AP2 vs MCP vs x402: The Agentic Commerce Difference

Commerce and payments vendors have started putting AP2, MCP and x402 on the same slide, usually under a heading like "agent ready." The three names do different jobs. A vendor that supports one of them has told you nothing about the other two, and that gap is where most of the vague promises sit.

This guide sorts the protocols by the question each one answers, using their own documentation, then lists what we would ask a payments or commerce vendor before signing.

The short version

Protocol	Layer	Who created it	What it answers	What it does not do
MCP	Tool and data access	Open-source standard (modelcontextprotocol.io)	How does an agent reach a catalog, a tool or a workflow?	Authorize or settle a payment
AP2	Purchase authorization	Announced by Google, 16 September 2025; v0.2 donated to the FIDO Alliance	Did the user give the agent authority for this exact purchase?	Move the money, or connect the agent to your catalog
x402	Payment settlement	Coinbase	How does the client pay for this resource right now?	Prove what the user intended
ACP	Product feeds into ChatGPT	OpenAI and Stripe	How do my products and promotions reach ChatGPT shoppers?	Run a standalone checkout inside ChatGPT (OpenAI is moving away from that)
UCP	Discovery, buying and post-purchase on Google	Google	How does checkout in AI Mode or Gemini talk to my backend?	Replace you as seller; you stay merchant of record

MCP is the connection layer

The Model Context Protocol is an open-source standard for connecting AI applications to data sources, tools and workflows. Its docs call it "a USB-C port for AI applications." Servers expose data and tools, clients connect to them, and the docs list Claude, ChatGPT, VS Code and Cursor among the clients that support it.

For a store, an MCP server is one way an agent could read inventory or call a stock-check tool. It is not a payment protocol, and it only carries a payment when something like x402's MCP transport rides on top. If a vendor's "agentic commerce" pitch is an MCP server alone, you are buying a connector.

AP2 is the permission layer

The Agent Payments Protocol exists to answer three questions a merchant or payment network has when an agent shows up with a card: did the user give the agent authority for this purchase, does the request reflect what the user actually wanted, and who is liable when it goes wrong.

It answers them with verifiable digital credentials, cryptographically signed records called mandates. The current docs (v0.2) name two, and each has an open and a closed stage:

Checkout Mandate. The items and purchase details agreed between agent and merchant. The open stage holds the user's constraints before a cart is final. The closed stage authorizes one specific finalized checkout.
Payment Mandate. Authorizes payment against a specific instrument. It is shared with the credential provider, the networks and the merchant's payment processor.

Earlier AP2 writing (v0.1) called these intent and cart mandates. If a vendor's documentation still uses those words, ask which version they built against.

AP2 is available as an extension for the Agent2Agent (A2A) protocol and Google's Universal Commerce Protocol. The initial version supports pull payments such as credit and debit cards, with e-wallets, push payments like UPI and PIX, and digital currencies on the roadmap. Version 0.2 has been donated to the FIDO Alliance, where standardization continues.

x402 is the money layer

x402 is an open payment protocol developed by Coinbase for instant stablecoin payments over plain HTTP, built on the old 402 Payment Required status code. In Coinbase's flow, the server answers a request with 402 and its terms in a PAYMENT-REQUIRED header. The client signs a payment and retries with a PAYMENT-SIGNATURE header. The server verifies it, locally or through a "facilitator," settles onchain and returns 200.

It is stateless, with no sessions or accounts, and supports EVM and Solana networks identified by CAIP-2 IDs such as Base. An MCP transport spec lets a paid MCP tool ask for payment, with the client retrying the call and attaching it in _meta["x402/payment"].

Where ACP and UCP fit

ACP, the Agentic Commerce Protocol from OpenAI and Stripe, is now mostly about getting products into ChatGPT. On 24 March 2026 OpenAI extended it to discovery: merchants share product feeds and promotions through ACP, and Shopify merchants are covered through Shopify Catalog. OpenAI said the original Instant Checkout "did not offer the level of flexibility" it wanted, so purchases now complete on merchant-owned sites or apps, per ChatGPT's merchant page.

UCP is Google's open standard for discovery, buying and post-purchase support, compatible with A2A, AP2 and MCP. It powers a Buy button on eligible listings in AI Mode and Gemini for select merchants, covering products eligible in the United States, Canada and Australia. You stay merchant of record, and customers pay with Google Pay.

How they stack in one purchase

Picture a shopper asking an assistant for running shoes under a set budget. The agent finds your product through an ACP feed, a crawl or your Merchant Center listing, then checks live stock by calling a tool, possibly over MCP. Under AP2, the budget and size become an open Checkout Mandate. Once the cart is agreed, the closed mandate authorizes that exact checkout and a Payment Mandate authorizes the card. Money moves over card rails, or over x402 in a stablecoin flow; AP2's samples include a "Human Not Present x402" case.

Few purchases will touch every layer. What matters is knowing which layer a vendor is actually selling you.

Questions to ask your payments or commerce vendor

On MCP

Which tools does your MCP server expose, and can any of them place an order or spend money?

On AP2

Did you build against v0.1 (intent and cart mandates) or v0.2 (Checkout and Payment Mandates)?
Do you verify mandates yourself, or pass them through to our processor?
What happens if the price changes between the open and closed Checkout Mandate, and who keeps the signed records for a dispute?

On x402

Which networks do you support, who runs the facilitator, and do you support the MCP transport?

On ACP and UCP

Can you deliver our feed to ChatGPT through ACP, with a stable id and at least one variant (with its own id and title) on every product?
Can you set native_commerce(checkout_eligibility) in Merchant Center (it defaults to FALSE), and are any of our products in a category Google lists as ineligible, such as subscriptions, pre-orders or freight items?

Who should care about which layer first

Physical goods sold in the US, Canada or Australia: UCP eligibility and an ACP feed come first.
Paid APIs, data or per-request digital goods: x402, whose account-free design suits that case.
Teams exposing a catalog or tools to agents: MCP.
Payments and risk teams: AP2, and specifically whether your processor plans to support v0.2.

The step none of these protocols covers

Every protocol above assumes the agent already picked your product. None of them puts you in front of it. Discovery comes first, and it is the layer you can measure today.

That is the job Promptwatch does. Its ChatGPT Shopping tracking follows the products you add and how their positions move over time. Prompt tracking and citation analytics show whether you appear across ChatGPT, Gemini, Claude, Perplexity and Google AI Mode, and which pages get cited. Agent Analytics logs which AI crawlers fetch your product pages, and visitor analytics ties AI-referred sessions to conversions. Crawler logs first appear on Professional ($245/mo); Essential is $95/mo with a 7-day trial. ChatGPT Shopping tracking is on Business ($579/mo). Our ChatGPT Shopping tracking guide covers setup.

Ask your payments vendor the questions above. Then check whether agents can find what you sell, at promptwatch.com.