Agentic Commerce Protocols Explained: ACP vs UCP vs MCP

ACP (Agentic Commerce Protocol) and UCP (Universal Commerce Protocol) are commerce protocols: they define how an AI agent and a merchant build a checkout, exchange buyer and fulfillment details, pass payment credentials and track orders. ACP is maintained by OpenAI and Stripe; UCP is co-developed by Google, Shopify and other commerce companies. MCP (Model Context Protocol) is not a commerce protocol. It is a general way for AI applications to call tools and read data, and commerce protocols can run over it.

They are not interchangeable. ACP and UCP overlap in purpose and come from different ecosystems; MCP is a lower layer both can use. Merchants mostly meet them through their commerce platform rather than by implementing them by hand. For the business context, see agentic commerce.

ACP is an Apache 2.0 open specification maintained by OpenAI and Stripe (ACP repository). Its core is a checkout API the merchant implements and the agent calls: create a checkout session, update it (items, address, fulfillment option), retrieve it, complete it with a payment credential and cancel it. Every POST must carry an Idempotency-Key; requests also carry an API-Version and authorization, with optional request signatures. The merchant stays the system of record and the merchant of record: OpenAI states it is not the merchant of record in ACP.

ACP's Delegated Payment Spec lets the agent platform obtain a one-time payment token, limited by a maximum amount and an expiry, from the merchant's payment provider; Stripe's Shared Payment Token was the first compatible implementation. Later releases added capability negotiation, extensions, discounts and payment handlers, and the 2026-04-17 release added cart, feed, orders, authentication and MCP. In ChatGPT itself, OpenAI scaled back native Instant Checkout in March 2026 and shifted shopping towards discovery, with purchases completed in merchant apps or on merchant sites (CNBC); ACP development continues.

UCP is an Apache 2.0 open specification published at ucp.dev. A business publishes a profile at /.well-known/ucp listing its supported version, services, capabilities and payment handlers. Capabilities use reverse-domain names: dev.ucp.shopping.checkout, dev.ucp.shopping.cart, dev.ucp.shopping.order and dev.ucp.common.identity_linking, with extensions such as fulfillment and discounts. The platform (the agent side) sends its own profile, both sides negotiate the capabilities and version they share, and the conversation runs over one of several transports: REST, MCP, A2A or an embedded binding.

UCP checkout has explicit states (incomplete, requires_escalation, ready_for_complete, complete_in_progress, completed, canceled), and when a checkout needs something the agent cannot supply, the business returns a continue_url so the buyer finishes on the merchant's site. Webhooks from business to platform must be signed with HTTP Message Signatures. Payment handlers are modular, including wallets and an AP2 mandates extension for autonomous agents. Google uses UCP for checkout on eligible listings in AI Mode in Search and the Gemini app for selected merchants, and Shopify's agent tooling supports it.

MCP is a general protocol that lets AI applications discover and call tools, read resources and receive prompts from servers. It knows nothing about carts or payments. In commerce it plays two roles: a tool layer, where a store or platform exposes product search, availability, cart or order status as MCP tools, and a transport, as in UCP's MCP binding. MCP Apps, its official UI extension, also lets a store show interactive product or checkout views inside assistants that support it. See the Model Context Protocol guide and AI assistant app UX.

A realistic setup combines them. A merchant's product data reaches agents through feeds and catalogs (AI product feeds). An agent platform that speaks ACP calls the merchant's ACP checkout endpoints; one that speaks UCP reads the merchant's /.well-known/ucp profile and uses the negotiated capabilities, possibly over MCP. Payment credentials arrive through the protocol's payment mechanism and are processed by the merchant's payment provider (see AI commerce payments). Order updates flow back by webhook. The merchant's own systems remain the source of truth for price, tax, inventory and order state throughout. For the full flow, see agentic checkout.

'MCP is a commerce protocol.' It is a general tool protocol that commerce protocols can use. 'ACP and UCP are the same thing.' They overlap but differ in structure, governance and ecosystem. 'The AI company becomes the seller.' In both ACP and UCP the merchant remains the merchant of record. 'One protocol will win soon.' Possibly, but planning for one winner is risky; abstract your checkout logic instead. 'Implementing a protocol guarantees visibility.' Eligibility, product data quality and the agent platform's own ranking still decide what shoppers see.

ACP and UCP are the two main open commerce protocols for agent-to-merchant checkout and orders, from different ecosystems; MCP is the general tool and transport layer underneath many AI integrations, including commerce. Understand each at its own layer, keep your merchant systems authoritative, build capabilities behind your own API and let your platform carry most of the protocol work. The full architecture is in the agentic commerce stack.