AI - Good or Bad?

I managed to implement a Steam Lobby into my indiegame Bomberfun Tournament including slash commands and more. I didn't code it myself however, but had Claude Code do the work. It needed specific instructions though as well as a thorough understanding of the concepts and strategies used in the existing game code... which is more than 20 years old. However, when starting to wire up everything against the online networking for actual gameplay,... well, there is a german saying that goes like this: "Alles ging den Bach runter."

Turns out that the networking layer and the workflow connected to it has been too complex for Claude Code to understand it in a way that would allow for some serious modifications.

Every code change lead to a more and more instable version... until I stopped and had it go back to the last working commit... basically dropping a day and a half worth of token.

However... I will deep dive into my two decades old code (that actually I wrote back then) so that I can either extend it on my own, or give Claude Code better instructions.

What's your experience in working with AI to modify or migrate legacy code?