The spec needs to be explicit about edge and corner cases.
At some point such a spec converges to the actual code you’d have written.
I've actually tried that and it helps. First I create a PRD type doc, then I have the AI break it down in a task doc, including code snippets where relevant. This helps it to think through edge cases before it starts implementing (oh we need X now, but that means we should have done task 3 differently to allow that).