First you have to be very specific with what you mean by idiomatic code - what’s idiomatic for you is not idiomatic for an LLM. Personally I would approach it like this:

1) Thoroughly define step-by-step what you deem to be the code convention/style you want to adhere to and steps on how you (it) should approach the task. Do not reference entire files like “produce it like this file”, it’s too broad. The document should include simple small examples of “Good” and “Bad” idiomatic code as you deem it. The smaller the initial step-by-step guide and code conventions the better, context is king with LLMs and you need to give it just enough context to work with but not enough it causes confusion.

2) Feed it to Opus 4.5 in planning mode and ask it to follow up with any questions or gaps and have it produce a final implementation plan.md. Review this, tweak it, remove any fluff and get it down to bare bones.

3) Run the plan.md through a fresh Agentic session and see what the output is like. Where it’s not quite correct add those clarifications and guardrails into the original plan.md and go again with step 3.

What I absolutely would NOT do is ask for fixes or changes if it does not one-shot it after the first go. I would revise plan.md to get it into a state where it gets you 99% of the way there in the first go and just do final cleanup by hand. You will bang your head against the wall attempting to guide it like you would a junior developer (at least for something like this).