s402

🧭

Superset of x402 and MPP

Speaks x402 V1/V2 and MPP as payment-in formats, plus five schemes they structurally can't express. An agent that picks s402 gets every option the others offer, plus the ones they don't.

💸

Same price or cheaper for agents

Wire-compatible schemes match competitors at parity. Prepaid on Sui hits $0.014 per 1,000 calls — 500× savings on session-based access. No reason for an agent not to route through s402.

🧩

Six payment schemes, one protocol

Exact for one-shot. Upto for metered. Prepaid for high-frequency. Escrow for trustless commerce. Stream for per-second billing. Unlock for pay-to-decrypt. Sui happens to be the most expressive chain — every scheme lands natively.

🤖

Built for agents that self-recover

Every error includes `retryable` and `suggestedAction`. Your agent knows whether to retry, top up, or switch schemes — no human needed. Zero runtime deps. TypeScript, Python, Go.

See It in Action

An AI agent hits a paid API. The server says "pay me." The agent pays and gets the data. Three HTTP requests, zero human intervention.

typescript

import {
  extractRequirementsFromResponse,
  encodePaymentPayload,
  S402_HEADERS,
} from 's402';

async function agentFetch(url: string, buildPayment: PaymentBuilder) {
  const res = await fetch(url);

  if (res.status !== 402) return res;

  // 1. Decode what the server wants
  const requirements = extractRequirementsFromResponse(res);
  if (!requirements) throw new Error('Invalid 402 response');

  // 2. Build and sign a payment (you bring the chain SDK)
  const payment = await buildPayment(requirements);

  // 3. Retry with payment attached
  return fetch(url, {
    headers: { [S402_HEADERS.PAYMENT]: encodePaymentPayload(payment) },
  });
}

s402 defines the wire format — what gets sent over HTTP. You bring your own chain SDK for the how (PTB builders, signers, RPC calls).

Start Where You Are

🤖 I'm building an AI agent Need to pay for APIs, tools, or inference. Start with the Quick Start to wire a client in 5 minutes, then read Which Scheme Do I Need? to pick between Exact, Upto, and Prepaid.

🏗 I run a paid API Want agents to pay you directly — no billing dashboards, no API keys. See the server tutorial for a 402 endpoint in 20 lines, then Fee Ownership & Trust for who pays gas.

🦊 I build a wallet or SDK Need to speak s402 from a client library. Start with the Wire Format Spec and the 161-vector conformance suite. TypeScript, Python, Go adapters exist today.

⚙️ I operate a facilitator Running verify/settle infra for a chain or scheme. Read the Architecture page for the facilitator contract, then Security Model for invariants you must preserve.

🔎 I'm auditing or researching Evaluating s402 for production or academic work. Start with the Whitepaper, Threat Model, and the three-way comparison against x402 and MPP.

🔁 I'm migrating from another protocol Already running x402 or MPP. See Migrating from x402 (one-line middleware swap) or Migrating from MPP (coexistence pattern, no rip-and-replace).

Python

python

import httpx
from s402 import decode_payment_required, encode_payment_payload, S402_HEADERS

url = "https://api.example.com/premium-data"
res = httpx.get(url)
if res.status_code == 402:
    requirements = decode_payment_required(res.headers[S402_HEADERS["PAYMENT_REQUIRED"]])
    payment = build_payment(requirements)  # you bring the chain SDK
    res = httpx.get(url, headers={S402_HEADERS["PAYMENT"]: encode_payment_payload(payment)})

bash

pip install s402   # Python — zero dependencies
npm install s402   # TypeScript — zero dependencies

v0.5.0 · Six payment schemes · 831 tests · 161-vector conformance suite · Apache-2.0 · npm

s402Your AI agent needs to pay for things.