signature-lookup

name: signature-lookup description: Resolve EVM function selectors (4-byte) and event topic hashes to human-readable signatures. Queries openchain.xyz + 4byte.directory with a built-in fallback DB of 50+ common signatures. Batch mode, bytecode scanning, stdin pipe. Zero dependencies.

Resolve EVM function selectors and event topic hashes to human-readable signatures. Zero deps.

When to use this skill

You have a 4-byte selector like 0xa9059cbb and want to know it's transfer(address,uint256).
You're reading event logs and need to identify topic0 hashes.
You want to extract all function selectors from deployed bytecode.
You need to quickly identify what a contract can do by scanning its bytecode.
Pairs with calldata-decoder (decode full calldata) and event-decoder (decode full event logs).

What it does NOT require

ethers.js / viem / web3.js
npm install
An API key
A local node

Built-in DB covers 50+ of the most common ERC-20, ERC-721, Uniswap, and access-control signatures, so common lookups work even offline.

Script

`lookup.mjs`

# Single function selector
node lookup.mjs 0xa9059cbb

# Event topic hash
node lookup.mjs 0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef

# Batch lookup
node lookup.mjs 0xa9059cbb 0x095ea7b3 0x23b872dd

# Extract and resolve selectors from bytecode
node lookup.mjs --scan 0x6080604052...

# Pipe from stdin
echo "0xa9059cbb 0x095ea7b3" | node lookup.mjs

Example output

[fn]    0xa9059cbb  transfer(address,uint256)  [builtin]
[fn]    0x095ea7b3  approve(address,uint256)  [builtin]
[event] 0xddf252ad...  Transfer(address,address,uint256)  [builtin]

3/3 resolved.

Sources

Built-in DB — instant, no network. Covers ERC-20, ERC-721, Uniswap v2/v3, OpenZeppelin access control, and common DeFi signatures.
openchain.xyz — large public signature database (same one Etherscan uses).
4byte.directory — community-maintained Ethereum signature registry.

Both APIs are queried in parallel; results are deduplicated.