# x402 payment protocol

> How the HTTP 402 Payment Required status code enables on-chain micropayments between agents via the Agent Gateway.
x402 defines a machine-readable payment handshake over standard HTTP. An agent and a server can negotiate, settle, and verify an on-chain payment without any out-of-band coordination.

## How x402 works

x402 extends HTTP with two message components:

HTTP 402 Payment Required response: When a resource requires payment, the server returns a 402 response with a machine-readable payment payload. The payload specifies the amount, accepted token or currency, the payment network, and the destination address.

PAYMENT-REQUIRED response header: When a resource requires payment, the gateway returns a HTTP 402 response with the payment requirement in the PAYMENT-REQUIRED header. The client makes the required on-chain payment and attaches the signed payment proof in the payment proof header on a retry of the original request.

```mermaid
sequenceDiagram
    participant A as Agent
    participant G as Gateway
    participant B as Blockchain

    A->>G: GET /tool
    G-->>A: 402  PAYMENT-REQUIRED: {amount, network, address}
    A->>B: Pay on-chain
    B-->>A: Confirmed
    A->>G: GET /tool + payment proof header
    G-->>A: 200 + response
```

The protocol is transport-agnostic and works over any standard HTTP endpoint. No dedicated payment intermediary is required between the two parties.

## Networks

x402 payments settle on-chain. The Agent Gateway supports the following networks:

- Base: the default network for x402 payments in the gateway.

- Ethereum: Ethereum mainnet and standard testnets.

- Solana: Solana mainnet.

Network support and accepted tokens are configured per gateway deployment in x402.json.

## Related

- [AP2 protocol](/products/affinidi-trust-fabric/agent-gateway/concepts/protocols/ap2.md): Agent Payments Protocol: the surface-level payment protocol for agent-initiated commerce flows.

- [MCP protocol](/products/affinidi-trust-fabric/agent-gateway/concepts/protocols/mcp.md): Model Context Protocol: how the gateway serves and proxies MCP tool servers.