Confidence, Not Coverage

Tests are often justified using coverage metrics. Coverage is a weak proxy for what tests actually provide: confidence.

Painful tests usually indicate deeper design problems. Tight coupling, unclear boundaries, and leaky abstractions reveal themselves quickly during testing.

Different tests answer different questions:

• Unit tests validate logic
• Integration tests validate boundaries
• End-to-end tests protect critical flows

Over-investing in one category creates blind spots. Mock-heavy unit tests can pass while systems fail. End-to-end tests alone are slow and fragile.

Testing lessons that repeat themselves:

• Tests should describe intent, not implementation
• If refactoring breaks many tests, tests are too coupled
• Mock behavior, not structure
• Tests are most valuable when they fail for the right reasons

Testing enables refactoring. Without tests, change feels dangerous. With them, change becomes routine. This makes testing foundational to long-lived systems.

Good tests act as documentation. They show how the system is expected to behave under real conditions.