Testability

Tests should target observable behavior rather than implementation internals to allow refactoring without unnecessary test failures. Testability gives coding agents guardrails by forcing progress through checkable steps. Manual review and AI-assisted issue prediction do not compensate for weak test coverage under production load. Pure functions are easy to test because their behavior is determined by their inputs and they have no side effects. Automated testing becomes the main limit on safe delivery speed when agents generate code faster than humans can read it. Dependency injection enables tests to substitute test doubles for real services. Hard-coded dependencies make mocking and meaningful unit testing impossible. Without strong testability, production becomes the first real feedback environment.