I've been making a macos app with opus 4.8-5 and at first it was great, everything materialized in a week, but when I started tuning stuff and fixing performance problems I have spent a very frustrating month refactoring code where I had to constantly catch llm red-handed and explain and sometimes push obvious ways how to make things work properly (a general knowledge from a completely different stack). In the process CLAUDE.md and memory grew exponentially explaining what it should and what it should never do.

were you letting it run for hours like the OP, or doing short tasks and reviewing/testing each one? every once in a while I also ask it to consolidate/summarize docs and stuff like that. we'll see what happens in a week though