Calldata decoder (ABI or selector DB)

Decode EVM smart-contract transaction calldata into the function name and typed parameters. Send POST /api/calldata-decode with the required field data and pay $0.003 per call over x402 or MPP (there is no free tier). It returns a JSON object with selector, decoded, name, signature, params and 1 more.

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.

Category: Crypto & onchain data · Tags: crypto calldata decode abi selector evm transaction smart-contract

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Parameters

Example request

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

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

Example response

{
  "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"
}

From an MCP client

catalog.call {
  "slug": "calldata-decode",
  "params": {
    "data": "0xa9059cbb000000000000000000000000abf4fabd7c416fb67202e5f9002389fc75e2a9d000000000000000000000000000000000000000000000000000000000000f4240"
  }
}

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

Errors and behavior

• data 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/calldata-decode 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/calldata-decode", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "data": "0xa9059cbb000000000000000000000000abf4fabd7c416fb67202e5f9002389fc75e2a9d000000000000000000000000000000000000000000000000000000000000f4240"
  }),
});

Part of these workflows

Calldata decoder (ABI or selector DB) is one step in this skill pack, each sold as a single call:

• 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.

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