Preserve existing behavior
Tell the agent that everything that works today must keep working exactly the same way, except for the specific thing you asked to change.
Treat all current user-visible behavior outside the requested change as a requirement, including quirks users may rely on. Changes should be additive or tightly targeted, and verified against existing tests or manual checks.
Asking a tailor to shorten your sleeves. You expect the same jacket back with shorter sleeves, not a new collar and different buttons too.
Agents sometimes improve things you didn't ask about: defaults, error messages, sort orders, validation rules. Users notice even small changes. Saying this turns current behavior into an explicit rule.
Added date sorting. Existing behavior is preserved: alphabetical is still the default, and pinned favorites stay on top. The existing list tests still pass.
Add date sorting to the recipe list. Preserve existing behavior: alphabetical stays the default and pinned recipes stay on top. Tell me if anything else would change.
Assuming the agent knows which current behaviors matter. It can't read your users' minds. Name the behaviors that matter most.