Testing

---
name: testing
description: "Elixir testing patterns — ExUnit, Mox, factories, LiveView test helpers. Use when working on *_test.exs, test/support/, factory files, or fixing test failures."
effort: medium
user-invocable: false
paths:
  - "test/**/*_test.exs"
  - "test/support/**/*.ex"
  - "**/*factory*.ex"
---

# Elixir Testing Reference

> **Ash projects**: Use `DataCase` with `Ash.Test` helpers; test actions via domain code interfaces, not direct `Repo` calls. See `ash-framework` skill.

Quick reference for Elixir testing patterns.

## Iron Laws — Never Violate These

1. **ASYNC BY DEFAULT** — Use `async: true` unless tests modify global state
2. **SANDBOX ISOLATION** — All database tests use Ecto.Adapters.SQL.Sandbox
3. **MOCK ONLY AT BOUNDARIES** — Never mock database, internal modules, or stdlib
4. **BEHAVIOURS AS CONTRACTS** — All mocks must implement a defined `@callback` behaviour
5. **BUILD BY DEFAULT** — Use `build/2` in factories; `insert/2` only when DB needed
6. **NO PROCESS.SLEEP** — Use `assert_receive` with timeout for async operations… install to load the full skill