I'm finding my agents generate code that conforms to Black quite effectively, I think it's probably because I usually start them in existing projects that were already formatted using Black so they pick up those patterns.
I still quite often have even Opus 4.5 generate empty indented lines (regardless of explicit instructions in AGENTS.md not to (besides explicitly referencing the style guide as well), the code not containing any before and the auto-formatter removing them), for example. Trailing whitespace is much rarer but happens as well. Personally I don't care too much, since I've found LLMs to be most efficient when performing roughly the work of a handful commits at most in one thread, so I let the pre-commit hook sort it out after being done with a thread.
