UCP, ACP, AP2, MCP and A2A: which ones your store needs

In less than a year, agentic commerce collected an alphabet: UCP, ACP, AP2, MCP, A2A, and WebMCP. Each was announced as the way AI agents will shop, and vendor posts tend to frame them as a protocol war.

They are mostly not competing. Each answers a different question, and a single purchase by an AI agent can touch several of them. This post explains what each one does, who runs it, and which ones a merchant actually needs to act on, as of October 2026.

What problem does each protocol solve?#

Each one answers a single question in an agent's purchase, from "who are you talking to?" down to "did a person approve this payment?" Seen as a stack, they fit together.

• A2AAgent2Agent
• MCPModel Context Protocol
• AP2Agent Payments Protocol
• ACPAgentic Commerce Protocol
• UCPUniversal Commerce Protocol
• Your storeThe website and its data

UCP

How does an agent browse, build a basket and check out with you?

Who runs it

Co-developed by Google and Shopify; announced January 2026.

What it means for a store

Powers checkout in Google AI Mode and Gemini. You publish a profile; you stay the merchant of record.

What is UCP, and who uses it?#

UCP is an open standard for an agent to discover a store, browse its catalogue, build a basket and check out, with the merchant remaining the seller. Google announced it with Shopify at NRF in January 2026, with Etsy, Wayfair, Target and Walmart among the co-developers. It is published under the Apache 2.0 licence at ucp.dev.

A store that supports UCP publishes a profile describing what it can do. The specification groups those abilities into capabilities: catalogue search, cart, checkout, identity linking (OAuth 2.0, so an agent can act for a signed-in customer), and orders with status updates. UCP is designed to sit on existing standards: it uses REST and JSON-RPC, and states built-in support for AP2, A2A and MCP.

Its main use today is agentic checkout in Google AI Mode and Gemini. To take part, a merchant needs a Merchant Center account in good standing, return policies and customer support details set, and products opted in with the native_commerce(checkout_eligibility) attribute. Google's guidance excludes subscriptions, pre-orders, personalised goods and age-restricted items. If the product IDs in your feed differ from the ones your checkout expects, merchant_item_id tells Google which to send.

What is ACP, and what happened to ChatGPT checkout?#

ACP is the standard OpenAI and Stripe built for listing products in ChatGPT and, originally, for buying them inside the chat. It defines a product feed format, a checkout API and a delegated payment specification. Its latest stable version is dated April 17, 2026, and it is still developed in the open on GitHub.

ACP launched in September 2025 with Instant Checkout, which let shoppers pay without leaving ChatGPT. Stripe's Shared Payment Token was the first implementation of its delegated payment spec: a token scoped to one merchant and one basket total, so the agent never holds the card.

In March 2026 OpenAI scaled Instant Checkout back. Reporting at the time put the number of merchants who went live in the dozens. OpenAI said it would focus on product discovery and let merchants use their own checkouts. In practice, ACP is now mostly how your catalogue appears in ChatGPT, and purchases complete on your store. We cover the change and what to do about it in a separate post.

What is AP2?#

AP2 gives a merchant cryptographic proof that a real person authorised an agent's purchase, and on what terms. It answers the question every payment raises when the buyer is software: who said this was allowed?

AP2 expresses authority as signed mandates: a record of what the shopper asked for, the basket the merchant offered, and the payment the shopper approved. Each is a verifiable credential, so a merchant, payment provider or bank can check it rather than trust the agent's account of events. Google introduced AP2 in 2025 as an extension of A2A, and in April 2026 contributed it to the FIDO Alliance. FIDO, the body behind passkeys, formed a payments working group chaired by Mastercard and Visa to develop it, with Mastercard contributing its Verifiable Intent framework.

For most merchants AP2 is not something to implement directly. It is handled by payment providers and wallets. What matters is that your checkout accepts the payment methods that carry it, and that nothing in your flow demands a human gesture the mandate was meant to replace.

What are MCP and WebMCP?#

MCP lets an AI assistant call a tool directly; WebMCP lets a web page offer tools to a browser agent. Both replace guessing at a website with explicit, named actions.

The Model Context Protocol was started by Anthropic in 2024 and is now supported by most major AI clients. A store or platform can run an MCP server that exposes functions such as "search products" or "check order status". An assistant calls those functions instead of reading your pages. Several commerce platforms now ship MCP servers; for a merchant, it is an additional door, not a replacement for the website.

