TL;DR
- ACP (Agentic Commerce Protocol) is an open standard, co-developed by OpenAI and Stripe, that defines how AI agents discover products, run checkout sessions, and complete delegated payments with merchants.
- Instant Checkout ("Buy it in ChatGPT") launched in September 2025 with Etsy live day one; by March 2026 OpenAI deprioritized standalone in-chat checkout in favor of product discovery + merchant-owned checkout—the ACP protocol itself continues on agenticcommerce.dev.
- Stable spec 2026-04-17 adds cart, product feed, orders, authentication, and MCP bindings; merchants implement five
/checkout_sessionsREST endpoints and accept Shared Payment Tokens (SPT) atcomplete. - Shopify announced 1M+ merchants in the pipeline, but only roughly 12–30 Shopify brands ever went live for in-chat checkout before the pivot—feed syndication (Shopify/Etsy auto-integrated as of mid-2026) matters more than raw pipeline size.
- ACP is retail agent commerce—distinct from x402/MPP API micropayments and from UCP's Google-led full-journey model.
What Is ACP? — A Working Definition
The Agentic Commerce Protocol (ACP) is an interaction model and open standard for connecting buyers, their AI agents, and businesses to complete purchases seamlessly. OpenAI and Stripe announced ACP alongside Instant Checkout in September 2025, describing it as "the language that lets AI agents and businesses work together to complete a purchase for a user."
ACP specifies merchant-side checkout APIs, delegated payment mechanics, product feeds, and order notification webhooks—not LLM prompts or agent reasoning. The agent surface (ChatGPT today, potentially others) calls a merchant's ACP-compliant gateway; the merchant remains seller of record with authoritative cart state, tax, and inventory.
The canonical spec lives at agenticcommerce.dev with OpenAPI and JSON Schema artifacts versioned by date. Stable snapshot 2026-04-17 covers checkout, cart, product feed, orders, delegate authentication, and MCP bindings. The protocol is marked beta—implement against published specs, not assumptions about universal consumer adoption.
How ACP Works: Checkout Sessions and Delegated Payment
ACP's merchant integration centers on checkout sessions—stateful objects representing a cart negotiation between agent and merchant.
| Method | Endpoint | Purpose |
|---|---|---|
| POST | /checkout_sessions | Create session |
| GET | /checkout_sessions/{id} | Retrieve authoritative state |
| POST | /checkout_sessions/{id} | Update line items, buyer info |
| POST | /checkout_sessions/{id}/complete | Charge via delegated payment token |
| POST | /checkout_sessions/{id}/cancel | Cancel session |
Every request requires Bearer authentication, an API-Version header, and Idempotency-Key on POSTs. Responses return session objects with integer-cent totals, status, policy links, and messages—merchants own price integrity; agents cannot unilaterally discount.
The complete endpoint is the payment moment. The platform sends payment_data containing a Shared Payment Token (SPT)—a bounded credential (max_amount, currency, merchant_id, expires_at, reason: one_time) minted by the buyer's wallet or Stripe—plus optional billing address. The merchant validates the session, applies tax from buyer location, charges the token, marks the session completed, and fires order webhooks.
Declines map to structured errors: payment_declined, requires_3ds, invalid. The merchant never sees the buyer's full PAN; the agent never holds spendable credentials outside the token envelope.
Instant Checkout, the February 2026 Push, and the March Pivot
On September 29, 2025, OpenAI and Stripe launched Instant Checkout with U.S. Etsy sellers live the same day. Shopify announced a one-million-plus merchant pipeline (Glossier, SKIMS, Spanx, Vuori among cited brands), but operational reality lagged marketing scale: industry reporting placed roughly 12–30 Shopify merchants live for in-chat checkout before OpenAI changed course.
On February 16, 2026, OpenAI expanded "Buy it in ChatGPT" to Free, Plus, and Pro U.S. users—widening demand-side access while checkout remained single-item with multi-item carts still on the roadmap. PayPal and Walmart partnership announcements reinforced the agentic commerce narrative through early 2026.
By March 2026, OpenAI deprioritized standalone Instant Checkout, citing merchant-side complexity—inventory accuracy, tax remittance, and catalog freshness—as bottlenecks harder than demand generation. The product shifted to product discovery in ChatGPT with buyers completing purchases on merchant-owned checkout (in-app browser or merchant sites). Per OpenAI's merchant page as of mid-2026, Shopify and Etsy catalogs are auto-integrated for discovery; implementing ACP alone does not guarantee ChatGPT placement without platform approval.
