Agentic Checkout: How AI Agents Complete Ecommerce Purchases

Agentic checkout is a purchase an AI agent completes for a shopper through the merchant's own checkout system. The agent creates a checkout session with the selected items, the merchant returns authoritative prices, tax and fulfillment options, the agent adds buyer and delivery details, the shopper confirms, the agent completes the session with a payment credential, and the merchant charges, creates the order and sends updates by webhook.

The merchant is in charge throughout: it calculates totals, decides whether to accept the order, processes payment and remains merchant of record. The engineering challenges are the classic ones made sharper by automation: idempotency, authentication, signatures, retries, clear errors and order consistency.

The sequence below follows the two main open specifications, the Agentic Commerce Protocol (ACP, checkout API in the 2026-04-17 release) and the Universal Commerce Protocol (UCP, checkout capability in the 2026-08-25 release). Names differ slightly; the shape is the same.

The single most important rule: the merchant's checkout session is the source of truth. The agent may have seen a price in a feed an hour ago; the session's totals are what the shopper pays. UCP states that the checkout state reported by the business is authoritative, and ACP describes the merchant validating orders, calculating tax and fulfillment, assessing risk, charging payment and accepting or declining orders. Agents must display the merchant's totals for confirmation and never compute their own.

Both specifications use explicit statuses so the agent knows what to do next. ACP's checkout session statuses include incomplete, not_ready_for_payment, requires_escalation, authentication_required, ready_for_payment, pending_approval, complete_in_progress, completed, canceled, in_progress and expired (ACP specification). UCP uses incomplete, requires_escalation, ready_for_complete, complete_in_progress, completed and canceled (UCP checkout). Map your internal checkout states to these precisely; ambiguous states are where duplicate charges and stuck orders come from.

Agents, networks and platforms retry. ACP requires an Idempotency-Key header on every POST. UCP is precise about the complete operation: the platform must not start a new complete while the status is complete_in_progress; if a response is lost, it should poll the session with bounded backoff; only if the outcome is still unknown may it resend the identical request with the same key; and reusing a key with a different payload returns a 409 conflict. Implement idempotency at the payment and order-creation layers too, not only at the HTTP edge.

The merchant must know which agent platform is calling and that requests were not altered. ACP requests carry authorization and an API version, with optional request signature and timestamp headers. UCP uses HTTP Message Signatures (RFC 9421) with published keys, and requires webhooks from business to platform to be signed. Verify signatures and timestamps, reject replays, rotate keys, and scope each platform's credentials to the capabilities it needs. Agent identity is separate from shopper authorization and from payment authorization; see AI commerce payments and verifying AI agent traffic.

Return errors an agent can act on: a type, a machine-readable code, a human-readable message and the field concerned. Distinguish recoverable problems the agent can fix (invalid address, variant out of stock, price changed: show new totals) from those that need the buyer (authentication required, policy acceptance, age verification), which should escalate with a continue URL rather than fail silently. Never let an agent complete a session whose totals changed since the shopper confirmed; return to confirmation.

After completion, the agent platform, the shopper and your systems must agree on the order. Create the order atomically with the payment outcome, return the order ID in the completion response, send signed webhooks for every state change, make webhook handling idempotent on the receiving side, and provide an order lookup so either side can reconcile. Mark agent orders with their channel for operations and analytics; see orders placed by AI agents.

Agentic checkout moves the checkout form from your website into an agent's conversation, but not the responsibility. Keep the merchant's session authoritative, map statuses precisely, enforce idempotency and signatures, return actionable errors, escalate to the shopper when needed and keep orders consistent with signed webhooks. For the API around it, see agent-ready ecommerce API, and for protocol differences, ACP vs UCP vs MCP.