MCP vs UCP vs AP2 vs ACP: The Agentic Commerce Protocols Explained

Anyone trying to follow agentic commerce in 2026 runs into the same problem: there are four three-letter protocols flying around — MCP, UCP, AP2, ACP — and most coverage either treats them as competitors or smashes them into a single confused blob. They are not competitors and they are not the same thing. They live at different layers of the stack and they do different jobs.

This piece is the unglamorous answer to "what is each of these for, who runs it, and which ones do I actually have to care about?"

The One-Sentence Version

• MCP (Model Context Protocol) is the wire format that lets AI agents talk to tools. Generic. Created by Anthropic, now stewarded by the Linux Foundation.
• UCP (Universal Commerce Protocol) is the commerce-specific application protocol that defines what tools a store exposes (Cart, Checkout, Catalog, Order). Co-developed by Google and Shopify, governed by the UCP Tech Council.
• AP2 (Agent Payments Protocol) is the authorization layer that lets an AI agent prove the user authorized a specific payment. Led by Google, used inside UCP as an extension.
• ACP (Agentic Commerce Protocol) is OpenAI's protocol for plugging merchants into ChatGPT. Originally for in-chat checkout; now mostly for product discovery.

Now the longer version, which is what you came here for.

Comparison Table

MCP: The Wire

Model Context Protocol is the simplest of the four to explain, even though it has the longest history. Anthropic announced MCP in November 2024 as a generic protocol for connecting AI assistants to external data sources and tools — the same kind of role HTTP plays for web browsers. An MCP server exposes tools (think functions); an MCP client (an AI agent) calls them.

MCP is deliberately generic. The same protocol that lets Claude read your local filesystem, query Postgres, or call a Slack API is the protocol that Shopify uses to expose product search to AI shoppers. There is nothing commerce-specific about MCP itself.

A few things changed about MCP in late 2025 and early 2026. Anthropic donated MCP to the Linux Foundation in December 2025, where it sits inside the new Agentic AI Foundation alongside Block's goose and OpenAI's AGENTS.md. Anthropic announced the move as ensuring MCP stays "open, neutral, and community-driven." Founding partners of the AAIF include Anthropic, Block, OpenAI, Google, Microsoft, AWS, Cloudflare, and Bloomberg — a lineup that effectively retires the question of whether MCP is "an Anthropic thing."

The community-driven MCP Registry, launched in preview in September 2025, now indexes 10,000+ servers. It is the discovery layer for MCP-the-transport: it tells you what tools exist, not what protocol they speak on top.

For commerce, MCP's role is "the pipe." UCP can run over MCP. So can ACP-style merchant tools, when a vendor chooses to. So can countless internal tools. The right way to think about MCP is the same way you think about TCP — useful infrastructure that almost nothing meaningful happens at, but everything depends on.

UCP: The Commerce Application Protocol

UCP is what you get when you stop talking about transports and start talking about commerce specifically. The protocol defines four primary capabilities — Cart, Checkout, Order, and Identity Linking — plus a Catalog capability for searching and looking up products, plus extensions like discounts, fulfillment, loyalty, and AP2 mandates.

Everything in UCP lives at /.well-known/ucp on a merchant's domain. That manifest is the merchant's profile: which capabilities the store implements, which transports each capability uses, which payment handlers are configured, and which public keys sign responses. An agent's profile is symmetric — it lives at a known URL on the agent's domain and declares the same shape from the agent's side.

UCP supports four transports: REST (HTTP/JSON over OpenAPI), MCP (JSON-RPC with OpenRPC schemas), A2A (Agent-to-Agent using Agent Cards), and Embedded (host-embedded interfaces with OpenRPC events). A merchant can pick which transports each capability uses. Shopify's Storefront Catalog MCP, for instance, runs UCP Catalog over MCP. Google's AI Mode checkout flow runs UCP Checkout over REST.

The protocol was launched at NRF 2026 in January with Google and Shopify as co-developers and Etsy, Wayfair, Target, Walmart, and 16+ payment and retail partners as founding endorsers. The April 24 expansion of the UCP Tech Council added Amazon, Meta, Microsoft, Salesforce, and Stripe to the technical body that governs the spec. As of the v2026-04-08 release, UCP supports multi-item carts, live catalog queries, and identity-linked personalization — the things a real storefront does.

UCP is the protocol most merchants need to think about, because it is the one that puts your store inside an AI surface as a real commerce destination, not just a search result.

