An untested codebase doesn't just lack tests. It actively teaches every contributor, human or agent, that tests aren't part of the work.

Coding agents pattern-match on what they find. If the convention is no tests, that's what they'll write. And the longer it goes on, the more the codebase is shaped in ways that make testing harder, which entrenches the pattern further. New post on why testing is no longer optional in the new world of agentic coding, how to start when you have zero tests, and what to do when the code resists being tested at all.