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0xGasless Developer Documentation

x402 Protocol

x402 is an open protocol that turns the long-dormant HTTP status code 402 Payment Required into a real, usable handshake for crypto micropayments. A client calls an endpoint, the server responds with 402 and a price, the client signs a stablecoin transfer authorisation, and the server settles it on-chain. Total round-trip: under a second.

This page is the protocol-level documentation. If you’re looking for the TypeScript client that sits on top of it, see Agent SDK.

The handshake

Section titled “The handshake”
client                          server                          chain
  │                                │                              │
  │── GET /premium-data ──────────►│                              │
  │ ◄── 402 Payment Required ─────│  { price: "0.10 USDC",       │
  │      + payment requirements    │    network: "avalanche",     │
  │                                │    recipient: "0x..." }      │
  │                                │                              │
  │── sign EIP-3009 authorization ─┐                              │
  │                                │                              │
  │── GET /premium-data ──────────►│                              │
  │   X-Payment: <signed payload>  │  forward to facilitator      │
  │                                │── POST /settle ─────────────►│
  │                                │                              │ tx submitted
  │                                │ ◄── tx hash ─────────────────│   gas paid by
  │ ◄── 200 OK ────────────────────│      X-Payment-Response: …   │   facilitator
  │      + the actual response     │                              │

The protocol is chain- and token-agnostic, but in practice today most x402 traffic is USDC on EVM chains using EIP-3009 transferWithAuthorization. That’s the canonical scheme 0xGasless implements.

Why it’s interesting

Section titled “Why it’s interesting”
  • No accounts, no API keys for payment. Anyone with a stablecoin balance can pay any x402 server. No subscription, no signup.
  • No invoices, no chargebacks. Settled on-chain in seconds.
  • Tiny amounts work. Sub-cent payments are economical because the facilitator pays gas, not the payer.
  • Composable. Bots, agents, browsers, and humans all use the same flow.

This is the right shape for an agentic web: when an autonomous AI agent needs to call an API, it shouldn’t have to pre-arrange billing with the API provider. It just pays per call.

What 0xGasless provides

Section titled “What 0xGasless provides”
PieceURL
Public facilitatorhttps://x402.0xgasless.com
Agent SDK (TypeScript)@0xgasless/agent
Dashboarddashboard.0xgasless.com

You can use the facilitator without signing up for anything:

  • Anyone can POST /verify to check a signature.
  • Anyone can POST /settle to submit a payment on-chain. The facilitator pays the gas.
  • No API key required for the public endpoints.

You need an API key only if you want server-managed agent wallets (the Agent SDK feature). For direct x402 use with your own EOA, just hit the facilitator.

Quick start by role

Section titled “Quick start by role”
You areRead
A merchant / API provider — you want to charge per callAccept payments
A client — you want to pay an x402 endpointPay endpoints
A self-hoster — you want to run your own facilitatorSelf-hosting
Building an autonomous agent that paysAgent SDK Quickstart

Reference

Section titled “Reference”
  • Facilitator API — full REST contract for /verify, /settle, /list, /health.
  • Supported chains — chain IDs, USDC addresses, relayer contracts.
  • Self-hosting — run the open-source facilitator on your own infrastructure.

Standards

Section titled “Standards”
  • EIP-3009 — transferWithAuthorization. USDC implements this natively.
  • x402 spec — the canonical protocol definition maintained by Coinbase.