I've built very large structured programs with claude. Talking about the structure is indeed an important part of the exercise. It's also an important factor in success. Context is limited and separation of concerns is an essential part in the LLM being able to do it at all. The chunk of "what needs to be done" needs to be small enough for it to be able to recall and reason about. Bad architecture will result in spinning your wheels constantly changing spaghetti soup that never meets spec.

Building a CAD kernel one of the essential pieces in getting from vaguely working to closing an extremely large number of gaps was some rather strict separation of concerns – without it we were just stuck on perpetual rearchitecting switching from methodology to methodology opening new gaps with each attempt to close others.