Test-Driven Development (TDD)

When to use

• Starting implementation of new feature
• Starting implementation of bugfix
• Writing new production code

Skip when

• Reviewing/modifying existing tests
• Exploratory/spike work — TDD is for known requirements, not exploration.

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

The Iron Law