The Asymmetry of Change: Why Your Tests Are Looking the Wrong Way A passing build is often treated as a certificate of correctness. In reality, it's a narrow contract. It doesn't prove your code is right. It proves that the assertions you wrote in the past, against behaviors you anticipated back then, still hold true today. When you open a pull request, your unit tests ask: "Does the system still behave the way it used to?" The question you actually need to answer is different: "Is the new behavior I just introduced safe?" Those aren't the same thing. And that gap is exactly where production incidents live. The Wrong Question Here's the problem: tests are a snapshot of past understanding. Your code changed. Your tests didn't. And somehow the build is still green. A guard clause disappears. No test explicitly covered it because the guard was the coverage. A condition gets narrowed. An exception handler gets swapped. A state transition loses a validation step.…