That depends on the model and the toolkit it uses. In my experience from using Claude Code (Max, Opus 4.5+) intensely for the past six months, I maybe had 3 instances where the implementation broke functionally. And all of these breaking changes were resolved by Claude.

Obviously, this won't apply to every context: I work primarily with well-known langs (e.g., Python, JS), small to medium codebases (<500k LoC, for sure), and relatively few co-developers.