Transaction simulation (eth_call + gas)

Dry-run a prospective smart-contract call or transaction without broadcasting it: executes eth_call and eth_estimateGas against the latest block over the same keyless multi-endpoint public RPC pool as /api/evm-rpc (ethereum, base, polygon, arbitrum, optimism). Send POST /api/tx-simulate with the required field to and pay $0.005 per call over x402 or MPP (there is no free tier). It returns a JSON object with network, success, returnData, gasEstimate and revertReason.

Returns the return data and a gas estimate on success, or {success:false} with the revert reason when the call would fail. Strictly read-only - nothing is signed or broadcast.

Category: Crypto & onchain data · Tags: crypto simulation eth_call gas transaction evm dry-run smart-contract

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Parameters

Example request

curl -i -X POST https://agent402.tools/api/tx-simulate \
  -H "Content-Type: application/json" \
  -d '{"network":"base","to":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","data":"0x70a08231000000000000000000000000abf4fabd7c416fb67202e5f9002389fc75e2a9d0"}'

Without payment this returns HTTP 402 Payment Required with the exact price for tx-simulate; any x402 v2 or MPP client pays it and retries.

Example response

{
  "network": "base",
  "success": true,
  "returnData": "0x00000000000000000000000000000000000000000000000000000000000f4240",
  "gasEstimate": 31234,
  "revertReason": null
}

From an MCP client

catalog.call {
  "slug": "tx-simulate",
  "params": {
    "network": "base",
    "to": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "data": "0x70a08231000000000000000000000000abf4fabd7c416fb67202e5f9002389fc75e2a9d0"
  }
}

The hosted connector at https://agent402.tools/mcp needs a payment for tx-simulate; the stdio package pays it from a wallet or from AGENT402_CREDITS_KEY. Local install: npx -y agent402-mcp.

Errors and behavior

• to is required. An input the tool rejects returns an HTTP 4xx whose body carries error, tool, expected, required and example, so the caller can correct it.
• A paid call that ends in any status of 400 or above is not charged over x402, MPP or a prepaid credits key: settlement is cancelled when the tool fails. The exception is a Tempo push credential, a transfer the buyer sent before the call: it settles before the tool runs, so if the tool then fails the payment is recorded as a refund owed to the paying wallet.
• Wallet-only: this tool reaches the network or stored state, so it has no proof-of-work tier. A prepaid card-credits key issued earlier (Authorization: Bearer a402_...) also pays it.
• A GET or HEAD to /api/tx-simulate returns the same 402 quote, so the price can be read without a body.
• An Idempotency-Key header makes a retried paid call replay the first 200 instead of charging again (an answer larger than 1 MB is not replayed).

Paid call (JavaScript agent)

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

const client = new x402Client();
client.setSpendControls?.(false); // keep your own spending ceiling in code
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/tx-simulate", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "network": "base",
    "to": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "data": "0x70a08231000000000000000000000000abf4fabd7c416fb67202e5f9002389fc75e2a9d0"
  }),
});

Part of these workflows

Transaction simulation (eth_call + gas) is one step in this skill pack, each sold as a single call:

• Contract audit - Triage a smart contract before an agent interacts with it: verified Solidity source, heuristic vulnerability scan, known-address check, function-selector resolution, and a read-only dry-run of the exact call you plan to make.

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