Edge case
An input at the limit of what a program expects: nothing at all, exactly one, zero, the largest allowed, the same thing twice, things in the wrong order. Bugs gather here because the usual case is what everybody pictures.
Models handle the edges everyone has heard of and miss the ones particular to your domain. In our runs all 15 vague attempts handled a repeated webhook, and 11 of 15 got an old event arriving after a newer one wrong. Listing the edges costs one sentence.
Say it to your agent
“List the edge cases for this function before you write it: empty input, one item, duplicates, items out of order, very large values. Give the result you would expect for each, and ask me about any you are unsure of.”