AP2 vs MCP vs x402: The Agentic Commerce Difference
Which agent actions MCP, AP2 and x402 actually enable, what still needs a person to sign off, and where OpenAI's ACP and Google's UCP fit. Uses AP2 v0.2 Checkout and Payment Mandates.
We judge tools here by how much work they take off your plate. Protocols deserve the same test. MCP, AP2 and x402 each let an agent do something it could not do before, and each leaves a specific step for a person. Knowing which is which tells you how far "agentic commerce" really goes today.
Our earlier explainer, Agentic commerce and the Agent Payments Protocol, described AP2 with intent and cart mandates. That was the v0.1 vocabulary. The current AP2 docs (v0.2) call them the Checkout Mandate and the Payment Mandate, each with an open and a closed stage. This post uses the v0.2 names throughout.
Each protocol, judged by the action it gives an agent
| Protocol | Layer | Who created it | Agent action it enables | What it does not do | Where a person still decides |
|---|---|---|---|---|---|
| MCP | Tools and data | Open-source standard | Call tools and read data on an external system | Authorize or settle payment | Which tools the agent gets, and which can write |
| AP2 | Authorization | Announced by Google, 16 Sept 2025 | Buy within limits the user signed | Move money | Setting the constraints in the open Checkout Mandate |
| x402 | Settlement | Coinbase | Pay for a resource over HTTP, with no account | Capture what the user wanted | Spending limits, which sit outside the protocol |
| ACP | Discovery feed | OpenAI and Stripe | Read merchant feeds and promotions in ChatGPT | Complete checkout inside ChatGPT | The buyer finishes on the merchant's site or app |
| UCP | Discovery to checkout | Check out on AI Mode or Gemini against your backend | Make Google the seller | Merchant opt-in; you stay merchant of record |
MCP: the agent can call tools
The Model Context Protocol is an open-source standard for connecting AI applications to data sources, tools and workflows. The docs describe servers that expose tools and clients that connect to them, and list Claude, ChatGPT, VS Code and Cursor among supporting clients.
In commerce terms, MCP is how an agent might read a catalog, check stock or file an order request through a merchant's tool. It does not move money and it does not prove consent. It can carry a payment only when x402's MCP transport is layered on top.
The human step is scoping. Someone decides which tools a server exposes and which credentials the agent holds. Promptwatch's own MCP server is a useful example of that split: a read-only key hides the write tools, so an agent can call listShoppingTrackedProducts or getShoppingProductPositionAnalytics but cannot publish anything.
AP2: the agent can spend within signed limits
The Agent Payments Protocol targets authorization (did the user give the agent this authority), authenticity (does the request match the user's real intent) and accountability (who is liable when it goes wrong). It works through verifiable digital credentials that are cryptographically signed.
The v0.2 mandates map neatly onto human and agent steps:
- Checkout Mandate, open stage. The user's constraints before any cart is final. This is the person's part: budget, product type, whatever limits they set.
- Checkout Mandate, closed stage. Authorization of one specific finalized checkout. The agent and merchant have agreed on items and details.
- Payment Mandate. Authorizes payment against a specific instrument and is shared with the credential provider, payment networks and the merchant's processor.
AP2's site also publishes a "Human Not Present x402" sample, an autonomous transaction that pays through x402. So the design allows a purchase to complete without a person watching. The person still had to sign the constraints first. That is the part no agent can do for you.
AP2 ships as an extension for the Agent2Agent (A2A) protocol and Google's Universal Commerce Protocol. The initial version supports pull payments like credit and debit cards, with e-wallets, push payments (UPI, PIX) and digital currencies on the roadmap. Version 0.2 has been donated to the FIDO Alliance, where standardization continues.
x402: the agent can pay per request
x402 is Coinbase's open payment protocol for stablecoin payments directly over HTTP. A server answers with status 402 and a PAYMENT-REQUIRED header. The client signs a payload and retries with PAYMENT-SIGNATURE. The server verifies it, locally or through a facilitator, settles onchain and returns 200 with PAYMENT-RESPONSE (how it works).
This is the most autonomous layer in the stack. Coinbase describes it as stateless, with no sessions or accounts, and blockchain-agnostic through facilitators, supporting EVM and Solana networks. The MCP transport brings the same loop to tool calls: a paid tool returns isError: true with payment requirements, and the client retries with payment in _meta["x402/payment"].
Nothing in that flow asks a person to approve each payment. The client signs and retries. Any spending limit has to come from outside the protocol, for example an AP2 mandate wrapped around the transaction.
ACP and UCP decide where the purchase happens
ACP, the Agentic Commerce Protocol from OpenAI and Stripe, now centers on discovery. On 24 March 2026 OpenAI extended it so merchants share product feeds and promotions through ACP. It also said its original Instant Checkout "did not offer the level of flexibility" it wanted. ChatGPT's merchant page says purchases complete on merchant-owned sites or apps. So in ChatGPT, the agent helps find and compare, and the buyer finishes on your site.
UCP is Google's open standard for discovery, buying and post-purchase support, compatible with A2A, AP2 and MCP. It puts a Buy button on eligible listings in AI Mode and Gemini for select merchants, with products eligible in the United States, Canada and Australia. Customers pay with Google Pay; you remain merchant of record. Google also has a separate "agentic checkout" feature that buys on the merchant's own website at the customer's direction.
What still needs a human, in order
- Being found. None of the protocols chooses your product. Your feeds, pages and citations do that.
- Granting tools. Someone decides what an MCP server exposes and with which key.
- Signing constraints. The open Checkout Mandate starts with a person.
- Opting in. UCP checkout means expressing interest in Google's early access program and setting
native_commerce(checkout_eligibility)in Merchant Center. ACP discovery means sending a feed. - Handling disputes. AP2 makes the record auditable. A person still resolves the claim.
The step you can hand to an agent today
Step one is the one you can measure and act on now. Promptwatch covers it end to end, which is why it sits first in our directory. ChatGPT Shopping tracking follows the products you add and their positions over time. Prompt tracking and citation analytics show where you appear across ChatGPT, Gemini, Claude, Perplexity and Google AI Mode, and which pages get cited. Agent Analytics crawler logs show which AI crawlers fetch your product pages. Visitor analytics ties AI-referred buyers to conversions.
It also acts on what it finds. Content gap analysis feeds Content Agents, which draft pages and publish them to Webflow, Framer or WordPress through a review inbox, so a person approves before anything goes live. And because the MCP server exposes the shopping and citation tools, your own agent can pull positions into its workflow.
Essential is $95/mo with a 7-day trial. Professional ($245/mo) adds crawler logs. ChatGPT Shopping tracking starts on Business ($579/mo). Try it at promptwatch.com.