Ask them to debug. LLMs are acceptable at writing code, but they're really good at spotting bugs in code that's already been written. What kind of results do you get if you tell them to look for bugs with a clean context subagent?

In my experience, LLMs are excellent at finding concurency bugs.

Debugging concurrency issues isn't a syntactic process so they have to resort to println debugging. This works but isn't exhaustive and burns a lot of tokens.