We covered the basics of how AI agents buy — structured catalogs, readable terms, a completable path to checkout. Since then the protocol layer has gone from sketch to turf war, and the most instructive event so far is a retreat.
The map
ACP (Agentic Commerce Protocol) — launched by OpenAI and Stripe in September 2025 alongside Instant Checkout in ChatGPT. It standardizes how an agent and a merchant pass structured purchase data. The spec is on its fifth revision, PayPal has joined as a payment provider, Salesforce supports it, and Shopify supports it alongside UCP.
UCP (Universal Commerce Protocol) — Google and Shopify's broader counter, spanning the full journey: discovery, cart, checkout, order management, returns, loyalty, merchant rules. Over a million Shopify merchants are coming online through it. Where ACP standardizes the checkout handshake, UCP is a merchant-control argument: be usable by agents without dissolving into anonymous inventory inside someone else's assistant.
AP2 (Agent Payments Protocol) — Google's authorization layer, built around mandates: structured records of what a user actually permitted an agent to do. The insight is that authorization is not payment. Moving money proves nothing about whether it should have moved; when a purchase is disputed, the evidence has to show what was asked, what was allowed, and whether the agent stayed inside those limits.
x402 and stablecoin rails — Coinbase's revival of HTTP's dormant 402 "payment required" status, plus Stripe's machine-payments work. These target a different transaction shape entirely: software paying software — per-query data access, per-task API charges, agent budgets — payments too small and too frequent for anything resembling a checkout page.
The Instant Checkout lesson
Instant Checkout was the boldest bet: complete the purchase inside the chat. Within months, OpenAI scaled it back — and the numbers behind that retreat, reported by Wired from Walmart's own experiment, are the most useful data in agentic commerce so far.
Walmart put roughly 200,000 products into ChatGPT. In-chat purchases converted at about one-third the rate of the flow where ChatGPT sent the shopper back to Walmart's site. And yet — same experiment, same reporting — ChatGPT drove roughly twice the rate of new-customer acquisition compared to search engines.
Read those together and the story isn't "agentic commerce failed." It's that discovery and transaction want to live in different places. Buyers were happy to let the assistant find and shortlist; they wanted the merchant's own surface — cart, bundles, loyalty, trust signals — to finish. The checkout button was the wrong center.
The durable asset survived the retreat: the data protocol, not the checkout button. ACP kept shipping revisions after the product it launched with was walked back. What every camp agrees on is the substrate — structured product data, current inventory, explicit terms, machine-verifiable identity, and a callable path from intent to order.
No single winner, and that's fine
These protocols answer different questions, so layered coexistence is far more likely than a winner-take-all outcome. A retailer may plausibly need UCP for merchant-controlled journeys, ACP for assistant-surface transactions, AP2-style mandates for delegated consent, and never touch x402 — while an API business needs exactly the reverse.
Which means the right strategy for most brands is not to bet on a protocol. It's to build the layer every protocol consumes:
- Complete
Product/Offerschema — price, currency, availability, identifiers — on everything you sell. - Shipping, returns, and policy terms published as data, not prose.
- Inventory and pricing that are current, because agents act on what they read.
- AI fetchers allowed in, and a machine-readable manifest of what you sell and how to transact.
Do that, and whichever handshake wins your category, you're already speaking the part that doesn't change.
Commerce readiness is one of the eight dimensions the Legible Readiness Index scores. Run a free report to see whether an agent could discover, price, and complete a purchase from your site today — before a protocol makes the answer public.