ACP
CheckoutStripe · OpenAI · Meta
Product feed, checkout session, delegated payment, orders and webhooks.
7 specs, side by side
Protocols
They are not competing for the same job. Read the layer first: a single purchase can use a checkout spec, a payment spec and a transport at once.
UCP
CheckoutGoogle · Shopify
Catalogue, cart, checkout, order lifecycle, and what a merchant supports.
AP2
PaymentSigned intent, cart and payment mandates, for human-present and autonomous flows.
x402
Paymentx402 Foundation
A 402 response carrying payment terms, and the retry that proves payment.
MCP
TransportAnthropic · community
Tool discovery and invocation between a client and a server.
A2A
TransportLinux Foundation
Agent cards, tasks and messages between independent agents.
REST
Checkouteveryone, already
Nothing new — which is the point. An OpenAPI document makes it discoverable.
Let an agent complete a purchase against a merchant that it does not have an account with.
- Standardises
- Product feed, checkout session, delegated payment, orders and webhooks.
- Leaves alone
- Move money itself, or decide whether the agent is allowed to spend.
- Shape
- REST + JSON, dated spec versions, OpenAPI and JSON Schema published.
The same job as ACP, with capability discovery first and more than one transport.
- Standardises
- Catalogue, cart, checkout, order lifecycle, and what a merchant supports.
- Leaves alone
- Prescribe a payment rail; it carries whichever one the merchant accepts.
- Shape
- REST and MCP bindings, with SDKs and a conformance suite.
Prove that a human authorised what the agent is about to buy.
- Standardises
- Signed intent, cart and payment mandates, for human-present and autonomous flows.
- Leaves alone
- Define the storefront or the catalogue; it assumes a checkout already exists.
- Shape
- Signed credentials carried over A2A, with an x402 variant for on-chain settlement.
Charge for a single HTTP request, with no account and no checkout page.
- Standardises
- A 402 response carrying payment terms, and the retry that proves payment.
- Leaves alone
- Handle carts, tax, shipping or returns. It is one request, one price.
- Shape
- HTTP status 402 plus headers; settlement usually on-chain stablecoins.
Give a model tools and data, including a merchant's catalogue and cart.
- Standardises
- Tool discovery and invocation between a client and a server.
- Leaves alone
- Define anything about commerce; there is no official payments extension.
- Shape
- JSON-RPC over stdio or HTTP, with a dated spec.
Let two agents talk — a shopper's agent and a merchant's, for example.
- Standardises
- Agent cards, tasks and messages between independent agents.
- Leaves alone
- Carry payment on its own; the x402 extension adds that.
- Shape
- JSON over HTTP, with agent cards for discovery.
REST — Plain REST + OpenAPI
everyone, already · Checkout
The baseline. Most agents will meet an ordinary JSON API long before a named protocol.
- Standardises
- Nothing new — which is the point. An OpenAPI document makes it discoverable.
- Leaves alone
- Give you mandates, scoped tokens or a shared vocabulary between merchants.
- Shape
- Your endpoints, described so a model can call them without guessing.