As someone that agenticly codes A LOT. Detailed specs are not required, but certainly one way to use the systems.

If you are going to do a big build out of something, spec up front at least to have a clear idea of the application architectural boundaries.

If you are adding features to a mature code base, then the general order of the day is: First have the Ai scout all the code related to the thing you are changing. Then have it give you a summary of its general plan of action. Then fire it off and review the results (or watch it, less needed now though).

For smaller edits or even significant features, I often just give it very short instructions of a few sentences, if I have done my job well the code is fairly opinionated and the models pick up the patterns well and I don't really have to give much guidance. I'll usually just ask for a few touchups like introdusing some fluent api nicities.

That being said, I do tend to make a few surgical requests of the AI when I review the PR, usually around abraction seams.

(For my play projects I don't even look at the code any more unless I hit a wall, and I haven't really hit a wall since Opus 4.5, though I do have a material physics simulator that Opus 4.5 wrote that runs REALLY slow that I should muck around in, but I'm thinking of seeing if Opus 4.6 can move it to the GPU by itself first.)

So if I were doing an interview with an interview question. I would probably do a "let's break down what we know", "what can we apply to this", "ok. let's start with x" and then iterate quickly and look at the code to validate as needed.