Function selector / event topic lookup

Resolve a 4-byte function selector or a 32-byte event topic hash to its known human-readable signatures, via the openchain.xyz signature database with 4byte.directory as fallback. Unknown selectors return {found:false} with an empty list, not an error.

Input

Example output

{
  "selector": "0xa9059cbb",
  "kind": "function",
  "found": true,
  "signatures": [
    "transfer(address,uint256)"
  ],
  "source": "openchain.xyz"
}

Try it — see the 402 challenge (free)

curl -i -X POST https://agent402.tools/api/selector-lookup \
  -H "Content-Type: application/json" \
  -d '{"selector":"0xa9059cbb"}'

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/selector-lookup", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "selector": "0xa9059cbb"
  }),
});

Part of these workflows

This tool is one step in 2 curated multi-tool workflows — agents can fetch the whole sequence as an MCP prompt or call https://agent402.tools/api/skill-packs/{slug}/prompt.

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