Tx Preflight

{
  "confidence": 1,
  "data_versions": {
    "abi_packs": "2026-08-03.1",
    "chain": "eip155:8453",
    "denylist": "2026-08-03.1",
    "policy": "2026-08-03",
    "signature_index": "2026-08-03.1"
  },
  "evidence": [
    {
      "code": "RECIPIENT_MISMATCH",
      "data": {
        "actual": "0x5D3e1a9b7c2f4E8A0b6d3c5F9E1A7b4D2C8f0a6E",
        "expected": "0x1a2B3c4D5E6F7a8B9c0D1e2F3A4b5C6d7E8F9a0b",
        "field": "payTo"
      },
      "detail": "Pays 0x5D3e1a9b7c2f4E8A0b6d3c5F9E1A7b4D2C8f0a6E, but the quote names 0x1a2B3c4D5E6F7a8B9c0D1e2F3A4b5C6d7E8F9a0b. Signing this pays someone other than the party you are buying from.",
      "severity": "critical",
      "source": "quote-comparison"
    },
    {
      "code": "RPC_UNAVAILABLE",
      "detail": "No Base RPC endpoint is configured, so chain-dependent fields are unknown rather than assumed. See result.unresolved.",
      "severity": "medium",
      "source": "rpc"
    }
  ],
  "ok": true,
  "policy_version": "2026-08-03",
  "request_hash": "sha256:6d5c717eb5eede165dada2645ad0944430ff492739cf58d0c2dad2b812f0ddb4",
  "result": {
    "approvals": [],
    "authorization_checks": [],
    "candidate_signatures": [],
    "chain": "eip155:8453",
    "denylist_hits": [],
    "expected_transfers": [
      {
        "amount": "10000",
        "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
        "asset_kind": "erc20",
        "from": "0x9f2C5E4a8DB1C3A7e6b0d4f8C1e2A5b7d9c0E3f4",
        "source": "decode",
        "to": "0x5D3e1a9b7c2f4E8A0b6d3c5F9E1A7b4D2C8f0a6E",
        "token_id": null
      }
    ],
    "input_shape": "unsigned_tx",
    "intent": {
      "args": [
        {
          "name": "to",
          "type": "address",
          "value": "0x5D3e1a9b7c2f4E8A0b6d3c5F9E1A7b4D2C8f0a6E"
        },
        {
          "name": "amount",
          "type": "uint256",
          "value": "10000"
        }
      ],
      "calls": [],
      "decode_source": "bundled-abi",
      "depth": 0,
      "function_name": "transfer",
      "kind": "erc20_transfer",
      "selector": "0xa9059cbb",
      "signature": "transfer(address,uint256)",
      "standard": "erc20",
      "summary": "Transfer 10000 base units of token 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913 to 0x5D3e1a9b7c2f4E8A0b6d3c5F9E1A7b4D2C8f0a6E.",
      "target": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "truncated": false,
      "value_wei": "0"
    },
    "permit_scope": null,
    "proxy": {
      "address": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "admin": null,
      "beacon": null,
      "detail": "no RPC endpoint is configured, so no slot could be read",
      "implementation": null,
      "kind": "unknown",
      "slots_probed": [],
      "upgradeable": null
    },
    "quote_comparison": {
      "detail": "2 of 3 comparable fields match",
      "fields": [
        {
          "actual": "0x5D3e1a9b7c2f4E8A0b6d3c5F9E1A7b4D2C8f0a6E",
          "expected": "0x1a2B3c4D5E6F7a8B9c0D1e2F3A4b5C6d7E8F9a0b",
          "field": "payTo",
          "matches": false
        },
        {
          "actual": "10000",
          "expected": "10000",
          "field": "amount",
          "matches": true
        },
        {
          "actual": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
          "expected": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
          "field": "asset",
          "matches": true
        },
        {
          "actual": null,
          "expected": "8453",
          "field": "chainId",
          "matches": null
        },
        {
          "actual": null,
          "expected": "USD Coin",
          "field": "extra.name",
          "matches": null
        },
        {
          "actual": null,
          "expected": "2",
          "field": "extra.version",
          "matches": null
        }
      ],
      "matches": false,
      "present": true
    },
    "rpc": {
      "configured": false,
      "endpoints": [],
      "reachable": null
    },
    "simulation": {
      "asset_changes": [],
      "block_number": null,
      "detail": "simulation was not requested; send simulate: true to run one",
      "gas_used": null,
      "requested": false,
      "status": "not_requested"
    },
    "spender": {
      "address": null,
      "detail": "this transaction grants no allowance, so it has no spender",
      "is_contract": null
    },
    "unresolved": [
      "proxy status of 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913 is unknown: no RPC endpoint is configured, so no slot could be read"
    ]
  },
  "risk_codes": [
    "RECIPIENT_MISMATCH",
    "RPC_UNAVAILABLE"
  ],
  "verdict": "block",
  "warnings": []
}

