AP2 vs MCP vs x402: The Agentic Commerce Difference
Three protocols, three layers: MCP for tool access, AP2 for signed purchase mandates (v0.2), x402 for stablecoin settlement over HTTP 402. Layer table, verdicts, and where ACP and UCP sit.
Three acronyms. Three layers. They get compared as if they compete, but they answer separate questions. MCP connects an agent to tools. AP2 proves the user approved the purchase. x402 moves the money.
Two more names sit next to them: ACP from OpenAI and Stripe, and UCP from Google. Both live closer to the shopper. Below is the stack in one table, then a verdict per protocol. Every protocol fact comes from the vendor's own documentation.
The stack in one table
Read it top to bottom, in the order a purchase hits each layer.
| # | Layer | Protocol | Who created it | Answers | Does not do |
|---|---|---|---|---|---|
| 1 | Discovery | ACP (ChatGPT), UCP (Google) | OpenAI and Stripe; Google | Can the agent see my products? | Rank you. A feed is eligibility, not placement |
| 2 | Tool access | MCP | Open-source standard | How does the agent call a tool or read data? | Authorize or pay |
| 3 | Authorization | AP2 | Announced by Google, 16 Sept 2025; v0.2 donated to FIDO Alliance | Did the user authorize this exact purchase? | Settle funds |
| 4 | Settlement | x402 | Coinbase | How does the client pay right now? | Record user intent |
Layer 1 is the only one that touches who gets picked. Hold that thought for the end.
MCP
What it is. The Model Context Protocol is an open-source standard for connecting AI applications to data sources, tools and workflows. The docs call it "a USB-C port for AI applications." Servers expose tools, clients connect.
Who supports it. The docs list Claude, ChatGPT, VS Code and Cursor, among others.
Commerce role. Plumbing. An agent might read your catalog or check stock through an MCP server. MCP is not a payment or checkout protocol. It carries a payment only when x402's MCP transport rides on it.
Verdict: General agent plumbing, neutral for commerce. Supporting it tells a buyer nothing about checkout.
AP2
What it is. The Agent Payments Protocol, announced by Google on 16 September 2025. It targets three problems: authorization, authenticity and accountability.
How it works. Verifiable digital credentials, cryptographically signed, called mandates. Current docs (v0.2) name two types. Each has an open and a closed stage.
| Mandate (v0.2) | Covers | Shared with |
|---|---|---|
| Checkout Mandate | Items and purchase details agreed by agent and merchant. Open: user constraints before the cart is final. Closed: one finalized checkout | Agent and merchant |
| Payment Mandate | Payment against a specific instrument | Credential provider, networks, merchant payment processor |
v0.1 writing called these intent and cart mandates. Anyone still using those names is describing the older version.
Scope. Extension for A2A and UCP; its principles call it an open extension for A2A and MCP. Initial version: pull payments such as credit and debit cards. Roadmap: e-wallets, push payments (UPI, PIX), digital currencies. v0.2 now sits with the FIDO Alliance for standardization.
Verdict: The only layer that answers "who is liable." Cards today, x402 in the samples. Merchants wait on their processor here.
x402
What it is. x402 is Coinbase's open payment protocol for instant stablecoin payments over HTTP. It revives status code 402 Payment Required.
Flow, per Coinbase's docs:
- Client requests a resource.
- Server returns 402 plus a
PAYMENT-REQUIREDheader. - Client signs a payload, retries with
PAYMENT-SIGNATURE. - Server verifies (locally or via a facilitator), settles onchain, returns 200 with
PAYMENT-RESPONSE.
Design. Stateless, no sessions or accounts. Blockchain-agnostic through facilitators. CAIP-2 network IDs (Base, for example). EVM and Solana.
Over MCP. The MCP transport spec has a paid tool return isError: true with payment requirements. The client retries with payment in _meta["x402/payment"]. Settlement comes back in _meta["x402/payment-response"].
Inside AP2. The AP2 site ships a "Human Not Present x402" sample: an autonomous transaction paid through x402.
Verdict: Best fit for machine-to-machine, pay-per-request access. Says nothing about what the human wanted. Pair it with AP2 when that matters.
ACP and UCP
ACP. Agentic Commerce Protocol, OpenAI and Stripe. On 24 March 2026 OpenAI extended it to discovery: merchants share product feeds and promotions through ACP. OpenAI named Target, Sephora, Nordstrom, Lowe's, Best Buy, The Home Depot and Wayfair as integrated. Shopify merchants are covered via Shopify Catalog. Standalone Instant Checkout is being dropped; purchases complete on merchant sites or apps (chatgpt.com/merchants). Shopping in ChatGPT is live in the U.S.
UCP. Google's open standard for discovery, buying and post-purchase. Compatible with A2A, AP2 and MCP. Powers a Buy button on eligible listings in AI Mode and Gemini. Select merchants only, products eligible in the US, Canada and Australia. Merchant stays merchant of record; buyer pays with Google Pay.
Verdict: These two decide whether you are in the candidate set on ChatGPT (820M+ weekly active users) and Gemini (650M+ monthly active users). For most merchants, start here.
One purchase, every layer
- Shopper asks an assistant for a product within a budget.
- Agent finds candidates via ACP feed, crawl, or Merchant Center listing (layer 1).
- Agent checks stock or price through a tool, possibly MCP (layer 2).
- Budget becomes an open Checkout Mandate. Agreed cart becomes a closed one. A Payment Mandate covers the instrument (layer 3).
- Funds move on card rails, or over x402 for stablecoins (layer 4).
Not every purchase uses every layer. ChatGPT purchases, for example, now finish on the merchant's own site.
Verdicts, ranked by what a merchant should do first
| Priority | Protocol | Merchant action |
|---|---|---|
| 1 | ACP / UCP | Clean feeds, Merchant Center eligibility, native_commerce(checkout_eligibility) set where it applies |
| 2 | MCP | Expose tools only if you have something agents should call |
| 3 | AP2 | Ask your processor about v0.2 support |
| 4 | x402 | Test only if you sell paid APIs or per-request digital goods |
The layer none of them cover
None of these protocols ranks you. A feed makes you eligible. A mandate makes the payment trustworthy. Neither makes the agent choose your product over the next one. That happens in the answer, and you can measure it.
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