It is also important to emphasize that 100% test coverage of the core code is essential. Yes, it is a significant amount of work, and yes, it can feel like you are writing tests for situations that “should never happen.”
However, the current mindset of many programmers still assumes they are programming only with other humans. When AI is involved, that assumption breaks down. An AI often has visibility into the entire codebase and will actively use any available paths to satisfy the prompt—even if those paths lead to behavior the user never intended.
Achieving 100% test coverage helps keep the AI in check by clearly defining what is allowed, what is forbidden, and what must never occur. In that sense, exhaustive tests are no longer just about catching bugs—they are a critical part of constraining and guiding AI behavior.
Today the AI noted
That’s the second time today a test written for new behaviour caught a latent defect in old behaviour.