TL;DR
- Two checkout protocols lead. UCP (Google, Shopify and co-developers) covers the full journey from discovery to checkout (opens in new tab). ACP (opens in new tab) (created by Stripe, OpenAI and Meta (opens in new tab)) is the other.
- ChatGPT is discovery only. Since March 2026 ChatGPT helps people find products; the purchase happens on the merchant's own site or app (opens in new tab).
- Payments are a separate layer. AP2 proves the user approved a payment (opens in new tab). Visa TAP (opens in new tab) and Mastercard Agent Pay (opens in new tab) let merchants trust the agent. x402 moves machine payments over HTTP 402 (opens in new tab). Amex Intent Intelligence (opens in new tab) (published 13 Oct 2026) captures and authenticates the shopper's purchase intent before an agent pays, then scopes the payment credential with an Intent ID and Proof of Intent Token.
- MCP and A2A are the base. MCP connects agents to tools and data (opens in new tab); A2A connects agents to each other (opens in new tab).
- Product data decides if you show up. Google says "Accurate matching to products can't be assured when GTIN is missing (opens in new tab)". UCP on Google uses existing Merchant Center feeds (opens in new tab).
Audit your site
Checks a public website against the machine-checkable checklist items: discovery files, AI crawler access, Merchant Center crawlers and product markup. Read-only, at most 25 requests.
| Protocol | Summary | Backer | Status | Main link |
|---|---|---|---|---|
| UCP | Covers almost the whole journey: an AI surface finds your shop, reads your catalog, builds a cart, checks out and handles the order afterwards, while you stay merchant of record. Open standard co-developed by Google, Shopify and others. | Co-developed by Google, Shopify, Etsy, Wayfair, Target, Walmart, Amazon, Microsoft, Meta, Salesforce and Stripe (ucp.dev) | live Live in the US: UCP checkout via Google Universal Cart (AI Mode in Search and the Gemini app) since 15 Oct 2026, with Google Pay checkout or handoff to the merchant cart, merchant stays seller of record; Lodging draft specification available (not yet in a release); Food coming soon; Shopify Shop MCP (UCP over MCP) open to any agent since 13 Oct 2026; Shopify Checkout MCP and Cart MCP support the UCP buyer consent extension since 16 Oct 2026 | ucp.dev (opens in new tab) |
| ACP | Sits at product data, cart and checkout: lets an agent such as ChatGPT buy from you with payment credentials passed securely, while you stay merchant of record. Maintained by OpenAI and Stripe, per the ACP repo; Stripe's docs say it was created by Stripe, OpenAI and Meta. | OpenAI, Stripe (maintainers, per the ACP repo) | beta Beta; OpenAI says it is moving away from a standalone Instant Checkout experience (March 2026); purchases complete on merchant sites or apps | agenticcommerce.dev (opens in new tab) |
| Personal Agent Protocol (Poppy) | Sits before the purchase: a person's own AI agent finds your business, signs in to their account with you and acts on their behalf. Draft from Sierra and Meta, no payments yet. | Sierra, Meta (+ Genesys, Instinct, NiCE, Rocket, Shopify, Stripe, Walmart; 35 design partners including OpenAI) | draft Draft 0.1 published 9 Oct 2026; draft 0.2 in progress with breaking changes | personalagentprotocol.org (opens in new tab) |
| AP2 | Sits at payment: gives you signed proof that the shopper really authorized the agent to pay. Created by Google, now being standardized in the FIDO Alliance. | Google, now FIDO Alliance | draft Pre-release spec, not a live network | ap2-protocol.org (opens in new tab) |
| Visa Trusted Agent Protocol | Sits at trust: lets your site recognize a trusted Visa shopping agent and tell it apart from a malicious bot while it browses, and can carry consumer and payment details at checkout. | Visa | live Live in Europe since July 2026 with participating merchants and over 30 issuers; spec and sample code public. | developer.visa.com (opens in new tab) |
