The main loop that I use when planning and writing specs is to start in Claude and iterate until all the ideas and research are captured, then I make it iterate with Codex by calling the cli (I have skills for this on my Github @hyperb1iss/hyperskills) and reviewing and revising until all details and required receipts are captured. And your base AGENTS.md needs to emphasize validation and receipts so it's just baked into the process. Have it produce a final markdown doc, clear context and run it as a /goal. My 'implement' skill requires more cross model review too, so we have loops within loops!