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Payment Contract Reference

Every paid tool call on ASG follows the 402 Payment Required flow. This document describes the canonical response shape that agents must parse to build Solana payment instructions.

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402 Response Shape

When an agent calls a tool without payment, the gateway returns: 
{
  "code": "PAYMENT_REQUIRED",
  "message": "Payment required to execute tool",
  "request_id": "req_abc123",
  "quote": {
    "quote_id": "qt_xyz789",
    "tool": "inference_chat",
    "price_usdc_microusd": 2400,
    "price_display": "$0.0024",
    "ttl_seconds": 300,
    "expires_at": "2026-02-08T10:05:00Z"
  },
  "payment_instructions": {
    "network": "solana-mainnet",
    "asset": "USDC",
    "pay_to": "ATA_ADDRESS_HERE",
    "destination": "ATA_ADDRESS_HERE",
    "usdc_mint": "EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v",
    "memo_format": "asg:{quote_id}",
    "methods": ["transfer", "solana_pay"],
    "x402_header_format": "base64url(JSON({tx_signature, quote_id}))",
    "gasless_supported": true,
    "payment_methods": ["wallet", "gasless"]
  },
  "step_id": "step_001",
  "retry": {
    "method": "POST",
    "headers": { "X-Payment": "<base64url-encoded payment proof>" }
  }
}

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Field Reference

FieldTypeDescription
networkstringAlways solana-mainnet
assetstringAlways USDC
pay_tostringTreasury USDC token account (ATA)
destinationstringSame as pay_to (alias)
usdc_mintstringUSDC SPL token mint address
memo_formatstringRequired memo: asg:{quote_id}
methodsstring[]Supported: transfer, solana_pay
gasless_supportedbooleanWhether gasless relay is available
payment_methodsstring[]wallet (direct) or gasless (relay)

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Payment Flow

Agent                        ASG Gateway                    Solana
  │                              │                            │
  │── POST /mcp (tool call) ────►│                            │
  │◄── 402 + quote + pay_to ─────│                            │
  │                              │                            │
  │── Build USDC transfer ──────────────────────────────────►│
  │   (to pay_to, memo: asg:{quote_id})                      │
  │◄── tx_signature ────────────────────────────────────────│
  │                              │                            │
  │── POST /mcp (same call) ────►│                            │
  │   X-Payment: base64({        │                            │
  │     tx_signature, quote_id   │                            │
  │   })                         │── Verify on-chain ────────►│
  │                              │◄── Confirmed ──────────────│
  │◄── 200 + result + receipt ───│                            │

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X-Payment Header

The retry request must include X-Payment header with base64url-encoded JSON: 
{
  "tx_signature": "5Uj3...",
  "quote_id": "qt_xyz789"
}

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Important Notes

  • Network: Only solana-mainnet is supported. All payments must use Solana mainnet USDC.
  • Gasless: ASG covers Solana transaction fees. Agents only need USDC — no SOL required.
  • TTL: Quotes expire after ttl_seconds (default 300s / 5 min). Expired quotes return QUOTE_EXPIRED.
  • Idempotency: Re-submitting the same X-Payment header returns the cached result without re-charging.
  • Base URL: https://agent.asgcompute.com