I've refactored some files over 6000 loc. It was necessary to do it iteratively with smaller patches. "Do not attempt to modify more than one function per iteration" It would just gloss over stuff. I would tell it repeatedly: I noticed you missed something, can you find it? I kept doing that until it couldn't find anything. Then I had to manually review and ask for more edits. Also lots of style guidelines and scope limit instructions. In the end it worked fine and saved me hours of really boring work.