AP2: The Payment Authorization Layer

If UCP is the application protocol, AP2 is the authorization protocol that sits underneath payments. Google announced Agent Payments Protocol in late 2025 with more than 60 partners — Adyen, American Express, Ant International, Coinbase, Etsy, Forter, Intuit, JCB, Mastercard, Mysten Labs, PayPal, Revolut, Salesforce, ServiceNow, UnionPay International, Worldpay, and others.

AP2 solves three specific problems that come up the moment you let an AI agent move money:

• Authorization — proof that the user gave the agent permission to make this specific purchase.
• Authenticity — proof to the merchant that the agent's request reflects the user's actual intent.
• Accountability — a clear chain of evidence if a transaction turns out to be fraudulent or wrong.

AP2 implements all three using Mandates: cryptographically signed digital contracts. The CheckoutMandate represents the user's signed authorization of a specific Checkout state — these line items, this total, this currency. The PaymentMandate represents the user's authorization of a specific payment credential to be used for that exact checkout. Both mandates are verifiable credentials, signed with keys whose provenance can be checked.

The relationship to UCP is unambiguous: AP2 is plugged into UCP as the AP2 Mandates Extension. When a merchant and platform negotiate that extension during a UCP checkout, the session becomes "Security Locked" — neither side may revert to an unprotected flow. Mandate signing becomes part of every state transition.

The honest current status of AP2: the spec exists, the partner list is large, and the cryptographic plumbing works in test. As of April 2026, consumer-visible AP2 transactions in production remain limited. Most live UCP checkouts (Etsy and Wayfair items in Google AI Mode, for example) still use Google Pay tokens directly without enabling AP2 mandates. AP2 is real engineering, just not yet ubiquitous engineering.

ACP: OpenAI's Bet, Refocused

OpenAI's Agentic Commerce Protocol launched in September 2025 with Stripe as the payments backbone and Etsy as the first merchant. The original framing was full in-chat checkout: a ChatGPT user could complete a purchase from a participating merchant without ever leaving the conversation. OpenAI ran that version of Instant Checkout for about six months.

On March 24, 2026, OpenAI scaled it back. In a public update, the company wrote that "the initial version of Instant Checkout did not offer the level of flexibility that we aspire to provide" and announced it was letting merchants use their own checkout experiences while OpenAI focused ACP on product discovery. TechCrunch reported that the underlying issue was simple: ChatGPT users were not actually buying through Instant Checkout in meaningful volume, and e-commerce sites reported minimal sales from ChatGPT referrals.

The new ACP is mostly a product-discovery protocol. Merchants supply OpenAI with structured feeds — titles, descriptions, images, prices, availability, promotions — and ChatGPT renders those products visually inside the conversation, with side-by-side comparison, conversational filtering, and image-based lookalike search. Checkout still happens, but the user is routed to the merchant's own site (in-app browser on mobile, separate tab on desktop). Walmart's "Sparky" experience inside ChatGPT, announced March 25, follows that pattern: discover in ChatGPT, complete on Walmart.com.

Major retailers integrated for ACP discovery as of March 2026 include Target, Sephora, Nordstrom, Lowe's, Best Buy, The Home Depot, and Wayfair. ACP discovery now also supports integration through third-party providers like Salesforce and Stripe.

The interesting consequence is that ACP and UCP are converging on roles, not diverging. ACP's discovery layer overlaps with UCP's Catalog capability. ACP's pivot away from in-chat checkout means OpenAI is now ceding the checkout territory that UCP and Microsoft's Copilot Checkout are actively expanding into.

How They Fit Together

It helps to draw the stack from the bottom up.

At the wire layer is MCP (and REST, and A2A). These are how messages move.

On top of that wire is UCP, which defines what messages mean for commerce. A UCP Cart request can travel over MCP (Shopify's Storefront Catalog MCP) or over REST (Google's Merchant Center integration). The UCP layer says "this is a cart, here is what fields it has, here is what the response should look like."

Inside a UCP checkout, AP2 can be activated as an extension to provide cryptographic payment authorization. Without AP2, a UCP checkout still works — it just relies on the existing payment handler trust model (digital wallets, tokenized cards). With AP2, every payment carries a mandate.

