ACP defines how a shopping agent finds products — via structured merchant feeds — and shortlists them against a buyer's constraints. The spec (still in beta, date-versioned; the 2026-04-17 snapshot added cart, feed, orders, auth and MCP compatibility) is developed openly on GitHub, with PayPal joining Stripe as a payment provider.
Know the history: OpenAI's in-chat Instant Checkout launched September 2025 but was retired in March 2026 after weak merchant uptake, and ChatGPT shopping pivoted discovery-first — agents surface your products, then send buyers to your checkout. That makes structured feeds and machine-readable product data, not a checkout integration, the thing that decides whether you're surfaced. The merchant stays seller of record and owns fulfillment and the customer relationship.
Shopify merchants are largely auto-enrolled. Others should keep a clean structured product feed, ship schema.org Product/Offer markup, and allow OAI-SearchBot in robots.txt.