A requirement says what you want. Acceptance criteria are how you will know you got it.
Acceptance criteria
The specific checks that decide whether this piece of work is finished, agreed before it starts. Each one passes or fails. The best are written so that a program can run them.
An agent's own tests check what it built, not what you needed. Of the 11 broken vague attempts in our runs, 9 passed every test they wrote for themselves. Criteria written before the code, by someone who knows the job, can't be shaped by the code.
“Before you write code, add these to tests/webhook.test.ts and show me them failing: a repeat of the same event id ships once; refunded at 2 then paid at 1 leaves the status refunded and ships nothing. Then make them pass. Do not edit the tests.”