Agentic Commerce Standards: UCP vs ACP vs AP2

Agentic commerce standards — the open protocols that let AI agents browse, build a cart, and pay on a shopper’s behalf — went from a single experiment to a three-way standards contest in roughly nine months. By June 2026 a merchant deciding how to be sold by an AI agent faces three names: UCP, ACP, and AP2. They are not interchangeable, and choosing badly costs real engineering time.

What makes this hard is the noise. Most coverage treats UCP, ACP, and AP2 as three equally live competitors. They are not. They sit at different layers of the buying journey, they answer different questions, and one of them already watched its flagship consumer product get switched off. The honest version of this story is more useful than the press-release version.

This guide does three things. It separates the three standards by the layer each one actually owns. It tells the real history, including the quiet shutdown of OpenAI’s Instant Checkout in March 2026. And it ends with a decision matrix by merchant size, plus the case for the multi-standard adapter Adyen shipped four days before this post. Every figure here traces to a primary vendor source or is explicitly hedged where it does not.

The single most common mistake in agentic-commerce coverage is treating UCP, ACP, and AP2 as substitutes — as if a merchant must pick one. They are better understood as three layers of one transaction, each answering a distinct question. Get this mental model right and the rest of the decision becomes much simpler.

UCP answers what to sell and how to build the cart: it covers discovery, cart, checkout, orders, and post-purchase. ACP answers how to execute the payment inside an AI surface, originally OpenAI’s ChatGPT. AP2 answers who authorized the purchase — a cryptographic chain of proof so a merchant, bank, or network can trust that an agent acted with the user’s consent. The standards are designed to stack, not to replace one another.

The Universal Commerce Protocol was announced January 11, 2026 at NRF, the National Retail Federation’s annual show. It is co-developed by Google, Shopify, Etsy, Wayfair, Target, and Walmart, and launched with 20+ endorsing partners including Adyen, American Express, Mastercard, Stripe, Visa, The Home Depot, Macy’s, Best Buy, Flipkart, and Zalando. That breadth of retail-side backing is UCP’s defining advantage.

UCP is open-source under Apache 2.0 with its specification published at ucp.dev. There is no central approval committee: merchants and agents publish profiles via /.well-known/ucp JSON manifests, and the protocol uses three capability layers — a core Shopping service for transaction primitives, Capabilities for Checkout, Orders, and Catalog, and Extensions for domain-specific schemas. Crucially, UCP supports REST, Agent2Agent (A2A), and the Model Context Protocol (MCP) as transport options, so merchants are not locked to one communication layer.

The checkout state machine

UCP’s checkout uses three statuses: incomplete, requires_escalation, and ready_for_complete. When a transaction cannot proceed autonomously — a CAPTCHA, a missing address field — the merchant response includes a structured continue_url for clean handoff to a human. That graceful-degradation design is a meaningful practical detail: it means an agent that hits a wall hands the shopper back to the merchant rather than failing silently.

For deeper detail on the self-serve developer experience, see our walkthrough of the Shopify Spring ’26 Edition agentic commerce launch, and for the protocol’s origins, our overview of the Google Universal Commerce Protocol.

The Agentic Commerce Protocol launched September 29, 2025, co-developed by OpenAI and Stripe and open-sourced at agenticcommerce.dev. Its first live deployment was OpenAI Instant Checkout inside ChatGPT, with Etsy as the inaugural merchant and over a million Shopify merchants — including Glossier, SKIMS, Spanx, and Vuori — announced as coming soon. With 700M+ weekly ChatGPT users, the addressable surface looked enormous.

It did not work out. According to independent reporting from Digital Commerce 360 in March 2026, OpenAI shut down Instant Checkout after roughly five months with near-zero sales. The reasons were both technical and behavioral: OpenAI had no mechanism for sales-tax collection or real-time inventory sync; only around a dozen of the millions of eligible Shopify merchants actually integrated it; and consumers researched products in ChatGPT but reverted to familiar checkout flows the moment they had to enter payment details.

How ACP executes a payment

ACP’s core technical primitive is Stripe’s Shared Payment Token (SPT). An SPT lets an AI agent initiate a transaction without ever seeing the buyer’s payment credentials: each token is scoped to a specific merchant and basket total. After the buyer selects a payment method, ChatGPT receives the SPT and passes it to the merchant via API. Merchants not on Stripe can use the Delegated Payments Spec inside ACP. For merchants already on Stripe, OpenAI framed enabling agentic payments as taking as little as one line of code — a vendor-stated claim where real complexity depends on the existing stack. We dig into the token mechanics in our guide to Stripe’s Shared Payment Token for agentic checkout, and into the broader spec in our Agentic Commerce Protocol explainer.

What about the 4% fee?

