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To keep an AI agent from paying for the wrong offer, bind the payment authorization to the merchant’s authenticated, finalized checkout—not merely to the agent’s identity or a product listing. In Google’s AP2 model, the merchant signs the checkout, the user’s approval or delegated mandate authorizes that checkout, and the payment mandate is cryptographically linked to the same checkout.
What must the authorization be tied to?
It must be tied to the final checkout terms the merchant is prepared to honor. A product page or an agent’s summary is not enough: the accepted offer may depend on quantity, selected options, discounts, shipping, tax, currency, and the total due.
AP2 represents the merchant’s checkout details in a merchant-signed Checkout JWT. The closed Checkout Mandate is bound to that checkout, and the Payment Mandate refers to the same Checkout JWT using a cryptographic hash. The merchant-signed checkout is therefore the point of reference: if the checkout changes, the verifier must determine whether the existing authorization still covers it. A hash alone does not prove that a price is fair or that every relevant term was included; it binds authorization to the signed payload that actually exists.
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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →AP2 leaves the detailed contents of the checkout object to the commerce protocol. Implementations need to include the terms that matter to the purchase in the authenticated checkout data. AP2 also specifies a non-deterministic signature scheme for the Checkout JWT in the hash-binding design it describes.
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How does the checkout-to-payment flow work?
- Build the cart. The agent and merchant negotiate the items and terms. The agent should not treat an earlier listing or quote as the final offer.
- Authenticate the finalized checkout. The merchant returns a signed checkout object containing the purchase details it stands behind.
- Obtain the right kind of consent. A present user approves the closed checkout directly. For autonomous shopping, the agent acts under a user-approved mandate with constraints.
- Present the authorization. The agent supplies the closed Checkout Mandate and Payment Mandate associated with the checkout.
- Verify before payment. The merchant checks the checkout reference and applicable mandate constraints. The credential provider and, where applicable, the network check the payment authorization before releasing a credential. The merchant’s payment processor checks that the credential is scoped to the checkout.
- Record the result. Participants return signed receipts for acceptance or rejection, creating records that can be checked against the mandates.
AP2’s design does not make these checks an AI judgment call: mandate validation and processing must happen in deterministic code, even when an agent performs other parts of the workflow.
How do human approval and autonomous shopping differ?
When the user is present
A trusted user-facing surface shows the closed checkout and obtains the user’s informed approval. The resulting authorization applies to that finalized transaction. If material terms change after approval, the system should not silently treat the old approval as consent to the new checkout; it should evaluate the changed checkout against the authorization and obtain any required renewed approval.
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When the agent acts autonomously
The user approves constraints in advance rather than approving each finished cart in person. The agent then forms a closed checkout, and verifiers check that transaction against the open mandate and its constraints. In AP2, the closed mandate is what verifiers receive in both modes; the trust basis differs. Direct approval relies on the user’s signature, while autonomous authorization relies on the user-signed open mandate and the agent key.
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AP2’s checkout-mandate documentation identifies allowed merchants and line-item constraints. Other possible guardrails—such as amount, currency, expiry, or requiring the user to return for approval—must be represented and enforced by the applicable mandate and payment data in the implementation; AP2 does not standardize every such constraint for every deployment. The specification recommends short expiration for open mandates. It also says an agent must not reuse an open mandate for another checkout before receiving a rejection receipt for the prior one.
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Which protocol or control proves what?
Agent identity, payment-credential scope, and transaction-specific authorization address different risks. Knowing which agent sent a request does not establish that the user approved the offer in that request. A narrowly scoped token can limit where or how a credential is used, but that is distinct from cryptographically binding payment authority to a particular signed checkout.
| Protocol or control | What the cited material describes | What it does not establish by itself |
|---|---|---|
| AP2 | Mandates linked to a merchant-signed checkout, with verifiers checking mandate integrity, checkout hash, and applicable constraints. | That checkout contents are complete, or that every deployment implements the same constraints or dispute-retention procedures. |
| Visa Trusted Agent Protocol | Signed agent-recognition, linked consumer/device identity, and linked payment-container components for browsing and payment interactions. Visa’s developer guide describes verifying message signatures with the agent’s public key. | That the payment authorization matches the user’s accepted checkout. Message authentication is not, by itself, transaction-specific consent. |
| UCP | A common language for commerce interactions among agents, merchants, and payment providers; Google describes it as compatible with AP2, A2A, and MCP. | That a commerce-interaction protocol alone provides AP2’s mandate-to-checkout authorization binding. |
| Mastercard agentic-shopping controls | Mastercard describes agent-specific tokens with limits such as merchant category, purpose, spending limit, or time window, along with revocation and passkey or two-factor authentication options. | A universal capability or independently verified performance result for all agentic payment systems. |
These are different layers, not interchangeable products. Google also says its agentic checkout can take place on Google surfaces while the merchant remains merchant of record. A commerce protocol can coordinate shopping and checkout data; payment authorization must still establish the authority to pay for the resulting transaction.
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What evidence is available if something goes wrong?
AP2 defines Checkout and Payment Receipts returned after mandate acceptance or rejection. Its verification process includes checking mandate integrity, recomputing the checkout hash, and matching receipt references. Taken together, the mandates and receipts can support an integrity-preserving account of what was authorized and accepted.
That record is not a guaranteed refund, a determination of legal liability, or an automatic chargeback resolution. AP2 leaves detailed dispute procedures, evidence-retention rules, and retrieval requirements outside its specification. A deployment therefore needs its own documented approach for retaining and retrieving the signed checkout, mandates, and receipts if it expects to use them in an audit or dispute.
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What should an implementation check before charging?
- Verify the merchant’s checkout signature and validate the checkout schema and version.
- Recompute the checkout hash and compare it with the reference in the payment authorization.
- Check the authorized merchant, items, quantities, amount, currency, time window, and any other constraints actually represented in the mandate or payment data.
- Confirm the payment credential is scoped to the same transaction and has not expired or been revoked.
- Reject mismatches, invalid signatures, expired authorization, and replayed or reused mandates rather than asking an AI model to decide whether they are acceptable.
- Persist the signed checkout, mandates, and acceptance or rejection receipts under a defined retention and retrieval policy.
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