Calldata decoder (ABI or selector DB)

Decode EVM transaction calldata into the function name and typed parameters. Supply an ABI (from /api/contract-abi) or a signature like transfer(address,uint256) for a fully offline decode; with neither, the 4-byte selector is resolved via the openchain.xyz signature database (4byte.directory fallback) and each candidate signature is tried. Unknown selectors return a documented partial decode (selector + raw 32-byte words) instead of an error.

Input

Example output

{
  "selector": "0xa9059cbb",
  "decoded": true,
  "name": "transfer",
  "signature": "transfer(address,uint256)",
  "params": [
    {
      "type": "address",
      "name": null,
      "value": "0xabf4fabd7c416fb67202e5f9002389fc75e2a9d0"
    },
    {
      "type": "uint256",
      "name": null,
      "value": "1000000"
    }
  ],
  "source": "openchain.xyz"
}

Try it — see the 402 challenge (free)

curl -i -X POST https://agent402.tools/api/calldata-decode \
  -H "Content-Type: application/json" \
  -d '{"data":"0xa9059cbb000000000000000000000000abf4fabd7c416fb67202e5f9002389fc75e2a9d000000000000000000000000000000000000000000000000000000000000f4240"}'

The response is HTTP 402 Payment Required with exact payment requirements. Any x402 v2 client pays automatically and retries:

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();
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/calldata-decode", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "data": "0xa9059cbb000000000000000000000000abf4fabd7c416fb67202e5f9002389fc75e2a9d000000000000000000000000000000000000000000000000000000000000f4240"
  }),
});

Part of these workflows

This tool is one step in a curated multi-tool workflow — agents can fetch the whole sequence as an MCP prompt or call https://agent402.tools/api/skill-packs/{slug}/prompt.

• Transaction forensics — Explain what an EVM transaction actually did: confirmation status, the raw transaction from the chain, decoded calldata with typed parameters, the resolved function signature, and labeled counterparties.

Related tools