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ACP, UCP, AP2 and MCP: agentic commerce protocols explained
An agentic commerce protocol is a standard that lets an AI agent transact with a merchant directly, reading a catalogue and completing a purchase through a defined interface, rather than by driving a storefront designed for a human with a mouse. Four names come up: ACP, UCP, AP2 and MCP. They are not four candidates for the same job.
Four names come up, and the most useful thing to know first is that they are not four candidates for the same job. Two describe shopping, one describes payment authorisation, and one is the general mechanism the others reach for.
The four names you will see
ACP, the Agentic Commerce Protocol
Maintained by OpenAI and Stripe, published under Apache 2.0 in a public repository. Its own repository records an initial version dated 29 September 2025 and a revision dated 17 April 2026 that added cart, feed, orders, authentication and MCP. Retail Dive reported on 25 March 2026 that seven retailers were live on the updated protocol: Target, Sephora, Nordstrom, Lowe’s, Best Buy, The Home Depot and Wayfair, with Walmart selling through Sparky.
UCP, the Universal Commerce Protocol
Announced by Google at NRF on 11 January 2026, built with Shopify, Etsy, Wayfair, Target and Walmart, and described in Sundar Pichai’s remarks on Google’s company blog as endorsed by more than twenty further companies. Google’s developer blog the same day set out the technical shape: a merchant manifest at /.well-known/ucp, capabilities covering checkout, discovery, discount and fulfilment, and a stated relationship to AP2, A2A and MCP. Google’s Shopping blog added Cart, Catalog and Identity Linking over OAuth 2.0 on 19 March 2026, along with onboarding through Merchant Center; Search Engine Journal noted that Cart and Catalog are draft specifications while Identity Linking is stable.
AP2, the Agent Payments Protocol
Published by Google on 16 September 2025, with the announcement saying it was built with more than sixty collaborating organisations. AP2 is about authorisation rather than shopping: its Mandates model produces signed, verifiable records of what a user actually authorised an agent to buy and on what terms. That is the question a payment network asks, and it is a different question from "which product".
MCP, the Model Context Protocol
The general mechanism for exposing tools and data to a model, which the commerce protocols reach for rather than reinventing. Shopify’s developer documentation gives each store a Storefront Catalog MCP endpoint at https://{storedomain}/api/ucp/mcp, implementing the UCP Catalog capability. If you are on a hosted platform, this is the sort of thing that appears without you building it.
What a protocol replaces
Without a protocol, an agent shops the way a person does: it loads pages, reads what it can, clicks things, and hopes the checkout behaves. This works, and it is still how a great many agent purchases happen, but it inherits every fragility of a storefront built for a mouse.
A protocol replaces that with a defined exchange: here is the catalogue, here is the item, here is the order. Nothing has to be inferred from layout.
Why the fundamentals still decide it
Protocols standardise the transaction, but they still describe your product data. A protocol cannot invent a price you never published or a returns policy that only exists as a graphic. A store with poor data is poorly represented under every protocol, and a store with clean data is well represented under all of them and under none.
March 2026 made this concrete. When OpenAI retired the first version of Instant Checkout, as Retail Dive reported on 25 March 2026, the purchase moved back onto merchant checkouts. Merchants who had treated protocol support as the work found that the ordinary storefront was the work after all. The fallback path is not a legacy concern; it became the main path again inside six months.
What to do about it now
- Keep your product feed accurate and fresh, because every protocol reads from something like it.
- Keep structured data complete, because that is what fills the protocol’s fields.
- Keep your ordinary checkout drivable, because the fallback path is not going away and has already come back once.
- Watch what your platform ships, and let it do the adopting.
Protocol churn is a reason to have someone watching, not a reason to rebuild. The stores that will do well under agentic commerce are the ones whose data was already honest.
Questions
Do I need to adopt a protocol to be agent-ready?
No. Protocol support is an optimisation; agents will continue to fall back to driving your ordinary storefront, and that fallback path is where most stores are losing purchases today. Clean product data and a checkout that works without a mouse help under every protocol and in the absence of any.
What is the actual difference between ACP and UCP?
They come from different camps and cover different ground. ACP is maintained by OpenAI and Stripe under Apache 2.0 in a public repository, first published on 29 September 2025, with a revision on 17 April 2026 adding cart, feed, orders, authentication and MCP. UCP was announced by Google at NRF on 11 January 2026, built with Shopify, Etsy, Wayfair, Target and Walmart, and defines a merchant manifest at /.well-known/ucp with capabilities for checkout, discovery, discount and fulfilment. For most merchants the choice is made by their platform rather than by them.
Where does AP2 fit?
AP2 is the payments layer rather than the shopping layer. Google published it on 16 September 2025, saying it was built with more than sixty collaborating organisations, and it defines Mandates: signed, verifiable statements of what a user authorised an agent to buy and on what terms. Google’s UCP documentation describes UCP as relating to AP2, A2A and MCP, so these are layers of one stack, not rivals.
Which one should I back?
For most merchants this is decided by their platform rather than by them, and backing the wrong one is a smaller risk than being unreadable under all of them. Watch what your platform ships and keep the fundamentals in order in the meantime.