Why s402?

The Problem

HTTP has a status code for payments: 402 Payment Required. It has been reserved since 1999 but never standardized. Every API that charges money invents its own auth-key-plus-billing system, and every AI agent that needs to pay for an API call needs custom integration code for each provider.

That is a lot of glue code. And it doesn't compose.

What HTTP 402 Should Be

Agent                    API Server
  |--- GET /data -------->|
  |<--- 402 + "pay me" ---|    (standard format)
  |                       |
  |  (build payment)      |
  |--- GET + payment ---->|    (standard header)
  |<--- 200 + data -------|

A universal payment negotiation layer. The server says how much, the client pays, the server delivers. No API keys. No billing dashboards. No monthly invoices. Just HTTP.

Why Now

Three things converged:

1. AI agents spend money. LLM-powered agents make thousands of API calls per session. They need to pay programmatically, not through human-managed billing portals.

2. Sui has sub-second finality. Payment confirmation in ~400ms means the agent can pay and get data back in a single user-perceived request. On EVM chains, 12-second block times make this impractical.

3. Programmable Transaction Blocks. Sui's PTBs allow verify + settle in a single atomic transaction. No temporal gap between "looks valid" and "money moved." This eliminates an entire class of race conditions.

What s402 Is

s402 is a wire format for HTTP 402 on Sui. It defines:

• What goes in the headers — base64 JSON in three headers (payment-required, X-PAYMENT, payment-response)
• What the JSON looks like — typed requirements, payloads, and responses
• Five payment schemes — different economics for different use cases
• Error codes with recovery hints — so agents can self-recover without human intervention

s402 is not a payment SDK. It does not include Sui SDK code, transaction builders, or RPC clients. It is a 16 KB TypeScript package with zero runtime dependencies that defines the protocol layer. You build the Sui integration on top.

Five Schemes, One Protocol

Scheme	What It Does	Best For
Exact	One payment per request	Simple API calls, x402 interop
Prepaid	Deposit upfront, call off-chain	High-frequency APIs (500x gas savings)
Escrow	Time-locked vault with dispute resolution	Digital goods, freelance payments
Stream	Per-second micropayments	AI inference, video, real-time data
Seal	Pay-to-decrypt via Sui SEAL	Paywalled content, encrypted files

Not sure which one? See the scheme decision guide.

Wire Compatibility with x402

x402 by Coinbase established HTTP 402 payments on EVM. s402 uses the same HTTP headers and base64 encoding — an x402 client sending an exact payment can talk to an s402 server with zero code changes.

s402 extends the format with Sui-native capabilities (additional schemes, PTB-based settlement, mandate delegation) while maintaining backward compatibility. See the full comparison.