During the Instant Checkout era, ACP charged merchants a 4% transaction fee on each completed in-chat purchase, on top of standard Stripe processing of roughly 2.9% plus $0.30. On a $100 order that came to about $7.20 in combined platform and processing fees. That fee structure was tied to the in-chat Instant Checkout product specifically. Because that surface was shut down in March 2026, we do not assert the 4% fee as currently active on today’s ChatGPT App transactions — its present applicability is unconfirmed, and merchants should verify live fee terms with OpenAI and Stripe directly before modeling cost.

The Agent Payments Protocol was announced September 16, 2025 by Google, with 60+ launch partners including Mastercard, PayPal, American Express, Adyen, Coinbase, Etsy, Intuit, Revolut, Salesforce, UnionPay International, and Worldpay. Where UCP handles the cart and ACP handles execution, AP2 handles trust: it is the answer to the bank’s and the merchant’s most basic question about an agentic purchase — did the human actually authorize this?

AP2 works through three signed mandates that together represent every agent purchase. The Intent Mandate captures the user’s initial request with auditable context. The Cart Mandate is a cryptographically signed record of the exact items and price. The Payment step links the payment method to the verified Cart Mandate. For delegated or unattended purchases — where the agent buys later, on its own — all three are signed upfront with explicit rules around price limits, timing, and conditions.

The governance story matters as much as the technical one. On April 28, 2026, Google donated AP2 to the FIDO Alliance alongside a v0.2 release, transferring stewardship to an industry body to keep the protocol platform-agnostic and community-led; 60 organizations joined the donation. Mastercard co-developed a companion standard, Verifiable Intent, also donated to FIDO — a tamper-proof log of user-authorized agent actions to establish accountability. So while AP2 began as Google’s protocol, the accurate framing today is Google-initiated, now FIDO Alliance-governed.

Below is the comparison no single vendor will publish, because it includes the parts each would rather not lead with: the Instant Checkout shutdown, AP2’s governance handoff to FIDO, and Adyen Agentic as an aggregation layer that sits above all three. Every cell traces to a primary source listed at the foot of this guide.

Four days before this post, on June 16, 2026 in New York, Adyen announced Adyen Agentic — a modular API suite it bills as the universal translator for the next era of commerce. The differentiator is the whole point: it supports UCP, ACP, AP2, and Meta’s AI checkout simultaneously. A merchant integrates once and reaches every emerging agent surface without having to predict which standard wins.

Adyen Agentic has three layers. The Agentic Feed distributes real-time catalog, pricing, and availability. The Agentic Cart orchestrates checkout, tax, fulfillment, and order management — directly addressing the sales-tax and inventory gaps that helped sink Instant Checkout. The Agentic Payments layer handles authentication, tokenization, fraud management, and — importantly for enterprises — merchant-of-record preservation. Our dedicated Adyen Agentic multi-standard integration guide covers the rollout in depth, and for the wider landscape see our AI agent protocol ecosystem map for 2026.

The right standard depends almost entirely on merchant size and the stack you already run. No published guide segments the decision this way with realistic complexity and cost, so here is our framing. The cost figures below are deliberately conservative: we cite only Stripe processing of roughly 2.9% plus $0.30 — about $3.20 on a $100 order — because the 4% ACP platform fee belonged to the now-discontinued Instant Checkout product and its current applicability is unconfirmed.

One nuance the tiers hide: the standards stack, so these are not mutually exclusive. An enterprise using Adyen Agentic is still speaking UCP, ACP, and AP2 under the hood — the adapter just removes the integration burden. A mid-market UCP merchant can still benefit from AP2 mandates for trust on delegated purchases. Think in layers, not in either/or. If you want help mapping your own catalog, checkout, and trust requirements onto these standards, our ecommerce growth engagements start with exactly this kind of readiness assessment, and our AI transformation work covers the agent-integration build itself.

For a merchant investing engineering time, the standard’s governance model is a longevity signal that rarely makes the headlines but should weigh heavily. A protocol controlled by a single company can be redirected — or switched off — at that company’s discretion, as ACP’s Instant Checkout demonstrated. A protocol stewarded by a neutral industry body is harder to abandon and likelier to outlast any one vendor’s strategy shift.

On that axis the three standards rank clearly. AP2 is now governed by the FIDO Alliance — the strongest longevity guarantee of the three, backed by 60 organizations and a payment-network-grade standards process. UCP sits in the middle: a looser open coalition under Apache 2.0 with no single owner, which is durable but less formally governed. ACP is the weakest on this axis — still OpenAI- and Stripe-controlled, and already shown to be redirectable. None of this makes ACP a bad choice for the right merchant; it just means the risk-adjusted view favors the standards a single company cannot unilaterally retire.

Looking forward, the most likely 2026–2027 outcome is not a single winner but a settled division of labor: UCP-style discovery and cart standards, AP2-style verifiable authorization riding underneath, and execution layers that the major AI surfaces each adopt in their own way. The merchants who win will be the ones who treated agentic readiness as a data-and-integration problem early, rather than waiting for the standards war to resolve. Being present and readable beats being perfectly optimized for a protocol that may itself evolve.