ACP, UCP, AP2 and x402: which agentic commerce protocol do you need?
They are layers, not rivals for one job. ACP (OpenAI and Stripe) and UCP (Google with Shopify and large retailers) define how an agent and a merchant complete a purchase. AP2 (Google) defines how a payment is authorised and proved to be what the user approved. x402 (Coinbase) lets software pay for an HTTP request, usually in stablecoins. Most merchants will get the first two through their commerce platform, and should first make sure agents can read their catalog.
By the Axidort team · Updated
The stack at a glance
| Protocol | Layer | Backed by | What it standardises |
|---|---|---|---|
| ACP, Agentic Commerce Protocol | Checkout | OpenAI, Stripe | How an AI assistant creates and completes a checkout with a merchant, who stays the merchant of record. |
| UCP, Universal Commerce Protocol | Discovery to post-purchase | Google, Shopify and major retailers | The whole journey: catalog discovery, cart, checkout, order updates. |
| AP2, Agent Payments Protocol | Payment authorisation | Google with payment networks and partners | Signed mandates that prove what the user authorised an agent to buy. |
| x402 | Settlement | Coinbase, now a foundation | Paying for an HTTP request by answering a 402 Payment Required response. |
| MCP | Tool access | Open standard | How an agent calls your tools. Not a commerce protocol, but often the transport under one. |
ACP: checkout inside an assistant
ACP describes the conversation between an AI assistant and a merchant: the assistant sends the items, the merchant returns totals, tax, shipping options and accepts or declines, and payment is passed as a scoped token. The merchant keeps the customer relationship, fulfilment and returns. It was published by OpenAI and Stripe and is open for any assistant or payment provider to implement.
UCP: the whole journey
UCP covers more ground: how an agent discovers what a merchant sells, builds a cart, checks out and follows the order afterwards. It was announced by Google with Shopify and a group of large retailers, and is designed to work with AP2 for payment.
AP2: proof of what the user approved
When an agent pays, the merchant and the bank need to know a person authorised it. AP2 uses signed mandates: a record of what the user asked for and the limits they set, and a record of the cart that was approved. It is about trust and liability, not about the shop.
x402: machines paying per request
x402 revives the HTTP 402 Payment Required status. A server answers a request with a price, the client pays and retries, and the server delivers. It suits agents buying API calls, data or content in small amounts, and is settled mainly in stablecoins. It is less relevant to a store shipping physical goods.
What a merchant should do
- Check your platform first. If you sell on a major commerce platform, support for ACP and UCP is arriving as a platform feature. Turn it on there before building anything.
- Get your product data right. Every one of these protocols ingests your catalog. Complete, accurate structured data and a product feed with identifiers, price, unit and stock are the entry ticket.
- Keep the website usable by agents. Protocols cover agents inside partner assistants. Every other agent still arrives at your site in a browser, and in the Axidort Index 62% of leading commerce sites challenge it. See why AI agents fail at checkout.
- Add x402 only if you sell digital access by the request.
What about B2B?
These protocols were designed around consumer checkout: a card, a shipping address, a fixed price. B2B buying adds contract prices, approved catalogs, purchase orders, credit terms and approvals, which the consumer protocols do not yet model well. For B2B sellers the practical route today is an agent-usable site plus an authorised API or MCP server that returns customer-specific prices and accepts a purchase order. More in agentic commerce explained.
Frequently asked questions
01What is the difference between ACP and UCP?
Both let an AI agent buy from a merchant. ACP, from OpenAI and Stripe, focuses on the checkout step. UCP, from Google with Shopify and retailers, covers the journey from product discovery to order updates. A merchant can support both.
02Do I need to implement an agentic commerce protocol myself?
Usually not. Commerce platforms and payment providers are adding support, so most merchants enable it there. Building it yourself makes sense for large retailers with custom checkout.
03Is AP2 a competitor to ACP?
No. AP2 handles payment authorisation: proving that a user approved a purchase and on what terms. It sits beneath a checkout protocol such as UCP or ACP.