x402 Payments

x402 is an open protocol built on the HTTP 402 status code. Any web resource can require a small payment before serving content or data. AI agents pay automatically and retry — no registration, no billing portal, no API key exchange.

Quantoz supports x402 with two kinds of money:

EURO — the e-money token. Balances live in Quantoz managed accounts; settlement is off-chain and instant.
EURQ / USDQ — the EUR and USD stablecoins, live on Algorand, Ethereum and Polygon.

Payment schemes

	`euro` scheme	`exact` (Algorand)	`exact` (Ethereum / Polygon)
Asset	EURO (e-money)	EURQ / USDQ (ASA)	EURQ / USDQ (ERC-20)
Network id	`quantoz:mainnet`	`algorand:mainnet`	`eip155:1`, `eip155:137`
Settlement	Off-chain via Quantoz API	On-chain asset transfer	On-chain via EIP-2612 permit
Agent needs	Quantoz managed account	Whitelisted Algorand wallet holding EURQ/USDQ	EVM wallet holding EURQ/USDQ (no gas needed)
Merchant needs	Quantoz managed account	Whitelisted Algorand address	EVM address
Speed	Instant	~5 sec	~15 sec – 2 min
Facilitator	`mcp.ai.quantozpay.com/x402`	`x402algo.ai.quantozpay.com`	`x402algo.ai.quantozpay.com`

Start with the euro scheme unless you specifically need on-chain settlement. It requires no blockchain knowledge and settles instantly.

How x402 works (all schemes)

1. Agent requests a resource
   GET /premium-resource  (no payment)

2. Server responds 402 with payment options
   { accepts: [{ scheme: "euro", amount: "0.10", ... }, { scheme: "exact", ... }] }

3. Agent pays and retries with proof
   GET /premium-resource
   X-PAYMENT: <base64url proof>

4. Server verifies → serves resource
   200 OK

The withEurPayment() npm package handles steps 3–4 automatically for the euro and EVM paths. See the agent developer guide.

The `euro` scheme

Off-chain payments between Quantoz managed accounts. The facilitator handles payment creation and verification.

{
  "scheme": "euro",
  "network": "quantoz:mainnet",
  "asset": "EURO",
  "amount": "0.10",
  "payTo": "ACC_your_account",
  "paymentRequestCode": "pr_...",
  "expiresAt": 1234567890,
  "facilitator": "https://mcp.ai.quantozpay.com/x402"
}

Replay protection is handled by the facilitator — each payment request code can only be verified once.

The `exact` scheme on Algorand

On-chain EURQ/USDQ transfers on Algorand. The facilitator validates and submits transactions, and sponsors the network fee — the agent’s wallet needs no ALGO.

{
  "scheme": "exact",
  "network": "algorand:mainnet",
  "asset": "2768422954",
  "maxAmountRequired": "500000",
  "payTo": "YOUR_ALGORAND_ADDRESS",
  "maxTimeoutSeconds": 300,
  "facilitator": "https://x402algo.ai.quantozpay.com"
}

Amounts are in atomic units — EURQ and USDQ have 6 decimal places, so €0.50 = 500000.

The e-money and stablecoin rails are separate: EURO cannot be paid to a blockchain address, and EURQ/USDQ cannot be paid to a managed account. Paying this scheme requires an Algorand wallet holding the tokens.

The `exact` scheme on Ethereum and Polygon

EURQ/USDQ are ERC-20 tokens implementing EIP-2612 permit. The agent signs a gasless permit authorizing the facilitator as spender; the facilitator submits permit + transferFrom on-chain and pays the gas. The agent never needs ETH or POL.

{
  "scheme": "exact",
  "network": "eip155:137",
  "asset": "0xd571edb2ef29df10fcd6200fd6d0ed2389983db3",
  "maxAmountRequired": "500000",
  "payTo": "0xYOUR_MERCHANT_ADDRESS",
  "maxTimeoutSeconds": 300,
  "facilitator": "https://x402algo.ai.quantozpay.com",
  "extra": { "symbol": "EURQ", "spender": "0xFACILITATOR_ADDRESS" }
}

The extra.spender field tells the agent which address to authorize in the permit — it must be the facilitator’s hot-wallet address. Note this differs from the EIP-3009 flow used by e.g. USDC: EURQ/USDQ do not implement EIP-3009, so stock x402 EVM clients need the Quantoz client package (or the permit recipe in the agent guide).

Token deployments

Token	Network	Identifier	Decimals
EURQ	Algorand	ASA `2768422954`	6
USDQ	Algorand	ASA `2768603795`	6
EURQ	Ethereum (`eip155:1`)	`0x8dF723295214Ea6f21026eeEb4382d475f146F9f`	6
USDQ	Ethereum (`eip155:1`)	`0xc83e27f270cce0A3A3A29521173a83F402c1768b`	6
EURQ	Polygon (`eip155:137`)	`0xd571edb2ef29df10fcd6200fd6d0ed2389983db3`	6
USDQ	Polygon (`eip155:137`)	`0xb291996477504506bf5f583102b5b5ea5d1e40e0`	6

Next steps

Merchants — Protect a resource with x402
AI developers — Pay x402 resources from an agent