How do you keep coding agents from touching parts of the code they shouldn't?

I was dealing with an issue where automated scripts kept trying to "optimize" small stuff instead of just finishing the main task. You ask for a simple feature update, and suddenly it's changing file structures and refactoring background logic nobody asked for.

It feels like the more context you give them, the easier it is for them to get distracted by non essential tasks.

For people building with cursor or agent setups daily how do you look down scope so it stays focused on a single file or task?