TDD Playbook

HARD-GATE

• No implementation code is written until a test exists, is run, and fails for the right reason (missing behaviour, not syntax/config).
• Implementation requires explicit user approval.
• Refactoring preserves behaviour and keeps target tests green.
• The quality gate (mix format --check-formatted, mix credo --strict, mix dialyzer, mix test) must pass before opening a PR.

When to use

Building or changing Elixir behaviour where tests must gate implementation. Prefer pure-core unit tests first (FCIS); use DataCase only when persistence is the behaviour under test.

Atomic skills this playbook loads