The short version: Google's Universal Commerce Protocol (UCP) is a broad standard for AI agents to search catalogues, build carts, check out and manage orders, and it powers checkout in Google AI Mode and Gemini. The Agentic Commerce Protocol (ACP), from OpenAI and Stripe, connects merchants to AI agents and now feeds product discovery in ChatGPT. The Agent Payments Protocol (AP2) is narrower: a way to prove a user authorised an agent to pay, designed to plug into UCP.
If you sell on Shopify or another large platform, you may never touch any of them directly. But they decide where and how AI agents can buy from you, so it helps to know what they are. This guide is current as of September 2026, and all three are still changing.
Why protocols matter
When a person buys from your website, your checkout assumes a human is clicking. When an AI agent buys on someone's behalf, a lot of questions open up. How does the agent read your catalogue? How does it build a cart with the right variant and shipping option? How do you know the shopper actually approved the payment?
Protocols are shared answers to those questions. Without them, every AI platform would need a custom integration with every merchant. Google itself says in its search guidance that "Protocols like Universal Commerce Protocol (UCP) are emerging that will allow Search agents to do more" (Google Search Central, updated 10 July 2026).
For most merchants the practical question isn't "which protocol is best" but "which AI surfaces can reach my products, and on what terms."
At a glance
| UCP | ACP | AP2 | |
|---|---|---|---|
| Full name | Universal Commerce Protocol | Agentic Commerce Protocol | Agent Payments Protocol |
| Backed by | Google, co-developed with Shopify, Etsy, Wayfair, Target, Walmart, Amazon, Microsoft, Meta, Salesforce, Stripe and others | Stripe and OpenAI | Google; standardisation continuing in FIDO Alliance working groups |
| Scope | Discovery to checkout to order management | Checkout and, since March 2026, product discovery in ChatGPT | Payment authorisation and accountability |
| Where you see it | Checkout in Google AI Mode and the Gemini app | Product feeds in ChatGPT | Inside UCP and Agent2Agent (A2A) flows |
| Licence | Open standard | Open source, Apache 2.0 | Open protocol |
Sources: ucp.dev, agenticcommerce.dev and ap2-protocol.org, all accessed September 2026.
UCP (Google)
UCP describes itself as "the common language for platforms, agents, and businesses." Its site lists core capabilities including catalogue search and lookup, cart building, identity linking, checkout and order management. It supports REST and JSON-RPC, with AP2, Agent2Agent (A2A) and the Model Context Protocol (MCP) built in, and says businesses remain the merchant of record (ucp.dev, accessed September 2026). The site also says UCP is expanding into lodging and food.
Google's 16 September update is what made UCP news this month. Google said UCP "already enables direct checkout for hundreds of thousands of brands and retailers across Google." Merchants using the UCP integration hub in Merchant Center can now enable cart transfer to a merchant site and enhanced checkout flow testing, with analytics to come. Those new hub capabilities are rolling out gradually in the US, with Australia and Canada to follow early next year (Google, 16 September 2026).
Big brands are live. Tapestry announced that shoppers can now buy Coach and Kate Spade products through an inline "Buy" button in AI Mode and the Gemini app, paying with Google Pay via UCP (Tapestry via Business Wire on Yahoo Finance, 16 September 2026).
For Shopify merchants, UCP arrived without any setup. Eligible US stores are being automatically enrolled in native checkout in AI Mode and Gemini, with an opt-out under Sales channels > Agentic (Search Engine Land, 22 September 2026). Our Shopify agentic storefronts guide covers that in detail.
ACP and ChatGPT's pivot away from in-chat checkout
ACP was developed by Stripe and OpenAI as "an open standard for programmatic commerce flows between buyers, AI agents, and businesses." It is open source under Apache 2.0, works over REST or MCP, and keeps the business as merchant of record. Stripe is the first compatible payment provider, through its Shared Payment Token. The ACP site also makes a point worth underlining: implementing the protocol does not automatically list your products in any AI agent. Each platform runs its own participation process (agenticcommerce.dev, accessed September 2026).
ACP's best-known use changed this year. OpenAI originally used it for Instant Checkout inside ChatGPT. In March 2026 OpenAI said it was "allowing merchants to use their own checkout experiences" while it focused on product discovery (CNBC, 24 March 2026). At the same time OpenAI said it was extending ACP to support product discovery, with merchants sharing product feeds and promotions through it, and Shopify product data reaching ChatGPT through Shopify Catalog (OpenAI, March 2026).
