These three come up in every agent-commerce conversation, usually as if you have to pick one. You do not. They sit at different layers of the same stack: MCP is how an agent calls a tool, ACP is a checkout standard, and UCP covers the whole journey from discovery to checkout. Here is what each one actually standardizes, and how they compose in a single purchase.
The agent-commerce stack has more than three parts. MCP, ACP, and UCP are the ones people compare, but they only make sense next to the discovery, identity, and payment layers they sit on.
| Piece | Layer | What it standardizes | Where it fits |
|---|---|---|---|
| agents.txt | Discovery | Where a store's agent surface lives | The doorway |
| Know your agent | Identity | Who the agent is and who it acts for | The handshake |
| MCP | Transport | How an agent calls typed tools | The pipe |
| ACP | Checkout | Product feed, checkout API, payment token | The transaction |
| UCP | Full journey | Discovery through checkout, end to end | The whole flow |
| x402 | Payment | HTTP-native stablecoin settlement | The money |
Read down the table and the "vs" dissolves. MCP, ACP, and UCP never compete for the same job.
MCP, the Model Context Protocol, is how an agent calls a typed tool. It standardizes the request and response shape between an agent and the tools it can use, so a model can materialize a client and call a capability without a bespoke integration.
MCP is not a commerce protocol. It carries agent-commerce traffic the way HTTP carries a checkout, but it says nothing about discovery, identity, checkout, or settlement. A store can expose a commerce capability over an MCP endpoint, and the actual purchase still rides a checkout standard on top.
ACP, the Agentic Commerce Protocol, standardizes the transaction: a product feed the agent can read, a checkout API the agent can call, and a payment token the agent presents. Its focus is the checkout itself, the moment an agent turns an intent into an order.
ACP defines the checkout shape but leaves the rail open underneath. The payment token can settle through a card processor or another rail; ACP is about the checkout contract, not about how the money ultimately clears.
UCP, the Universal Commerce Protocol, covers the whole journey, from discovery through checkout, as one standard. Where ACP focuses on the checkout, UCP describes the full flow an agent walks: find the product, get a quote, reserve it, and settle.
Those four verbs, search, quote, reserve, and settle, are the primitive UCP composes. A store that speaks UCP exposes a checkout session an agent can drive end to end. It is the surface Facet advertises by default, and it settles on whatever payment rail the store and agent support.
Because they are layers, a single agent purchase can touch several at once. A concrete path:
No single protocol does all of that, and none of them has to. The store supports the layers its buyers use, and they stack. This is the layer Facet installs above an existing store: it rides the open rails rather than replacing them, and it is non-custodial by design, so funds settle to the store's own or connected account, never a Facet-held balance.
Facet installs above your existing store and speaks the open protocols for you: discovery, identity, checkout, and settlement, on any platform. About five minutes to mount.
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