Exact Scheme

One-shot payment. The client builds a signed transfer PTB, the facilitator verifies and broadcasts it atomically. One transaction per request.

This is the x402-compatible baseline. An x402 client can talk to an s402 server using this scheme with zero modifications.

When to Use

• One-off API calls
• Paying multiple different providers
• When you need x402 interoperability
• Low-frequency requests where per-call gas is acceptable
• Prototyping (simplest scheme to start with)

How It Works

Client                    Server                  Facilitator
  |--- GET /api/data ------->|                         |
  |<-- 402 + requirements ---|                         |
  |                          |                         |
  |  (build PTB, sign)       |                         |
  |--- GET + X-PAYMENT ----->|--- verify + settle ---->|
  |                          |<--- { success, tx } ----|
  |<-- 200 + data -----------|                         |

Step by step:

1. Client requests a paid resource
2. Server responds with HTTP 402 + payment requirements
3. Client builds a transfer PTB (SUI coin split + transfer), signs it
4. Client retries the request with the signed TX in the X-PAYMENT header
5. Server forwards to facilitator, which verifies and broadcasts
6. Facilitator returns the transaction digest
7. Server delivers the content with a payment-response header

Economics

	Cost
Per API call	~$0.007 (one TX on Sui)
1,000 calls	~$7.00
Latency	~400ms (on-chain settlement)

For high-frequency usage (more than ~10 calls per session), the Prepaid scheme is 500x cheaper because it amortizes gas across a batch.

Trust Model

• Atomic settlement: The PTB either succeeds entirely or fails entirely. No partial state.
• No state between calls: Each request is independent. No deposits, no meters, no streams.
• Facilitator must wait for finality: The facilitator calls waitForTransaction() before returning success. Without this, the server could grant access for a transaction that gets reverted.

Requirements

const requirements: s402PaymentRequirements = {
  s402Version: '1',
  accepts: ['exact'],
  network: 'sui:mainnet',
  asset: '0x2::sui::SUI',
  amount: '1000000', // 0.001 SUI
  payTo: '0xrecipient...',
  expiresAt: Date.now() + 5 * 60 * 1000, // 5-minute window
};

Payload

const payload: s402ExactPayload = {
  s402Version: '1',
  scheme: 'exact',
  payload: {
    transaction: '<base64 signed PTB bytes>',
    signature: '<base64 signature>',
  },
};

x402 Compatibility

Exact is the bridge between s402 and x402. The wire format is identical:

• Same HTTP headers (payment-required, X-PAYMENT, payment-response)
• Same base64 JSON encoding
• The detectProtocol() function distinguishes s402 from x402 by checking for s402Version in the decoded JSON

An x402 client sending an exact payment will work against an s402 server. The normalizeRequirements() function handles the conversion transparently.

Edge Cases

• What if the transaction fails on-chain? The facilitator returns { success: false, error: "..." }. The client receives the error in the payment-response header and can retry.
• What if requirements expire? The facilitator rejects with REQUIREMENTS_EXPIRED (retryable). The client re-fetches requirements from the server.
• What about double-spending? Sui's object model prevents double-spending at the consensus level. If the same coin is used in two transactions, one will fail.