Off to the side, ACP is OpenAI's parallel application protocol. It does not run on top of UCP. It does not use AP2. It is OpenAI's own discovery and (formerly) checkout protocol, primarily reaching merchants through OpenAI's curated merchant program. Some of the same merchants — Etsy, Wayfair — implement both UCP and ACP because they want presence in both ecosystems. Most large merchants will end up doing the same.

For platforms in the middle, the picture is even more layered. Microsoft's Copilot Checkout, announced January 2026, runs its own commerce flow inside Copilot powered by Stripe and PayPal — not on UCP, not on ACP. Microsoft joining the UCP Tech Council in April could change that, but as of late April 2026 Copilot Checkout is its own thing.

Which Protocols Actually Affect You

For a typical merchant, the practical answer is short.

• You already speak MCP if you are on Shopify (Storefront MCP rolled out to every store in 2025). You do not need to do anything to "support MCP."
• You should support UCP if you want to be present in Google AI Mode, Gemini, and the broader open agentic commerce ecosystem. This means publishing a /.well-known/ucp manifest. On Shopify, Salesforce Commerce Cloud, and Commerce Inc, the platform does this for you. Elsewhere, you do it yourself.
• You should consider ACP if you sell categories where ChatGPT product discovery moves the needle (apparel, beauty, home, gifts). ACP is now primarily a product-discovery channel, which makes the implementation cost lower and the upside narrower than the original Instant Checkout pitch.
• You will eventually adopt AP2 if you handle high-trust, high-value autonomous transactions. For most merchants today, AP2 is a thing your payment processor will turn on for you when the broader payments rails are ready.

For an AI agent builder, the picture inverts: you almost certainly want to speak MCP at the transport layer, UCP at the commerce layer, and ACP for ChatGPT-specific reach.

What the Convergence Looks Like

The thing nobody quite says out loud is that the four protocols are converging on different time horizons.

MCP-UCP convergence is already done — UCP runs over MCP as a first-class transport. UCP-AP2 convergence is in progress and effectively complete in spec, pending broader payment-network rollout. UCP-ACP convergence is unstable: ACP backed off from checkout in March, the UCP Tech Council added Stripe and Microsoft (both ACP-adjacent) in April, and the practical merchant integration patterns are starting to look similar (structured feeds + capability negotiation + merchant-of-record retention). Whether that turns into formal alignment or an indefinite parallel existence is the most interesting open question in agentic commerce right now.

For a longer breakdown of why UCP and ACP compete on different axes than they appear to, see our UCP vs ACP analysis. For a state-of-the-industry view of how all four protocols are landing in real merchant adoption numbers, see our April 2026 state of agentic commerce post.

Frequently Asked Questions

Are MCP, UCP, AP2, and ACP competing standards?

Mostly no. MCP is a transport for agents to talk to tools. UCP is a commerce-specific protocol that runs on top of MCP, REST, or A2A. AP2 is a payments authorization layer that plugs into UCP as an extension. ACP is OpenAI's checkout-and-discovery protocol used inside ChatGPT. UCP and ACP are the only two that meaningfully overlap.

Which protocol does my store need to support?

If you sell on Shopify, you already support MCP and UCP automatically. If you want ChatGPT discovery traffic, you separately want ACP via a product feed. If you handle high-value autonomous purchases, you eventually want AP2 mandates on top of your UCP checkout.

Is MCP being replaced by UCP?

No. MCP is a generic transport for any AI tool, donated to the Linux Foundation in December 2025. UCP is a commerce-specific protocol that uses MCP as one of its supported transports.

What is AP2 and how does it relate to UCP?

AP2 (Agent Payments Protocol) is Google's open protocol for authorizing AI agents to make payments. It uses cryptographically signed Mandates to prove the user authorized a specific transaction. AP2 plugs into UCP as the AP2 Mandates Extension.

Why does OpenAI use ACP instead of UCP?

OpenAI shipped ACP with Stripe in September 2025, four months before UCP existed. ACP is tightly integrated with ChatGPT's conversational interface and OpenAI's curated merchant program. After OpenAI scaled back Instant Checkout in March 2026, ACP is now mostly a product-discovery protocol.

Who governs each protocol?

MCP is governed by the Linux Foundation's Agentic AI Foundation. UCP is governed by the UCP Tech Council, which includes Google, Shopify, Etsy, Target, Wayfair, Amazon, Meta, Microsoft, Salesforce, and Stripe. AP2 is led by Google with 60+ payments and tech partners. ACP is owned by OpenAI in partnership with Stripe.