x402 Protocol Overview

What is x402?

x402 is a payment protocol built on HTTP 402 (Payment Required). When a client requests a paid resource, the server responds with 402 and includes payment requirements in the X-Payment-Required header. The client constructs a payment proof, attaches it as the X-Payment header, and retries the request.

FluxA extends the x402 protocol to support three payment schemes — EVM (USDC on Base), Credits, and XRP — enabling AI agents to pay for APIs, MCP tools, and other resources programmatically.

Payment Schemes

Scheme	Networks	Currency	Amount Unit	Signature	x402Version
EVM	base (mainnet), base-sepolia (testnet)	USDC	6 decimals (1000000 = 1 USDC)	EIP-712 TransferWithAuthorization	1, 2
Credits	fluxa-monetize-credits	FLUXA_MONETIZE_CREDITS	2 decimals (100 = 1 Credit)	EIP-712 CreditsMandateSpend	1
XRP	xrpl-mainnet	XRP	drops (1000000 = 1 XRP)	Signed XRPL Payment transaction	1

All schemes use scheme: "exact".

x402Version — protocol versions

The EVM scheme accepts both protocol versions; new integrations should prefer 2.

• x402Version: 1 — original payload. The network field uses the short chain name (e.g. "base").
• x402Version: 2 — same payload structure with the network field upgraded to a CAIP-2 identifier (e.g. "eip155:8453" for Base mainnet, "eip155:84532" for Base Sepolia). All other fields are unchanged; existing 1 clients continue to work.

The Credits and XRP schemes currently use x402Version: 1 only.

How It Works

1. Client sends a request to a paid resource
2. Server responds with HTTP 402 and X-Payment-Required header containing accepted payment schemes
3. Client selects a scheme, constructs a signed payment proof
4. Client retries the request with the X-Payment header
5. Server verifies and settles the payment, then delivers the resource

See Payment Flow for a detailed walkthrough.

Facilitators

Each scheme has a facilitator — a service that verifies payment proofs and executes settlement:

Facilitator	Schemes	Networks	x402Version	Settlement	Base URL
Base USDC Facilitator	EVM (USDC)	base (mainnet), base-sepolia (testnet)	1, 2	On-chain transferWithAuthorization on Base	https://facilitator.fluxapay.xyz/
Credits Facilitator	Credits	fluxa-monetize-credits	1	Off-chain balance debit with immutable ledger	https://walletapi.fluxapay-qa.xyz/api/facilitator
XRP Facilitator	XRP	xrpl-mainnet	1	On-chain XRPL Payment transaction	https://walletapi.fluxapay-qa.xyz/api/facilitator

The Base USDC Facilitator at facilitator.fluxapay.xyz is FluxA's production x402 settlement endpoint for USDC on Base. It is registered on x402scan and is currently the most active non-Coinbase x402 facilitator on Base by daily transaction count. It covers gas on the user's behalf and submits EIP-3009 transferWithAuthorization transactions, so funds settle directly user → user.

Credits and XRP share the same multi-asset facilitator service.

Next Steps

• Payment Flow — end-to-end walkthrough
• EVM Scheme — USDC on Base payment details
• Credits Scheme — FluxA Credits payment details
• XRP Scheme — XRP Ledger payment details