Pre-flight an x402 payment: decode the endpoint's challenge, report the cost on every rail

[
  {
    "scheme": "exact",
    "network": "eip155:8453",
    "payTo": "0x5253666c40432f009ebd99908b9cd2a7fd1019cb",
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "amount": "3000",
    "maxTimeoutSeconds": 300
  },
  {
    "scheme": "exact",
    "network": "solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp",
    "payTo": "FgcKNzsBXSEHGNZNWetPsjXiBZk1WFoU6sF2zVsnzXfa",
    "asset": "EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v",
    "amount": "3000",
    "maxTimeoutSeconds": 300
  }
]

{
  "bazaar": {
    "info": {
      "input": {
        "method": "GET",
        "queryParams": {
          "address": "0xc2a36e22ecbf353086cf83e7beea78bc28a64083",
          "url": "https://api.example.com/paid-endpoint"
        },
        "type": "http"
      },
      "output": {
        "example": {
          "affordability": {
            "address": "0xc2a3...4083",
            "can_afford": true,
            "cost_usdc": "0.005",
            "rail_checked": "evm",
            "usdc_balance": "1.234"
          },
          "cheapest": {
            "cost_usdc": "0.005",
            "network": "eip155:8453",
            "rail": "evm"
          },
          "is_x402": true,
          "payable": true,
          "rails": [
            {
              "asset_name": "USDC (Base mainnet)",
              "cost_usdc": "0.005",
              "network": "eip155:8453",
              "network_name": "Base mainnet",
              "pay_to": "0x...",
              "rail": "evm"
            },
            {
              "asset_name": "USDC (Solana)",
              "cost_usdc": "0.005",
              "network": "solana:5eykt4...",
              "network_name": "Solana mainnet",
              "pay_to": "...",
              "rail": "svm"
            }
          ],
          "url": "https://api.example.com/paid-endpoint",
          "warnings": []
        },
        "type": "json"
      }
    },
    "schema": {
      "$schema": "https://json-schema.org/draft/2020-12/schema",
      "properties": {
        "input": {
          "additionalProperties": false,
          "properties": {
            "method": {
              "enum": [
                "GET"
              ],
              "type": "string"
            },
            "queryParams": {
              "properties": {
                "address": {
                  "description": "Optional payer wallet to check Base affordability",
                  "pattern": "^0x[0-9a-fA-F]{40}$",
                  "type": "string"
                },
                "url": {
                  "description": "The x402 endpoint to pre-flight",
                  "format": "uri",
                  "type": "string"
                }
              },
              "required": [
                "url"
              ],
              "type": "object"
            },
            "type": {
              "const": "http",
              "type": "string"
            }
          },
          "required": [
            "type",
            "method"
          ],
          "type": "object"
        },
        "output": {
          "properties": {
            "example": {
              "type": "object"
            },
            "type": {
              "type": "string"
            }
          },
          "required": [
            "type"
          ],
          "type": "object"
        }
      },
      "required": [
        "input"
      ],
      "type": "object"
    }
  }
}

curl "https://quartermaster.surewhynot.app/v1/x402/simulate"
# -> 402 Payment Required, accepts[] lists how to pay
# retry with a PAYMENT-SIGNATURE (or PAYMENT header) once paid

const res = await fetch("https://quartermaster.surewhynot.app/v1/x402/simulate");
if (res.status === 402) {
  const { accepts } = await res.json();
  // pay one of accepts[] via an x402 client, then retry with the payment header
}

import httpx
res = httpx.get("https://quartermaster.surewhynot.app/v1/x402/simulate")
if res.status_code == 402:
    accepts = res.json()["accepts"]
    # pay one of accepts[] via an x402 client, then retry with the payment header