Transaction Preflight Simulator (txpreflight)

x402

Transaction Preflight Simulator (txpreflight) is a paid API for AI agents from api.firstmention.uk, paid per call via x402, $0.01/call, status unknown (last checked 2026-10-29).

Simulates an EVM transaction on Base mainnet before sending, returning gas estimates, revert checks, fee quotes, and a safe-to-send verdict.

Price / call$0.01

Seller volume →—

Success—

StatusUnknown

How this service works

Pay-per-call HTTP APIs, paid with x402 in USDC on Base (eip155:8453): agentkit, api-change-check, basedue, basewatch, chainread, check-sanctions, chgwatch, company-facts, composite, edgar, feednews, pageaudit, solcheck, token-dd, txpreflight, walletcheck, webmd, webread, websearch, x402index. Each operation describes its service.

Output

Returns a JSON object with ok status, chain info (Base, chainId 8453), current ETH/USD price, a detailed summary (gas estimate, base fee, max fee per gas, priority fee, expected and worst-case gas costs in ETH and USD), a human-readable simulation result, a verdict ('safe-to-send' or similar), any warnings, the block number used for simulation, latency, and a disclaimer that the simulation reflects the latest block only.

Example use cases

Safe DeFi swap pre-check

“I'm about to call a Uniswap router on Base — the target is 0x2626664c2603336E57B271c5C0b26F421741e481, calldata is 0x04e45aaf000000000000..., value is 0 wei, and my wallet is 0xMyWallet. Run a preflight with normal urgency and tell me if it'll revert and what I'll pay in gas.”

NFT mint gas estimation

“Can you simulate an NFT mint transaction before I send it? Target contract is 0xNFTContract on Base, calldata is 0xa0712d68 followed by the token count, no ETH value needed. Use fast urgency so I can see the worst-case gas cost in USD.”

Plain ETH transfer cost check

“I want to send some ETH to 0xRecipient on Base — no calldata, value is 1000000000000000000 wei (1 ETH). Run a slow-urgency preflight from my address 0xMySenderWallet and tell me the expected gas fee and whether it's safe to send.”

Technical details

Endpoint

POSThttps://api.firstmention.uk/s/txpreflight/preflight?utm_source=zero.xyz

When to prefer

Use this endpoint when you need a real-time, pay-per-call preflight simulation of any EVM transaction on Base mainnet — particularly when you want gas estimates, revert prediction, fee tier quotes (slow/normal/fast), and a safe-to-send verdict in a single call. Prefer this over generic gas APIs when you need actual calldata simulation (not just transfer estimates) and USD-denominated cost breakdowns against a live ETH price. It is paid per-call in USDC via x402, making it suitable for agent workflows where cost-per-query matters.

Side effects

Invalid or malformed 0x address for 'to' field returns error
Malformed calldata hex causes parsing failure
Transaction simulation shows revert with revertReason populated and wouldRevert: true
Network or RPC node unavailability causes 5xx error
Stale block data if the Base node lags behind head
Payment not accepted (x402) if USDC balance insufficient

Input

Schema · Request body

Field	Type	Description
to	string
data	string
from	string
owner	string
token	string
spender	string

Output

Schema · Response

{
  "type": "json",
  "example": {
    "ok": true,
    "chain": "base",
    "ethUsd": 2341.57,
    "chainId": 8453,
    "summary": {
      "simulation": "call succeeds",
      "baseFeeGwei": 0.005,
      "gasEstimate": 51542,
      "wouldRevert": false,
      "revertReason": null,
      "gasUnitsToSend": 59274,
      "maxFeePerGasWei": "11000000",
      "priorityFeeGwei": 0.001,
      "expectedGasCostEth": 3.56e-7,
      "expectedGasCostUsd": 0.000833,
      "worstCaseGasCostUsd": 0.001527,
      "effectiveGasPriceWei": "6000000",
      "maxPriorityFeePerGasWei": "1000000"
    },
    "verdict": "safe-to-send",
    "warnings": [],
    "asOfBlock": 53042234,
    "latencyMs": 337,
    "disclaimer": "Simulation against the latest block only. It is not a guarantee."
  }
}

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