If it wasn't clear, I don't ask the agent to "clean up my code". I ask it to "take this map[string]string you pass around with constant keys and turn that into a structure" or "extract this API provider out into an interface and make everything using it use this interface instead" or other concrete instructions.

LLMs, to a first approximation, already did as well as they could on the first pass. You can get a bit more out of them by asking them to just try harder, but not much. Whereas if you go in with specific changes they are pretty good at implementing them.

This especially matters because my personal style deviates from the common practices. This may also be why I can't get it to just happen by prompting for it. But if you walk it through it's perfectly capable of transforming the code into a better style, and it's still way faster than trying to write it from scratch.