| Mastercard Agent Pay | Sits at payment: a registered agent pays with a tokenized Mastercard credential instead of a raw card number. Agent Pay for Machines extends this to micro-payments. Agents are credentialed with Mastercard Verifiable Intent so they can be recognized across ecosystems. Since 30 September 2026, trust and intelligence services add a probability score that a transaction was initiated by an AI agent, in testing in the US. | Mastercard | beta Rolling out; Agent Pay for Machines launched June 2026; Agent Pay Trust Framework intelligence layer announced 30 Sep 2026, with an AI-agent probability score in US testing | developer.mastercard.com (opens in new tab) |
| Know-Your-Agent | Will sit at trust: Visa, Mastercard and Ant International will explore common principles, building on the SAFR framework, via BuildFin.ai; each has its own protocol (TAP, Verifiable Intent, Agentic Mobile Protocol). Announced 10 September 2026, no spec yet. | Visa, Mastercard, Ant International | announced Announced, no spec yet | ant-intl.com (opens in new tab) |
| x401 | Sits at trust: a site or API answers an agent request with a PROOF-REQUEST header, and the agent returns a verifiable credential (OpenID4VP over the W3C Digital Credentials API) proving who authorized it before access is granted. Payment stays with HTTP 402 protocols such as x402. Proof says it plans to submit x401 to the FIDO Alliance: "Proof will submit x401 to the FIDO Alliance's agentic authentication standards workgroup." (launch release, 25 Jun 2026). | Proof; Circle (co-endorser) | draft Draft spec v0.2.0, open (Apache-2.0); launched 25 Jun 2026 | x401.proof.com (opens in new tab) |
| x402 | Sits at payment for pay per request: your server answers an unpaid request with HTTP 402, the agent pays and retries. Built for small automated payments rather than a classic cart. | x402 Foundation | live Live; about $24M volume and 75M transactions in the last 30 days per x402.org (live counter, read 2026-10-12) | x402.org (opens in new tab) |
| MPP | Sits at payment for machine to machine purchases over HTTP 402. Payment methods per mpp.dev: Tempo stablecoins, Stripe, cards via encrypted network tokens, Bitcoin over Lightning, EVM, Solana, Stellar, XRPL, Monad, NEAR Intents, RedotPay, or custom; plus sessions and subscriptions. From Stripe and Tempo. | Stripe, Tempo | live Live since March 2026 | mpp.dev (opens in new tab) |
| PayPal Agent Ready | Sits at checkout and payment: a PayPal or Braintree integration makes your checkout available to agents in ChatGPT (ACP, via Braintree) and Google AI Mode and Gemini (UCP, via Braintree or PayPal Orders v2). An integration, not a protocol. | PayPal, Braintree | live Developer docs live | developer.paypal.com (opens in new tab) |
| MCP | Not a journey stage but plumbing: connects AI apps to your tools and data. UCP and ACP can both run over it. | Created by Anthropic; now hosted by the Agentic AI Foundation (Linux Foundation) | live Stable, widely used | modelcontextprotocol.io (opens in new tab) |
| A2A | Not a journey stage but plumbing between agents from different vendors, for example a shopper's agent and your shop's agent. Originally from Google, now at the Linux Foundation. | Google, now Linux Foundation (Agentic AI Foundation) | live Stable, widely used | a2a-protocol.org (opens in new tab) |
| WebMCP | Sits in the browser at cart and checkout: a web page registers structured tools, such as checkout, that an AI agent in the user's browser can call, via JavaScript (imperative API) or annotated HTML forms (declarative API). A proposed web standard in a Chrome origin trial, primarily designed for local browser workflows with a human in the loop. | Google Chrome | draft Proposed web standard (Chrome origin trial from Chrome 149) that lets a page expose structured tools such as checkout to in-browser AI agents via JavaScript or annotated HTML forms. Subject to change. Shopify checkout supports it since 28 Sep 2026. | developer.chrome.com (opens in new tab) |
Product data (Merchant Center, OpenAI feed, Microsoft Merchant Center, Schema.org, GTIN): see the feed comparison in the Tech view or the Do now list in the Business view.
All of this is also available to AI clients: MCP server and JSON API.
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