Some time ago I just needed to tweak what a compiler was emitting, and thought perhaps I should learn how to use the modern tools for producing code. And that really got out of hand as I love compiler and OS/hypervisor programming so decided to build a nimble compiler that would optimize but not too much, just enough not to be embarassing :D
I built it with Claude Opus 1M 4.6-4.8. Wasn't autonomous, I was directing it and helping with debugging. There were times when I was writing algorithms down with a pencil on a paper from the Dragon and the Dinasour books to make sure I understand, and I didn't have a computer, too, lol. So adjusting to the new brave world another time!