What is Universal Commerce Protocol (UCP)?

UCP addresses a fundamental gap in the agentic economy: AI agents can discover products (via A2A) and pay for them (via AP2), but there was no standard for the middle step, structuring a purchase. UCP defines a JSON schema for cart objects that includes line items with SKU references, quantity constraints, tiered pricing rules (volume discounts, loyalty tiers, contract rates), tax jurisdiction calculations, and fulfillment options. The protocol supports three cart types: simple (single-item immediate purchase), composite (multi-vendor aggregation where a single agent builds a cart spanning multiple suppliers), and subscription (recurring purchases with modification windows). Each cart object carries a cryptographic signature chain so that pricing commitments are verifiable and tamper-proof. The merchant agent validates inventory availability in real-time, applies business rules (minimum order quantities, geographic restrictions, credit limits), and returns a finalized cart with a time-limited price guarantee. This eliminates the need for traditional checkout flows entirely, an AI procurement agent can complete a complex multi-vendor purchase in seconds.