I read the three specs side by side, looking at what a PSP has to build, not what the announcements say.

ACP (OpenAI and Stripe) is the narrowest. The agent gets a scoped, single-use payment token and the merchant's PSP charges it. For you it's mostly a new credential type on an existing authorize call.

AP2 (Google) is about proof. Signed mandates record what the user authorized, either a cart or an intent with limits. The payment still travels on normal rails, but the mandate has to ride along so an issuer or a dispute reviewer can see it later.

UCP (Google) sits one level up. It covers checkout, order state and identity linking between agent and merchant, and it is meant to work with AP2 for the payment step.

My read: don't pick a winner. Build the layer all three need.…