The Web Model Context Protocol is a draft web standard that brings the same idea into the browser: the page itself declares the actions it supports. Shopify shipped WebMCP tools for checkout on September 28, 2026: get_checkout, update_checkout and complete_checkout. A browser agent can read and update the checkout and place the order after the buyer confirms, paying with a saved Shop Pay card. It is Chromium-only for now and does not cover every checkout type.

What is A2A?#

A2A is how one AI agent finds another and hands it a task. An agent publishes an "agent card" at a well-known address (/.well-known/agent-card.json in current versions) describing what it can do; other agents read it and send requests.

A2A was started by Google in 2025 and moved to the Linux Foundation. It matters to a merchant only if you run an agent of your own, such as a customer-service or order-tracking agent you want other assistants to reach. If you do, publish the card at the current path: older versions used /.well-known/agent.json, and a current client will not look there.

Which protocols does your store need?#

Start from where your customers' agents shop, and from what your platform already provides. For most stores the order of work is the same.

1. Everyone: be reachable and readable. No protocol helps if your bot management refuses agents or your product data is incomplete. This is the layer under every other one. See why AI shopping agents get a 403 at your checkout.
2. Selling through Google: opt products into agentic checkout in Merchant Center and confirm your platform publishes a UCP profile that points to live endpoints.
3. Visible in ChatGPT: make sure your product feed reaches OpenAI, through your platform or directly, and that it agrees with your pages on price and stock.
4. On Shopify: WebMCP checkout and UCP arrive with the platform. Check that your checkout customisations have not broken them.
5. Running your own agent: publish an A2A agent card, and consider an MCP server for order lookups.

How do you know your protocol support actually works?#

Test it the way an agent uses it: discover the profile, follow its endpoints, and check that the answers match your website. Publishing a protocol is not the same as supporting it.

The failures we see most often are not exotic. A UCP profile that points at an endpoint returning errors (your profile points somewhere dead). An agent card at the old path (your agent card is at the old address). A product feed whose prices disagree with the product pages (feed and page disagree on price). Each one looks fine on a dashboard that only checks whether a file exists.

Where a protocol publishes its own conformance tests, run them rather than writing your own: UCP has an upstream conformance suite, and tragent's conformance checks run that suite against a staging store instead of paraphrasing the spec. Then repeat it whenever your platform, theme or checkout changes. Protocols version quickly in 2026, and a store that conformed in spring can fall behind by autumn.

Frequently asked questions

Is UCP replacing ACP?

No. They serve different surfaces. UCP powers agentic checkout on Google AI Mode and Gemini; ACP is how products are listed in ChatGPT. Many merchants will support both, usually through their commerce platform.

Do I need to build UCP myself?

Usually not. Shopify co-developed UCP and generates the profile for its merchants, and other platforms are adding support. Custom stores can implement it from the specification at ucp.dev.

Can ChatGPT still check out on my store?

ChatGPT can list your products and send shoppers to your checkout. OpenAI scaled back in-chat Instant Checkout in March 2026, so purchases now complete on your own site or app.

What is the difference between MCP and WebMCP?

MCP lets an AI assistant call tools exposed by a server. WebMCP lets a web page declare actions a browser agent can call while it is on that page, such as Shopify's checkout tools.

Does AP2 replace my payment provider?

No. AP2 adds proof of the shopper's authorisation to a payment. Your payment provider still processes the transaction, and is usually where AP2 support is implemented.

Sources

1. Universal Commerce Protocol — UCP Authors
2. Prepare your Merchant Center account for UCP — Google for Developers
3. Agentic Commerce Protocol — OpenAI and Stripe (GitHub)
4. Stripe powers Instant Checkout in ChatGPT and releases Agentic Commerce Protocol — Stripe newsroom. September 2025
5. What it means that the leader in agentic commerce just pulled back — Forrester. Analysis of the March 2026 change
6. FIDO Alliance to develop standards for trusted AI agent interactions — FIDO Alliance
7. WebMCP support for checkout — Shopify developer changelog. September 28, 2026
8. Checkout WebMCP — Shopify developer docs

Written by the tragent team and checked against the sources listed above on October 19, 2026. See how we write and correct posts in our editorial policy.