I mean I have grabbed random non-greenfield projects and added features to them for my temporary / personal needs with Claude Code. The key thing is setting it up. The biggest thing is adopting good programming principles like breaking up godclasses. Things that help human devs consume code easier turns out it works for LLMs too.

I have done this sort of thing too. I’m curious about big, mature projects like numpy or the Linux kernel.

It seems the users of this are so varied that refactors like what you describe would be rolled out more gradually than the usual AI workflow.