TDD — Test-Driven Development

Build features and fix bugs using the test-first red-green-refactor cycle. Tests specify observable behavior through public interfaces — they survive internal refactors.

Core philosophy

Tests should verify behavior through public interfaces, not implementation details.

Good tests read like specifications. They tell you what the system does, not how it does it.

When to use this skill

• User requests test-first development or red-green-refactor
• Building new features where behavior should drive design
• Fixing bugs where a regression test should come first
• Any work where test design should precede implementation

When not to use this skill