Critical distinction: the ACP protocol did not sunset—checkout specs, delegate payment, and feed schemas continue on GitHub. Merchants building for agent commerce should plan for feed quality + optional checkout APIs, not assume a permanent in-chat "Buy" button for every brand.
Stripe's Agentic Commerce Suite (ACS) remains the fastest integration path for Stripe merchants, with platform partners including Wix, WooCommerce, BigCommerce, Squarespace, and commercetools.
Shared Payment Tokens and the Security Model
ACP's delegated payment model mirrors patterns across the agent stack:
- Humans authorize spend in a wallet or Stripe surface
- Platforms mint SPTs scoped to merchant and amount
- Merchants charge SPTs through familiar payment processors
- Agents orchestrate checkout but do not custody payment credentials
This aligns with MPP SPT demonstrations at Stripe Sessions 2026 and with the guardrails philosophy in agent payments: policy-time authorization, machine-time execution.
ACP also specifies product feeds (JSONL/CSV) so agents can discover catalog data, plus webhook events (checkout.session.created, checkout.session.completed, etc.) for order sync. Reference gateway implementations (for example, community ACP Checkout Gateway projects) show how to deploy compliant HTTP services on serverless infrastructure.
ACP vs UCP: Two Agent Checkout Camps
Two open protocols dominate retail agent checkout in 2026, from overlapping but distinct coalitions:
| Dimension | ACP | UCP |
|---|---|---|
| Maintainers | OpenAI, Stripe | Google, Shopify |
| Primary surfaces | ChatGPT discovery (+ historic Instant Checkout) | Google AI Mode, Gemini, Copilot |
| Scope | Checkout sessions + delegated payment | Full commerce journey (discovery, loyalty, post-purchase) |
| Transport | REST (OpenAPI) | REST, MCP, A2A, GraphQL |
| Payment handlers | Stripe SPTs, expanding | Negotiated handlers (Google Pay, Shop Pay, etc.) |
Both are pilot-stage as of mid-2026. Merchant groundwork overlaps: clean catalog data, accurate inventory, fresh pricing, webhook reliability. Brands on Shopify may implement UCP for Google and ACP for ChatGPT—not necessarily one exclusive protocol.
ACP does not replace your existing storefront; it adds an agent-addressable checkout API alongside human web checkout.
ACP vs x402, MPP, and AP2
- ACP vs x402/MPP: ACP is retail cart checkout with line items and orders. x402/MPP gate individual HTTP resources—API calls, files, tool invocations. A SaaS API company bills agents via MPP; a DTC brand sells via ACP.
- ACP vs AP2: AP2 provides mandate-based authorization evidence; ACP provides checkout session APIs. Future compositions may attach AP2 mandates to ACP
completecalls for audit-heavy enterprise retail.
If your product is B2B subscription software, ACP is rarely the first integration—unless you distribute through agent marketplaces selling packaged offerings. Your core billing remains subscriptions, smart routing, and human checkout; ACP is a new demand channel, not a billing replacement.
What ACP Means for Merchants and SaaS Teams
Retail and DTC brands on Stripe or Shopify should monitor ACP/UCP rollouts as discovery and conversion channels—analogous to early mobile commerce, but with agent-mediated intent.
Marketplace operators may expose ACP gateways so third-party sellers accept agent checkout without each seller building custom integrations.
SaaS and API companies should prioritize MPP or x402 for metered access; ACP only matters if you sell sku-shaped products through ChatGPT-class surfaces.
Compliance and ops teams should note: seller of record stays with the merchant; platform fees and dispute paths follow Stripe/OpenAI published terms—verify current fee schedules and pilot eligibility rather than assuming GA economics.
Conclusion
ACP is the open commerce protocol for ChatGPT-era agent retail—checkout APIs, delegated payment tokens, and product feeds that let merchants keep backend systems while agents handle discovery and (where enabled) purchase orchestration. After the March 2026 product pivot, the durable merchant play is feed quality and syndication, with checkout APIs ready when platforms re-enable in-chat completion.
Build ACP feeds if ChatGPT discovery is a channel you can serve with accurate catalog data. Build MPP/x402 if your buyers are agents consuming APIs. Build UCP alongside ACP if Google and Copilot surfaces matter too. The agent economy is multi-protocol by design—not a single checkout winner.