The idempotency angle matters far beyond domains: any agent action with a real cost needs a key that makes a timed-out retry safe. GoDaddy refusing to run without an Idempotency-Key is a good default, since the alternative is exactly the double-purchase you describe when a retry fires blind. For agents calling APIs that lack that guarantee, how are you enforcing idempotency yourself, a client-side key store or a check-before-act step?