tells you "Claude app not found", your Claude Desktop is probably the MSIX-packaged build. Its real config file may live outside the app package, e.g. at %LOCALAPPDATA%\Claude-Data\<package-id>\claude_desktop_config.json. The Edit config button in Settings always opens the true file — trust it, not the textbook paths. Back the file up before editing, merge the mcpServers entry in, and restart Claude fully (tray → Quit) so it re-reads the config.

Then just ask, in plain words:

• "We got 24 cm of fresh snow overnight, it's −8 °C, the wind is 8 km/h and visibility is perfect. How good a ski day is it?" → 96/100, "Champagne-powder alert"

• "How fast is 60 mph wind in km/h?" → 96.6 km/h

• "An American resort reports 10 inches of fresh snow, 20 °F, wind 10 mph, visibility 4 out of 5…" → Claude converts, then scores: 89/100

Why this exists

In April 2025 the author ran an experiment: an MCP add tool that quietly returned a − b under an honest "Add two numbers" docstring. The lesson stuck — a language model trusts a tool's description, not its code.

This server is the counterpart. Every tool's behaviour is pinned down by automated tests (test_server.py, pytest) that mirror the original Rust test suite and add converter and error-case checks, so the descriptions Claude reads are claims the code has been proven to keep. Trust, but verify — then earn the trust.

Scoring at a glance

Verdicts run from "Champagne-powder alert" (85+) down to "Hot-chocolate conditions" — with a safety override: at −30 °C or colder, or wind at 70 km/h or more, the verdict is always "Red-flag conditions — official advice first", whatever the score.

Tech

• Python + MCP Python SDK 2.3.0 (mcp[cli]), managed with uv

• Tests: pytest — uv run pytest -q

Licence

MIT (same as the Rust original).