What is an edge case?
An edge case is a situation at the boundary of what software expects: the empty list, the 10,000-character name, the duplicate submission, the user with no account, the file that's the wrong type. Individually rare, collectively constant, since with enough users someone hits every boundary.
Edge cases are where bugs concentrate because they're exactly what nobody thought about while building the main flow. The happy path got all the attention; the edges got assumptions.
How do builders handle edge cases?
By enumerating boundaries deliberately: for every input, the empty, huge, duplicated, and wrongly formatted versions; for every flow, the interrupted, repeated, and unauthorized attempts; for every list, the zero-item and thousand-item cases.
AI makes this tractable for non-engineers. A prompt like "list the edge cases for this form and handle each one" produces both the checklist and the fixes, and running it per feature is one of the cheapest quality upgrades a vibe-coded app can get.
