Prepaid Scheme

Deposit-based access. An agent deposits funds into an on-chain PrepaidBalance shared object, makes API calls off-chain, and the provider batch-claims accumulated usage. The Move contract enforces rate caps — no trust required.

When to Use

• High-frequency API access (100+ calls per session)
• AI agent API budgets
• Compute metering
• Any scenario where per-call settlement is too expensive

Economics

	Exact (per-call)	Prepaid (batched)
1,000 API calls	1,000 on-chain TXs	2 on-chain TXs
Gas cost	~$7.00	~$0.014
Per-call effective gas	$0.007	$0.000014
Improvement	—	500x cheaper

How It Works

Phase 1: Deposit (one on-chain TX)

The agent deposits funds into a PrepaidBalance shared object targeting a specific provider.

Agent → Sui: "Lock 1 SUI into PrepaidBalance for provider 0xABC"
Result: PrepaidBalance object created on-chain
Gas: ~$0.007

Phase 2: Usage (off-chain, zero gas)

The agent makes API calls, including the PrepaidBalance object ID in each request. The server:

1. Reads the on-chain balance (free — no transaction)
2. Verifies sufficient funds remain
3. Serves the response
4. Increments a local usage counter

Agent → Server: GET /api/data
  Header: X-Prepaid-Balance: 0x123...

Server checks balance → serves response → tracks usage locally

No on-chain transaction per call. The server tracks usage in its own database.

Phase 3: Claim (one on-chain TX)

The provider batch-claims accumulated usage from the balance.

Provider → Sui: "Claim 250 calls × 0.004 SUI each = 1.0 SUI from balance 0x123"
Gas: ~$0.007

Total gas for 1,000 calls: $0.014 (deposit + claim)

Trust Model

The provider tracks usage off-chain, which means they could theoretically over-report. The on-chain contract prevents this:

• claimed ≤ maxCalls × ratePerCall — can't claim more than the math allows
• claimed ≤ deposited amount — can't claim more than what's in the balance
• Withdrawal delay: The agent can withdraw unclaimed funds after withdrawalDelayMs passes

The agent doesn't need to trust the provider. The Move contract enforces the rules.

Requirements

const requirements: s402PaymentRequirements = {
  s402Version: '1',
  accepts: ['prepaid'],
  network: 'sui:mainnet',
  asset: '0x2::sui::SUI',
  amount: '10000000', // 0.01 SUI minimum deposit
  payTo: '0xprovider...',
  prepaid: {
    ratePerCall: '1000',     // 0.000001 SUI per call
    maxCalls: '10000',       // up to 10K calls
    minDeposit: '10000000',  // 0.01 SUI minimum
    withdrawalDelayMs: '3600000', // 1 hour delay
  },
};