I found that turning on every possible clippy lint and telling it that it has to run clippy as well as tests before it can claim it's finished helped a decent amount. Of course, if you have a decent-sized codebase of Rust you're happy with, it helps immensely, since it will tend to listen to follow instructions to follow existing patterns.

Be absolutely ruthless with technical debt. Opus is perfectly capable of producing idiomatic code in any mainstream language you please, but will seize on any opportunity to justify writing basically-python instead because that's "consistent" with the "convention". Deprive it of that excuse.

Give it coding guidelines. It'll largely try to do what you ask.

Left to itself, it often follows human developers who conceive of their goal as "get the program working, the end justifies the means." Which makes sense because there are a lot of systems like that in the training corpus.