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x402 Protocol

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What is x402?

• No API keys — Payment itself is the authentication
• Pay-per-use — No subscriptions, pay only for what you consume
• Machine-native — Designed for autonomous AI agents
• On-chain — All payments are verifiable blockchain transactions

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Protocol Flow

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1. Initial Request (No Payment)

GET /feeds/whale-alerts HTTP/1.1
Host: api.shadowfeed.app

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2. Server Returns 402

HTTP/1.1 402 Payment Required
payment-required: eyJ4NDAyVmVyc2lvbiI6Miwi...  (base64)
Content-Type: application/json

{
  "x402Version": 2,
  "resource": {
    "url": "https://api.shadowfeed.app/feeds/whale-alerts",
    "description": "Whale movements data",
    "mimeType": "application/json"
  },
  "accepts": [{
    "scheme": "exact",
    "network": "stacks:1",
    "amount": "5000",
    "asset": "STX",
    "payTo": "SP1DV3T4ST2A89ZZ07M73B2N4AR5XFMDCNPGKK6CS",
    "maxTimeoutSeconds": 300
  }]
}

• amount: Price in microSTX (5000 = 0.005 STX)
• payTo: Provider’s Stacks address
• network: stacks:1 (mainnet) or stacks:2147483648 (testnet)

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3. Agent Signs and Retries

1. Parses the payment-required header (base64 → JSON)
2. Creates a STX transfer transaction for the exact amount
3. Signs it with the agent’s private key
4. Encodes the signed TX as base64
5. Retries the request:

GET /feeds/whale-alerts HTTP/1.1
Host: api.shadowfeed.app
payment-signature: eyJwYXlsb2FkIjp7InRyYW5z...  (base64)

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4. Server Verifies and Settles

1. Decodes the payment-signature header
2. Sends to facilitator: POST /verify (validates amount, recipient, signature)
3. Sends to facilitator: POST /settle (broadcasts TX to Stacks via Hiro API)
4. Waits for TX confirmation
5. Returns the data with payment-response header

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5. Response with Data

HTTP/1.1 200 OK
payment-response: eyJzdWNjZXNzIjp0cnVl...  (base64)
Content-Type: application/json

{
  "alerts": [...],
  "summary": {...},
  "btc_price_usd": 104928
}

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Headers Reference

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x402-stacks SDK

• paymentMiddleware — Server-side middleware for Express/Hono
• wrapAxiosWithPayment — Client-side axios interceptor
• privateKeyToAccount — Wallet derivation from private key
• STXtoMicroSTX — Price conversion utility