x402

Overview

x402 revives the long-reserved 402 Payment Required HTTP status code. When an agent hits a priced endpoint without a payment, the server responds with a 402 carrying a payment challenge. The agent signs a USDC transfer, replays the request with a Payment header, and receives the response.

Custena speaks x402 out of the box — both as a seller (accepting settlements) and as a buyer (signing and retrying for your agent).

Flow

GET /tools/search?q=hello HTTP/1.1
Host: seller.example.com

HTTP/1.1 402 Payment Required
X-Payment-Required: {"amount":"0.01","currency":"USDC","network":"base-sepolia","payto":"0xabc..."}

Agent signs and retries:

GET /tools/search?q=hello HTTP/1.1
Host: seller.example.com
X-Payment: eyJ0eXAiOiJKV1Qi...

HTTP/1.1 200 OK
Content-Type: application/json

{ "results": [...] }

Settlement

• Network: Base mainnet (production) / Base Sepolia (test)
• Asset: USDC
• Facilitator: Custena's x402 facilitator verifies the signed transfer and releases the response. Sellers never need to run a node.

When to use it

• Agent already holds USDC in a managed or self-custodial wallet
• You want cryptographic proof of payment stamped into the request
• You need per-call settlement below card-fee viability