So in ChatGPT today, ACP is mainly about getting accurate product data in front of the model, not about taking payment. The ACP site still describes ChatGPT checkout, so expect some lag between the protocol documentation and how OpenAI actually uses it. For what this means in practice, see ChatGPT shopping after Instant Checkout and our ChatGPT Shopping explainer.
AP2 and the payment layer
AP2 tackles a narrower problem: proving that a person really authorised an agent's purchase. Its documentation frames the issue around authorisation, authenticity and accountability, and uses signed "verifiable digital credentials" called mandates. A checkout mandate records what was agreed with the merchant. A payment mandate authorises payment against a specific instrument. Each comes in an open form (the user's constraints, such as a budget) and a closed form (a specific, final transaction) (ap2-protocol.org, accessed September 2026).
AP2 is available as an extension for A2A and UCP. The site dates Google's original announcement to 16 September 2025, a year before this month's UCP news, and says standardisation is continuing within FIDO Alliance working groups. The initial version supports card payments, with real-time bank transfers and digital currencies on the roadmap.
For a merchant, AP2 is mostly invisible. It matters to payment providers and platforms. You will benefit from it indirectly if it makes agent purchases less prone to disputes.
Stripe Link and Shop Pay for agents
Protocols aren't the only rails. Wallets matter too, because an agent needs a way to pay that the shopper trusts.
- Link by Stripe. Meta's Muse agent, launched in the US on 8 September 2026, can open a browser, fill in forms and pay using Link by Stripe (Meta, 8 September 2026).
- Shop Pay. Meta listed Shop Pay as "coming soon" for Muse, and Shopify and Meta have since partnered on it. PYMNTS reported that Shop Pay has passed $400 billion in lifetime accelerated GMV (PYMNTS, 22 September 2026).
- Google Pay. Used for UCP checkouts such as the Coach and Kate Spade launch.
Stripe shows up in all three stories: as ACP co-developer, as a UCP co-developer, and as the wallet behind Muse. Shopify is similar, sitting on the UCP co-developer list while supplying catalogue data to ChatGPT and Shop Pay to Meta.
Which to adopt by platform
A practical guide, as of September 2026:
| Your setup | What to do |
|---|---|
| Shopify, US | You are likely already connected to UCP through Google checkout and to ChatGPT through Shopify Catalog. Review Sales channels > Agentic and your product data. |
| Shopify, outside the US | Keep your Merchant Center feed and catalogue clean. Google's new UCP hub capabilities reach Australia and Canada early next year. |
| Other large platform (e.g. Salesforce, commerce suites) | Ask your platform which protocols it supports. OpenAI names Salesforce and Stripe as delivery paths for ChatGPT feeds. |
| Custom stack on Stripe | Read Stripe's agentic commerce docs and the ACP spec before building anything. |
| Custom stack, Google-first | Start with Merchant Center and the UCP integration hub. |
| B2B SaaS | Watch rather than build. ACP says it supports digital goods and subscriptions, but agents buying software for a company is still early. Focus on being recommended first. |
Discovery still decides the sale
Every protocol in this post handles what happens after an agent has decided to buy. None of them decides which product the agent picks. That choice comes from what AI systems know and trust about your brand: your product data, your site, reviews and third-party coverage.
OpenAI's pivot says the same thing. It stepped back from checkout to focus on discovery. Google is pushing merchants to add conversational attributes and says, from lululemon testing, that those attributes were used 50% of the time in relevant AI Mode recommendations (Google, 16 September 2026).
This is where Citedify fits. It doesn't implement protocols. It asks ChatGPT (with web search), Claude, Perplexity and Google AI Overviews 20 buyer-intent questions about your brand, scores whether and how you are recommended, and shows the competitors named instead and the sources the AI cites. It re-tests monthly, so you can see whether your product-data and content changes are paying off. See e-commerce AI visibility and AI search optimization, or start with what agentic commerce is for the bigger picture.
Key takeaways
- UCP is Google's broad agentic commerce standard, co-developed with major retailers and platforms, and it powers checkout in AI Mode and Gemini.
- ACP is OpenAI and Stripe's open standard. In ChatGPT it now mainly carries product feeds for discovery.
- AP2 is a payment authorisation layer that plugs into UCP and A2A.
- Wallets like Link by Stripe, Shop Pay and Google Pay are the payment rails agents actually use.
- Most Shopify and large-platform merchants are already connected. Focus on product data and settings, not custom builds.
- Protocols handle the purchase. Recommendation decides whether there is one.
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