Function selector / event topic lookup

Resolve a 4-byte function selector or a 32-byte event topic hash to its known human-readable Solidity signatures, via the openchain.xyz signature database with 4byte.directory as fallback. Send POST /api/selector-lookup with the required field selector and pay $0.002 per call over x402 or MPP (there is no free tier). It returns a JSON object with selector, kind, found, signatures and source.

Unknown selectors return {found:false} with an empty list, not an error.

Category: Crypto & onchain data · Tags: crypto selector 4byte signature event topic evm smart-contract

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Parameters

Example request

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

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

Example response

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

From an MCP client

catalog.call {
  "slug": "selector-lookup",
  "params": {
    "selector": "0xa9059cbb"
  }
}

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

Errors and behavior

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

Part of these workflows

Function selector / event topic lookup is one step in these 2 skill packs, 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.
• 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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