I don't have a ton of experience with this, but every attempt I've made to quickly get an LLM to one-shot an AGENTS file has been too verbose in all the wrong areas. I'm not convinced LLMs are actually good at summarizing anything complex. Maybe some "blessed" prompts will bubble up in time that change my mind.

LLMs don't one-shot anything very well IMHO, but if you make several passes and work through each section it should end up ok. The key word I use is "optimize", but I also press it on what's actually effective. The goal is a small file, so just ruthlessly cut anything that doesn't have high value across the entire project.

Again, the goal is to let the agent know how to work with the project at a high level, not much else. Skills and docs cover the rest.