s402 vs x402
s402 extends x402 (Coinbase's HTTP 402 protocol) with Sui-native capabilities. They are complementary, not competing — x402 handles EVM well, s402 handles Sui.
At a Glance
| x402 (Coinbase/EVM) | s402 (Sui) | |
|---|---|---|
| Settlement | Two-step: verify then settle (temporal gap) | Atomic: verify + settle in one PTB |
| Finality | ~12 seconds (EVM L1) | ~400ms (Sui) |
| Payment models | Exact (one-shot) | Exact, Prepaid, Escrow, Stream, Seal |
| Coin handling | approve + transferFrom | Native coinWithBalance + splitCoins |
| Direct mode | No | Yes (no facilitator needed) |
| Receipts | Off-chain | On-chain NFT proofs |
| Agent auth | None | AP2 mandate delegation |
| Wire compatibility | — | x402 V1 wire-compat; V2 via compat layer |
High-Frequency API Payments
This is where the economics diverge most. When an AI agent makes thousands of API calls, the gas model matters:
| x402 Exact | s402 Prepaid | |
|---|---|---|
| 1,000 API calls | 1,000 on-chain TXs | 2 on-chain TXs |
| Gas cost | ~$7.00 | ~$0.014 |
| Latency per call | ~12s (block confirmation) | ~5ms (off-chain balance check) |
| Settlement finality | ~12 seconds | ~400ms |
Methodology (Important)
This comparison is not apples-to-apples. x402's Exact scheme settles every call individually. s402's Prepaid scheme amortizes costs by depositing upfront and batch-claiming later.
Exact-to-exact, the gas difference is more modest — roughly 10-20x from Sui's lower base gas cost. The 500x advantage comes from a payment model (Prepaid) that EVM architecturally cannot support because it relies on Sui's shared objects and Programmable Transaction Blocks.
We state this explicitly because transparent methodology builds trust.
Wire Compatibility
s402 uses the same HTTP headers as x402 V1:
| Header | Direction | x402 | s402 |
|---|---|---|---|
payment-required | Server → Client | ✅ | ✅ |
X-PAYMENT | Client → Server | ✅ (V1) | ✅ |
PAYMENT-SIGNATURE | Client → Server | ✅ (V2) | Via compat |
payment-response | Server → Client | ✅ | ✅ |
An x402 client sending an exact payment can interact with an s402 server. The detectProtocol() function auto-detects which protocol a server is using by checking for s402Version in the decoded requirements.
import { detectProtocol, normalizeRequirements } from 's402';
// Works whether the server sent s402 or x402
const protocol = detectProtocol(response.headers);
// Normalize either format into s402 types
const requirements = normalizeRequirements(decoded);Why Not Just Use x402 on Sui?
You can — s402's exact scheme is wire-compatible with x402. But you'd miss:
- Prepaid: Deposit-based access with off-chain API calls. 500x gas savings for high-frequency usage.
- Escrow: Time-locked trustless commerce with arbiter dispute resolution.
- Stream: Per-second micropayments via on-chain meters.
- Seal: Pay-to-decrypt via Sui SEAL + Walrus encrypted storage.
- Direct settlement: No facilitator needed — settle directly on-chain.
- On-chain receipts: Permanent, auditable proof of every payment as NFTs.
- AP2 mandates: Agent spending authorization with per-tx and lifetime caps.
These capabilities come from Sui's object model, PTBs, and sub-second finality. They can't be replicated on EVM — and they don't need to be. x402 already handles EVM well. s402 handles Sui.