What 100% Test Coverage Can't Measure

What 100% Test Coverage Can't Measure Customers started asking us: "How do you evaluate test quality? What does your evaluation look like?" We had coverage numbers - line, branch, function - and we were driving files to 100%. But we didn't have a good answer for what happens after 100%. Coverage proves every line was exercised. It doesn't say whether the tests are actually good. Coverage Is the Foundation Coverage tells you which lines ran during testing. That's important. A file at 30% coverage has obvious blind spots. Driving it to 100% forces tests to exercise error branches, conditional paths, and edge cases that might otherwise be ignored. We treat coverage as the primary goal and spend most of our effort getting files there. But coverage measures execution, not verification. A test that renders a payment form, types a valid card number, and clicks submit can hit every line and every branch. It proves the happy path works. It doesn't tell you whether the form handles an expired ca