A user asks an AI shopping agent for "a medium grey t-shirt." The agent finds the product. It picks a variant. It adds it to the cart. The merchant rejects the cart. The agent retries with a different variant. The merchant rejects that one too. The session ends in cart_created without a checkout — the user's $40 purchase quietly disappears, and nobody on the merchant side ever sees the failure. This pattern is the single largest source of agent checkout failures we see across the 4,500+ verified UCP stores in the directory . More than schema invalidity, more than tool errors, more than payment-handler problems. Variant mismatch — the agent and the merchant disagreeing on which SKU corresponds to "Medium" — is responsible for a meaningful fraction of the gap between "store has a UCP manifest" and "agent can actually buy from it." The good news: it's almost entirely fixable on the merchant side, in your variant data structure, without changing any tooling.…