The answer opens with a number, and it plays the same part here that the exit code played in the terminal.
Status code
Also called HTTP status
A three-digit number at the top of every response. 2xx means it worked, 3xx means look elsewhere, 4xx means the request was wrong, and 5xx means the server failed. It works like an exit code for the web.
This is the word that sharpens your instructions most. Left alone, agents often catch an error and return 200 with an error message inside, which reads as success to every client, cache and graph. Asking for the exact code fixes all of them at once.
“Add a 429 with a Retry-After header to POST /login after 5 failed attempts per IP address per minute. Return 401 for a wrong password and 422 for a missing email, never a 200 with an error in the body, and add a test for each status.”