Overview

HTTP 402 “Payment Required” is the status code the web reserved and never shipped — formally reserved in RFC 9110 §15.4.3. x402 revives it as a real payment handshake: a server answers a request with 402 and a machine-readable price, the client pays, and the same request is replayed and served.

Otomat is Solana’s x402 payment rail for machine-to-machine, per-call APIs. It settles each request in USDC on Solana through a spec-compliant facilitator and an on-chain settlement program.

Not Solana Pay. Solana Pay encodes a human scanning a QR to pay a merchant. Otomat is the opposite side of the market: software paying software per API call, negotiated entirely over HTTP headers with no human in the loop.

The three payment headers

The handshake rides on three headers. Each carries a base64(JSON) v2 document.

The flow

1. 1Client sends GET for the resource.
2. 2Server answers 402 with PAYMENT-REQUIRED (base64 JSON price + payTo + network).
3. 3Client decodes the requirements and builds a partially-signed Solana VersionedTransaction paying the amount.
4. 4Client replays the request carrying the signed transaction in PAYMENT-SIGNATURE.
5. 5The facilitator verifies the payment and settles it on Solana.
6. 6Server returns 200 with the resource and the settlement in PAYMENT-RESPONSE.

Quickstart

Price a route with one line of middleware. Pass the per-call price, the asset, and your payTo address.

import express from 'express';
import { otomat } from '@otomat/middleware';

const app = express();

// One line prices the route. Unpaid calls get 402; paid calls run the handler.
app.get(
  '/report',
  otomat.express({ price: '$0.002', asset: 'USDC', payTo: 'YOUR_SOLANA_ADDRESS' }),
  (req, res) => res.json({ report: buildReport() }),
);

app.listen(3000);

// app/api/report/route.ts
import { otomat } from '@otomat/middleware';

export const GET = otomat.next(
  { price: '$0.002', asset: 'USDC', payTo: 'YOUR_SOLANA_ADDRESS' },
  async () => Response.json({ report: buildReport() }),
);

import { Hono } from 'hono';
import { otomat } from '@otomat/middleware';

const app = new Hono();

app.get(
  '/report',
  otomat.hono({ price: '$0.002', asset: 'USDC', payTo: 'YOUR_SOLANA_ADDRESS' }),
  (c) => c.json({ report: buildReport() }),
);

Facilitator API

The facilitator is the stateless verifier and settler. It exposes three endpoints.

curl https://facilitator.otomat.fun/supported

# { "kinds": [ { "scheme": "exact", "network": "solana:mainnet" } ] }

On-chain program

Settlement is enforced by the otomat_rail Anchor program. The exact scheme commits a receipt per call; channels escrow a deposit and settle a run of off-chain checks in one transaction.

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References