The Hidden Risk in Codebases: Behavior With No Documented Proof

As software systems age, a dangerous gap grows between how code actually behaves and what the repository can formally prove about that behavior. Critical logic — such as fraud rules, retry handling, or edge-case workarounds — often exists only in a developer's memory, an old Slack thread, or a long-forgotten pull request comment. Tests help, but they only validate what someone remembered to assert, leaving many real user-facing behaviors entirely unprotected. The rise of AI coding tools has sharpened this risk, as agents can silently simplify or remove undocumented logic while tests continue to pass. The author argues that missing behavioral evidence should be treated as a warning signal, and that code reviews must ask not just whether code looks correct, but what behavior it claims to preserve and where the proof lives.