A few tips:

- Make tasks short and break them into smaller steps. E.g. don't say "Add a UI button, and a handler that does the thing". But first add the button, confirm its showing as expected then move on to a handler, and so on.

- Do give warnings like "Don't modify unrelated code" but again what the model considers related and what you consider may not be the same (this is much easier if the tasks are small so see point 1).

- If the model keeps making similar mistakes or repeating the same broken thing its because it doesn't know how to solve the problem. These models don't have a way to tell you "I don't know" so they will just keep producing busted code. Give the model additional information to help it like you would a coworker who can't seem to make progress.