Payment Schemes

Exact and Upto payment schemes for x402

Payment Schemes

x402 supports two payment schemes that cover different pricing models.

Exact Scheme

The payer authorizes a fixed amount that is settled in full.

Aspect	Detail
Amount	Fixed at authorization time
Settlement	Full authorized amount
Transfer methods	ERC-3009, Permit2, Solana
Use cases	Fixed-price APIs, one-time purchases

{
  "scheme": "exact",
  "amount": "1000000"
}

The on-chain transfer moves exactly 1,000,000 units (1 USDC with 6 decimals).

Upto Scheme

The payer authorizes a maximum amount, and the seller settles only the actual usage.

Aspect	Detail
Amount	Maximum authorized
Settlement	Any amount up to the maximum
Transfer methods	Permit2 only
Use cases	Usage-based APIs, AI token billing, metered resources

{
  "scheme": "upto",
  "amount": "5000000",
  "extra": {
    "assetTransferMethod": "permit2",
    "settlementAmount": "300000"
  }
}

The payer authorized up to 5 USDC, but the seller only settles 0.30 USDC.

Why Upto is Permit2 Only

ERC-3009 transferWithAuthorization binds the value directly in the signature hash. The on-chain contract verifies that the transfer amount matches the signed value exactly — partial settlement is rejected.

Permit2’s permitWitnessTransferFrom separates the authorized maximum (in the permit) from the requested transfer amount (in transferDetails). This allows the Facilitator to settle any amount up to the authorized limit.

Zero Settlement

If settlementAmount is "0":

• No on-chain transaction is executed
• The payer’s locked balance is fully released
• The Facilitator returns success: true immediately

This is useful when the buyer’s request consumed no billable resources.

Settlement Amount Validation

Condition	Result
settlementAmount <= amount	Accepted
settlementAmount > amount	Rejected with error
settlementAmount == "0"	No on-chain tx, immediate success
settlementAmount missing (upto)	Rejected — required for upto

Choosing a Scheme

Scenario	Recommended Scheme
Fixed-price API endpoint	exact
Pay-per-token AI inference	upto
Subscription-like recurring payment	exact
Metered compute / bandwidth	upto
One-time content purchase	exact