Def a pain point. Anecdotally Claude code and aider both can be of some help. My go to method is: dump everything in the code base into Gemini and ask for an architecture spec, then ask for implementation from aider or Claude code. This 90% works 80% of the time. Well maybe 90% of the time. Notably it can deal with cross codebase interfaces and data structures, in general, with good prompting.

Dumping it at Claude 3.7 with no instructions will 100% get random rewriting - very annoying.