Testing Skill for Claude
Write or repair Elixir tests with ExUnit, sandbox isolation, async reliability, Mox, ExMachina, and LiveViewTest. Use for test files, test setup, or failing/flaky tests. NOT for investigating an application bug outside the test suite.
Install
one command, takes just this skill from the repositorynpx skills add https://github.com/oliver-kriska/claude-elixir-phoenix --skill testingWhat comes with it
12 329 bytes besides the instructionThe instruction itself
10 sections, as written by the authorElixir 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
- ASYNC BY DEFAULT — Use
async: trueunless tests modify global state - SANDBOX ISOLATION — All database tests use Ecto.Adapters.SQL.Sandbox
- MOCK ONLY AT BOUNDARIES — Never mock database, internal modules, or stdlib
- BEHAVIOURS AS CONTRACTS — All mocks must implement a defined
@callbackbehaviour - BUILD BY DEFAULT — Use
build/2in factories;insert/2only when DB needed - NO PROCESS.SLEEP — Use
assert_receivewith timeout for async operations - VERIFY_ON_EXIT! — Always call in Mox tests setup
- FACTORIES MATCH SCHEMA REQUIRED FIELDS — Factory definitions must include all fields that have
validate_requiredin the schema changeset. Missing fields cause cascading test failures
Quick Decisions
Which Test Case?
| Testing | Use |
|---------|-----|
| Controller/API | use MyAppWeb.ConnCase |
| Context/Schema | use MyApp.DataCase |
| LiveView | use MyAppWeb.ConnCase + import Phoenix.LiveViewTest |
| Pure logic | use ExUnit.Case, async: true |
When to use async: true?
- ✅ Pure functions, no shared state
- ✅ Database tests with Sandbox (PostgreSQL)
- ❌ Tests modifying
Application.put_env - ❌ Tests using Mox global mode
Mock or not?
- ✅ Mock: External APIs, email services, file storage
- ❌ Don't mock: Database, internal modules, stdlib
build() or insert()?
- Use
build()by default for speed - Use
insert()only when you need DB ID, constraints, or persisted associations
Quick Patterns
# Setup chain
setup [:create_user, :authenticate]
# Pattern matching assertion
assert {:ok, %User{name: name}} = create_user(attrs)
# Async message assertion
assert_receive {:user_created, _}, 5000
# Mox setup
setup :verify_on_exit!
expect(MockAPI, :call, fn _ -> {:ok, "data"} end)
# LiveView async
html = render_async(view) # MUST call for assign_async
Common Anti-patterns
| Wrong | Right |
|-------|-------|
| Process.sleep(100) | assert_receive {:done, _}, 5000 |
| insert(:user) in factory | build(:user) in factory |
| async: true with set_mox_global() | async: false |
| Mock internal modules | Test through public API |
References
For detailed patterns, see:
references/exunit-patterns.md- Setup, assertions, tagsreferences/mox-patterns.md- Behaviours, expect/stub, asyncreferences/liveview-testing.md- Forms, async, uploadsreferences/factory-patterns.md- ExMachina, sequences, traits
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How to use it
Take oliver-kriska/claude-elixir-phoenix-testing from the repository into ~/.claude/skills for personal
use, or into .claude/skills inside a project.
The agent identifies a skill by the name field in its header. Two skills with the
same name cannot sit side by side — one of them will be ignored.