> ## Documentation Index
> Fetch the complete documentation index at: https://darwin.so/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Agentic payment protocols

> Understand how Darwin Pay adapts supported agent-commerce payment protocols.

Darwin Pay normalizes supported commerce protocols into the same bound payment interaction. The protocol describes how the merchant, agent, and provider coordinate; it never replaces user authority.

| Protocol | What Darwin adapts                                                                                                                           |
| -------- | -------------------------------------------------------------------------------------------------------------------------------------------- |
| UCP      | Capability discovery and interoperable commerce primitives between platforms, businesses, and payment or credential providers.               |
| ACP      | A seller-owned checkout session in which the seller calculates totals, manages inventory, processes payment, and remains merchant of record. |
| MPP      | An HTTP `402 Payment Required` exchange that lets a client satisfy a payment challenge and retry with a scoped credential.                   |

## Preserve the protocol boundary

Darwin selects only a protocol advertised by the exact capability revision and payment interaction. It retains the merchant's complete checkout state, handler identifiers, and confirmation result without translating free-form AI text into a payment proof.

## Keep approval separate

A protocol can carry payment credentials or cryptographic authorization, but Darwin still enforces its own interaction binding, expected Action revision, expiry, and user decision. Changed terms require a new review.

For an x402-specific exchange, see [x402](/docs/act/payments/x402).