ACP vs UCP: OpenAI's and Google's checkout protocols compared

ACP and UCP both let an AI assistant check out a buyer at a merchant’s store without the buyer leaving the chat. The difference is who designed them and where they run. OpenAI and Stripe maintain ACP, and it powers Instant Checkout in ChatGPT. Google built UCP with large retailers, and it powers checkout in AI Mode and Gemini. Under both, the merchant stays the seller of record. Neither protocol blocks the other, so a merchant can support both.

How we checked this

We read the published specifications and documentation for both protocols: the ACP GitHub repository, Stripe’s ACP docs and OpenAI’s developer guide for ACP, plus the UCP core-concepts documentation at ucp.dev. We also read the launch announcements (OpenAI, September 29, 2025; Google Developers Blog, January 11, 2026), OpenAI’s and Google’s help pages for buyers and merchants, and one payment processor’s 2026 outlook. Facts checked on September 24, 2026. Protocol versions and merchant rollouts change often; for the current status of all six agent-payment protocols, see our agentic commerce protocol tracker.

ACP vs UCP side by side

What each protocol standardizes

ACP. Stripe’s documentation describes ACP as “composable building blocks for agent-driven commerce” (Stripe). The five blocks are agentic checkout (create, update and complete a checkout session), cart and feed (browse a catalog, manage a cart), delegated payment (pass payment tokens between buyer, agent and business), delegated authentication (OAuth 2.0 so an agent can act for a buyer at a business), and orders and webhooks (confirmation, shipping, delivery, refunds). The GitHub repository shows how the spec grew. It started with checkout and delegated payment on September 29, 2025. Fulfillment came in December 2025, then capability negotiation, then extensions, discounts and payment handlers in January 2026. Cart, feed, orders, authentication and MCP arrived in April 2026 (GitHub).

UCP. Google says UCP “standardizes the full commerce journey - from discovery and consideration to purchase and order management” (Google Developers Blog). The unit of design is the “capability,” a versioned feature with a reverse-domain name like dev.ucp.shopping.checkout. Extensions add to a capability and are dropped automatically if the capability they depend on isn’t used (ucp.dev). A merchant publishes its profile at /.well-known/ucp. The agent sends its own profile in a request header, and the merchant’s server works out which capabilities both sides share. UCP names four roles: the platform (the agent), the business, the credential provider that holds payment data, and the payment service provider.

The two specs have moved closer together. ACP added capability negotiation, extensions and payment handlers in early 2026, all of which UCP had from the start. UCP started with discovery and identity linking, which ACP added in April. The biggest remaining difference is discovery. In ACP, the merchant sends a product feed to the agent platform. In UCP, the agent reads a profile the merchant hosts on its own domain.

Which PSPs and merchants support each

ACP. OpenAI’s developer guide calls Stripe’s Shared Payment Token “the first compatible implementation” of delegated payment and says more PSPs are expected (OpenAI Developers). Checkout.com said in January 2026 that it had “adopted protocols like OpenAI’s ACP” (Checkout.com). At launch in September 2025, OpenAI said Instant Checkout worked for US Etsy sellers. It named Shopify merchants including Glossier, SKIMS, Spanx and Vuori as “coming soon” (OpenAI). OpenAI’s developer guide says “building with the Agentic Commerce Protocol is open to all,” but Instant Checkout itself is “currently available to approved partners,” who apply through chatgpt.com/merchants.

UCP. Google lists more than 20 endorsers, including Adyen, American Express, Best Buy, Flipkart, Macy’s Inc., Mastercard, Stripe, The Home Depot, Visa and Zalando (Google Developers Blog). Endorsing a standard is not the same as running it in production. Google’s Merchant Center help page says UCP checkout is “available for select merchants at this time.” It covers products eligible in the United States, Canada and Australia (Google Merchant Center Help).

Some companies show up on both lists. Stripe co-created ACP and endorses UCP. Etsy was ACP’s launch merchant and co-developed UCP. Shopify appears on both as well. So for many merchants the question is not ACP or UCP. It is which one their platform or processor turns on first. Our tracker keeps a running list of who supports what.

What the buyer sees

In ChatGPT. Per OpenAI’s launch post, the buyer asks for a product and ChatGPT shows results. For eligible items, the buyer taps “Buy,” confirms order, shipping and payment details, and completes the purchase without leaving the chat (OpenAI). Subscribers can use their card on file or “other card and express payment options.” OpenAI says the service “is free for users, doesn’t affect their prices,” and the merchant pays a fee on completed purchases. OpenAI’s help center says Instant Checkout covers “some eligible products and merchants,” and that product results “are not ads” (OpenAI Help Center).

