# x402, AP2 and send21: AI agent payments explained

Three different answers to "how does an AI agent pay?". x402 lets the agent pay for a request itself. AP2 lets an agent buy on your behalf with a signed mandate. send21 lets the agent prepare the payment, and a person or the operator's own wallet signs it.

## x402: the agent pays per request

x402 is an open standard built on the HTTP status code 402 Payment Required. A server that wants payment answers a request with 402; the client, for example an AI agent, pays and retries. Payments are typically in stablecoins, on EVM chains, Solana and more. It was introduced by Coinbase, and an x402 Foundation at the Linux Foundation now stewards it. Details: [x402.org](https://www.x402.org/).

It fits machine-to-machine payments: an agent paying for an API call, a dataset or a page. The agent needs a wallet with funds it can spend.

## AP2: the agent buys with your mandate

AP2, the Agent Payments Protocol, is an open protocol published by Google under the Apache 2.0 license, with standardization continuing in FIDO Alliance working groups. It uses verifiable digital credentials: a Checkout Mandate shared with the merchant and a Payment Mandate shared with the payment side, so everyone can check what the person authorized. The first version covers card payments; e-wallets, bank transfers and digital currencies are on its roadmap. Details: [ap2-protocol.org](https://ap2-protocol.org/).

It fits agents shopping at merchants with cards, inside the existing card networks.

## send21: the agent prepares, a person signs

With send21 the agent never holds money. It creates a crypto pay link or payment draft over the [MCP server](https://send21.io/docs/mcp) or the API, and the payment is signed in a wallet the agent does not control. The agent can then track it and react to a signed webhook. Pay links only go to receiving addresses the owner approved.

It fits agents that bill people (invoices, checkouts, pay links), agents that hit their budget and hand a payment to a human, and teams that do not want an agent holding a funded wallet. send21 does not implement x402 or AP2 today.

## Side by side

|  | x402 | AP2 | send21 |
| --- | --- | --- | --- |
| Who pays | The agent, from its own wallet | The person, through their payment credentials and mandate | A person or the operator's wallet signs |
| What the agent holds | A funded wallet | Mandates, not the money | A scoped API key that cannot move funds |
| Typical use | Pay per API call or resource | Agent shopping at merchants | Billing people, pay links, budget hand-offs |
| Money | Stablecoins on several chains | Cards first | BTC, Lightning, USDC, USDT, EURC, SOL, ETH |
| Human approval | Optional, set by the agent's owner | Through the signed mandate | Always: a person signs each payment |

## Questions

### Can an AI agent pay with crypto?

Yes, in two ways: with its own funded wallet (as with x402), or by preparing a payment that a person signs (as with send21). The second keeps spending under human control.

### Does send21 support x402?

Not today. send21 is for agents that prepare payments for a person to sign, and for getting paid into your own wallet.

### Is x402 or AP2 better?

They solve different problems: x402 is pay-per-request between machines, AP2 is agent shopping with card payment credentials. Many agents will use more than one.

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Source: https://send21.io/x402-ap2-agent-payments