How x402 Works

Overview

x402 is an HTTP payment protocol for machine-to-machine transactions. Named after HTTP status code 402 (Payment Required), it enables instant micropayments without accounts, subscriptions, or API keys.

Protocol Flow

1. Client sends GET /price/ETH
2. Server returns 402 with PAYMENT-REQUIRED header
3. Client decodes: pay 0.01 USDC to receiver on Base
4. Client signs EIP-712 typed data with wallet key
5. Client retries GET with X-PAYMENT header (signed proof)
6. Server verifies signature via facilitator
7. Server returns 200 + data

Why x402?

No API keys or signup forms
Native crypto payment (USDC on Base)
Sub-second settlement
Works with any HTTP client
Open source protocol
Built for AI agents

Key Concepts

Payment Requirements (Server -> Client)

When you request a paid endpoint, the server responds with 402 and includes payment details:

{
  "version": "1.0",
  "amount": "10000",
  "token": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
  "network": "base",
  "recipient": "0x4SEC_RECEIVER_ADDRESS",
  "nonce": "unique_nonce_abc123"
}

Amount is in USDC base units (6 decimals): 10000 = $0.01

Payment Proof (Client -> Server)

The client signs the payment and retries with proof:

Headers:
X-PAYMENT: {
  "signature": "0x_signed_data...",
  "nonce": "unique_nonce_abc123",
  "amount": "10000",
  "payer": "0x_your_wallet_address"
}

Facilitator

The facilitator is a trusted service that verifies payments on-chain. Default: https://x402.org/facilitator

You can also self-host a facilitator for lower latency.

Comparison: x402 vs Traditional APIs

Feature	Traditional API	x402
Authentication	API key	Wallet signature
Payment	Credit card, subscription	USDC per request
Signup	Email, form	None
Latency	Auth check on every request	One-time payment
AI-friendly	Requires key management	Autonomous payment
Global access	Region restrictions	Permissionless

Resources

x402 Protocol Spec: https://x402.org/spec
x402 Client SDK: https://github.com/x402/client
Facilitator: https://x402.org/facilitator

How x402 Works ​

Overview ​

Protocol Flow ​

Why x402? ​

Key Concepts ​

Payment Requirements (Server -> Client) ​

Payment Proof (Client -> Server) ​

Facilitator ​

Comparison: x402 vs Traditional APIs ​

Resources ​