x402, AP2 Or ACK-Pay: Which Standard Fits?

x402, AP2 and ACK-Pay address different parts of agent payments. x402 handles settlement for an individual request, AP2 records whether a user authorized a purchase, and ACK-Pay connects an agent to its accountable entity and payment records.

x402 uses the HTTP 402 Payment Required status. A client requests a protected endpoint and receives the seller’s payment requirements. It signs a payment payload and sends the request again. The server or a facilitator verifies the payment before completing the request.

The protocol is designed for software-to-software transactions, including paid API calls, data access and tool use. It supports stablecoin payments on EVM-compatible networks and Solana. Facilitators can verify payment payloads and broadcast transactions, so sellers do not need to operate their own blockchain infrastructure.

x402 operates as a Linux Foundation project with its own foundation and working groups. During 2026, its documentation changed the names of several HTTP headers. Current documentation refers to a payment-required header in the 402 response, a payment-signature header on the repeated request and a payment-response header with settlement details. Tutorials using the older X-PAYMENT header describe an earlier draft.

AP2 addresses whether a user authorized an agent to make a purchase. Its current specification uses a Checkout Mandate for items and purchase details shared with a merchant, and a Payment Mandate for authorizing a payment instrument with credential providers and processors.

Each mandate has an open stage that records conditions such as spending limits and permitted instruments. A closed stage authorizes the final amount. The signed credentials can create an audit trail when the user is not directly involved. Earlier AP2 documents used the names Intent Mandate and Cart Mandate.

AP2 was developed for pull payments such as card transactions. Support for push payments and digital currencies remains on its roadmap, while the specification includes an x402-based example. Google publishes AP2 under the Apache 2.0 license. Standardization is moving into FIDO Alliance working groups focused on agentic authentication and payments. AP2 extends the Agent2Agent, or A2A, framework.

ACK-Pay focuses on agent identity, payment requirements and proof of payment. It is paired with ACK-ID, which uses decentralized identifiers and verifiable credentials to connect an accountable legal entity with the agent acting for it.

ACK-Pay can communicate payment requirements, initiate payment through a payment service and issue a confirmation as a verifiable credential receipt. It is designed for different payment rails, including card-network programs and 402-style stablecoin payments. An ACK-Pay transaction can therefore use x402 for settlement while adding identity information and a receipt. Catena Labs developed ACK-Pay and contributed it as an open project under the MIT license.

The standards emerged as lower-cost payments made smaller transactions practical. Card fees can make payments below $1 uneconomic, while transaction costs that scale with the amount can support payments for individual API requests. This allows products to charge per action instead of by user or subscription.

None of the standards sets an agent’s daily spending limit. Companies must enforce those limits through their own policy systems. They must also account for blockchain settlement finality, the lack of a card-style chargeback process and fees that may apply after facilitator allowances are exceeded.

Products that need settlement for paid requests may use x402. Products that need proof of user authorization may use AP2. Services that need agent identity and an auditable receipt may add ACK-Pay. The standards can be combined, with identity, authorization and settlement handled by separate layers.