Testing Skill

Stub-Driven Test-Driven Development and layer boundary testing for functional core and effectful edge architecture.

Core Principle: Stub-Driven TDD

Test-Driven Development workflow for the functional core / effectful edge pattern:

1. Stub   → Create minimal interface/function signatures
2. Test   → Write tests against stubs
3. Implement → Make tests pass with real implementation
4. Refactor  → Improve code while keeping tests green

Key insight: Write interface signatures first, test against those, then implement—not the other way around.

See [references/stub-driven-tdd.md] for complete workflow examples.