# Implementation Plan: 001-weather-cli

**Tech Stack**: Python 3.12+, `httpx` (HTTP client), `argparse` or `click` (CLI), `pytest` (Testing).

## 1. Research
- **API Choice**: Use `openweathermap.org` (reputable, simple).
- **Client**: `httpx` for async-capable HTTP requests.
- **Contract**: Expected JSON response structure from OpenWeather API current weather endpoint.

## 2. Design
- **Entities**: `CityWeather` dataclass (temp, condition, unit).
- **Contract**: `WeatherService` interface.
- **CLI Interface**: `fetch-weather --city [CITY] --units [metric|imperial]`.

## 3. Constitution Check
- **Article II: Test-First**: Plan includes `pytest` for all units and services.
- **Article V: Library-First**: Logic resides in `weather_fetcher/core.py`, CLI is a wrapper in `weather_fetcher/cli.py`.

## 4. Integration Scenarios
- [ ] `WEATHER_API_KEY` set → Fetch weather for "London" → Success.
- [ ] `WEATHER_API_KEY` missing → Fetch weather → Error message.
- [ ] Invalid city "Asdfghjkl" → Error message.