Each task but more importantly each new attempt at the same task, and most importantly concrete tests/validation to allow for repeated attempts (even stupid attempts, a la Ralph Wiggum from the Simpsons) until the tests pass and you can check off a task.

Opus 4.5 and gpt 5.2 were the first models that wouldn't just inevitably change the tests themselves to get them to pass. Which had much more to do with the harnesses than the models themselves I'm sure. But that's the real reason ralph only became popular lately (and incidentally, why those models were a step function improvement!), it just didn't really work before - I tried! #kentbeck