# bash or zsh: see the payment challenge. Nothing is charged.
curl -sS -g -i -X POST 'https://txguard.schemasure.com/v1/tx/preflight' \
  -H 'content-type: application/json' \
  --data-raw '{"expected_quote":{"amount":"10000","asset":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","extra":{"name":"USD Coin","version":"2"},"maxTimeoutSeconds":60,"network":"eip155:8453","payTo":"0x1a2B3c4D5E6F7a8B9c0D1e2F3A4b5C6d7E8F9a0b","scheme":"exact"},"unsigned_tx":{"data":"0xa9059cbb0000000000000000000000005d3e1a9b7c2f4e8a0b6d3c5f9e1a7b4d2c8f0a6e0000000000000000000000000000000000000000000000000000000000002710","from":"0x9F2C5E4A8db1C3a7E6B0d4f8C1E2a5b7D9C0e3F4","to":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","value":"0"}}'

# x402 v2 sends the challenge base64-encoded in the PAYMENT-REQUIRED header (v1 sends it in the body).
# To read the header:
curl -sS -g -o /dev/null -D - -X POST 'https://txguard.schemasure.com/v1/tx/preflight' \
  -H 'content-type: application/json' \
  --data-raw '{"expected_quote":{"amount":"10000","asset":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","extra":{"name":"USD Coin","version":"2"},"maxTimeoutSeconds":60,"network":"eip155:8453","payTo":"0x1a2B3c4D5E6F7a8B9c0D1e2F3A4b5C6d7E8F9a0b","scheme":"exact"},"unsigned_tx":{"data":"0xa9059cbb0000000000000000000000005d3e1a9b7c2f4e8a0b6d3c5f9e1a7b4d2c8f0a6e0000000000000000000000000000000000000000000000000000000000002710","from":"0x9F2C5E4A8db1C3a7E6B0d4f8C1E2a5b7D9C0e3F4","to":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","value":"0"}}' \
  | grep -i '^payment-required:' | cut -d' ' -f2 | tr -d '\r' | base64 --decode

// npm i @x402/fetch @x402/evm viem
import { wrapFetchWithPayment, x402Client } from "@x402/fetch";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

// Pays at most $0.02 per call.
const client = new x402Client().setSpendControls({ maxAmountPerPayment: "$0.02" });
registerExactEvmScheme(client, { signer: privateKeyToAccount(process.env.EVM_PRIVATE_KEY) });
const pay = wrapFetchWithPayment(fetch, client);

// Not sure it's safe to pay? ToolVet checks it first for $0.005. Pay for that from a separate
// client: this one is capped at $0.02 per payment. GET https://toolvet.app/api/v1/check?url=https%3A%2F%2Ftxguard.schemasure.com%2Fv1%2Ftx%2Fpreflight&method=POST
const res = await pay("https://txguard.schemasure.com/v1/tx/preflight", {
  method: "POST",
  headers: { "content-type": "application/json" },
  body: JSON.stringify({
    "expected_quote": {
      "amount": "10000",
      "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "extra": {
        "name": "USD Coin",
        "version": "2"
      },
      "maxTimeoutSeconds": 60,
      "network": "eip155:8453",
      "payTo": "0x1a2B3c4D5E6F7a8B9c0D1e2F3A4b5C6d7E8F9a0b",
      "scheme": "exact"
    },
    "unsigned_tx": {
      "data": "0xa9059cbb0000000000000000000000005d3e1a9b7c2f4e8a0b6d3c5f9e1a7b4d2c8f0a6e0000000000000000000000000000000000000000000000000000000000002710",
      "from": "0x9F2C5E4A8db1C3a7E6B0d4f8C1E2a5b7D9C0e3F4",
      "to": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "value": "0"
    }
  }),
});
console.log(res.status, await res.text());

# pip install "x402[requests,evm]"
import os
from eth_account import Account
from x402 import x402ClientSync
from x402.http.clients import x402_requests
from x402.mechanisms.evm import EthAccountSigner
from x402.mechanisms.evm.exact.register import register_exact_evm_client

# Pays at most $0.02 per call.
client = x402ClientSync().set_spend_controls({"max_amount_per_payment": "$0.02"})
register_exact_evm_client(client, EthAccountSigner(Account.from_key(os.environ["EVM_PRIVATE_KEY"])))

with x402_requests(client) as session:
    res = session.post(
        "https://txguard.schemasure.com/v1/tx/preflight",
        json={
            "expected_quote": {
                "amount": "10000",
                "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
                "extra": {
                    "name": "USD Coin",
                    "version": "2",
                },
                "maxTimeoutSeconds": 60,
                "network": "eip155:8453",
                "payTo": "0x1a2B3c4D5E6F7a8B9c0D1e2F3A4b5C6d7E8F9a0b",
                "scheme": "exact",
            },
            "unsigned_tx": {
                "data": "0xa9059cbb0000000000000000000000005d3e1a9b7c2f4e8a0b6d3c5f9e1a7b4d2c8f0a6e0000000000000000000000000000000000000000000000000000000000002710",
                "from": "0x9F2C5E4A8db1C3a7E6B0d4f8C1E2a5b7D9C0e3F4",
                "to": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
                "value": "0",
            },
        },
    )
    print(res.status_code, res.text)