Code that is cheap to change correctly. Two programs can pass the same tests today and differ hugely in what next week's request costs. Working is the minimum, and how easily it changes is what separates them.
A codebase with an agent in it gets changed far more often, and each session starts by rereading it from nothing. Every change pays for the shape the last one left behind, and no test reports that price going up.
Say it to your agent
“Before you add this, tell me how many files the next change like it would touch. If it is more than two, first restructure so the rule lives in one place, keeping the tests green, and then make the change.”