can I ask what language you are using AI for, there is also a difference in performance for AI in different languages

TypeScript with NextJS. I've also used AI tools with C and Zig, and AI is much better at writing TS. But even though TS works much better, it's still not that great. This is largely because the quality of the code that AI writes is not good enough, so then I have to spend a decent chunk of time fixing it.

Everyone I know trying to use AI in large codebases has had similar experiences. AI is not good enough at following the rules of your codebase yet (i.e., following structure, code style, library usage, re-using code, refactoring, etc...). This makes it far less useful for writing code changes and additions. It can still be useful for small changes, or for writing first drafts of functions/classes/interfaces, but for more meaningful changes it often fails.

That is why I believe that right now, if you want to maintain a large codebase, and maintain a high bar for quality, AI tools are just not good enough at writing most code for you yet. The solution to this is not to get AI to write even more code for you to review and throw out and iterate upon in a frustrating cycle. Instead, I believe it is to notice where AI is helpful and focus on those use-cases, and avoid it when it is not.

That said, AI labs seem to be focusing a lot of effort on improving AI for coding right now, so I expect a lot of progress will be made on these issues in the next few years.