In Google AI Mode and Gemini. Google’s help page says checkout “happens directly on Google’s surfaces while keeping you the merchant of record.” Payment uses “standard Funding Primary Account Numbers (FPANs) that users have stored on their Google Wallet” (Google Merchant Center Help). In practice, the buyer pays with Google Pay using a card already saved in Google Wallet.

In both cases the order comes from the merchant, not the platform. Returns, support and fulfillment go through the merchant’s own systems. OpenAI says merchants keep “fulfillment, returns, and support.” If something goes wrong, the buyer usually deals with the store, not OpenAI or Google. We cover that path in An AI agent bought the wrong thing. Who pays?

Does the buyer need an OpenAI or Google account?

For ChatGPT, yes. OpenAI’s launch post limited Instant Checkout to US ChatGPT Plus, Pro and Free users, so a free account is enough (OpenAI). For Google, payment comes from cards stored in Google Wallet, so the buyer needs a Wallet with a card in it. The buyer does not need an account with the merchant under either protocol. Both specs let the buyer link a merchant account through OAuth 2.0, for example to use loyalty benefits. That is an option, not a requirement.

What a merchant must choose

Neither protocol requires exclusivity. Both are Apache 2.0 open-source specs, and both keep the merchant as merchant of record. Both expose checkout as API calls against the merchant’s existing systems. The choices come down to four questions.

1. Which surface matters to your buyers? ACP is how you reach checkout in ChatGPT. UCP is how you reach checkout in Google AI Mode and Gemini. The protocol follows the audience.

2. What does your payment processor already support? In ACP, delegated payment runs through a compatible PSP token, and Stripe’s was first. In UCP, the payment handler is declared in your profile. Google’s own implementation also requires a Google Pay & Wallet Console account (Google Merchant Center Help). If your processor supports one protocol and not the other, that often decides the first integration.

3. How do you want to be discovered? ACP relies on a product feed (CSV or JSON with identifiers, prices, inventory, media and fulfillment options) shared with OpenAI (OpenAI Developers). Google’s UCP checkout depends on Merchant Center product data and a native_commerce product attribute that turns on the Buy button. UCP also has the /.well-known/ucp profile on your domain. If you already keep good Merchant Center feeds, UCP builds on that work.

4. Can you pass the approval gate? Neither protocol is self-serve. OpenAI approves Instant Checkout partners, and Google asks merchants to submit an interest form and meet technical requirements. The spec is open, but the platform decides who gets the Buy button.

Merchants on a hosted platform like Shopify or Etsy may never need to make these choices themselves. Both companies are involved in both protocols, so the platform may handle the integration. Our guide to accepting payments from AI agents without code covers that route. Checkout.com advises merchants to “proactively connect” to agentic infrastructure in 2026 (Checkout.com). That is a processor’s sales view, not neutral advice. Whether to build now depends on how many of your buyers shop through these assistants.

What to watch out for

• Spec versions move fast. ACP shipped five dated spec versions between September 2025 and April 2026. An integration built on an early version may lack cart, orders or MCP support. UCP also uses date-based versions (ucp.dev).
• “Endorsed” is not “live.” UCP’s list of 20-plus endorsers includes card networks and processors. Google’s own help page still describes checkout as limited to select merchants in three countries.
• Platform status can change faster than the spec. OpenAI’s help center describes Instant Checkout as covering only “some eligible products and merchants.” Check the tracker for the latest before investing engineering time.
• Fees are not in the specs. OpenAI says merchants pay “a small fee” on completed purchases. Get terms in writing from the platform and your processor.
• Buyer disputes still follow card rules. Both protocols keep the merchant as seller. Refunds and chargebacks run through the merchant and the card issuer, as they do for any other online order. For contract or liability questions, talk to a payments lawyer.

Go deeper

• Agentic payments, explained: how AI agents pay and who is behind it (hub)
• Agentic commerce protocol tracker: ACP, UCP, AP2, TAP, Agent Pay, x402 — who supports what
• Accepting payments from AI agents as a small business, without code
• An AI agent bought the wrong thing. Who pays? Refunds, chargebacks and liability
• Visa Trusted Agent and Mastercard Agent Pay: what changes for cardholders
• More in Agentic payments