Calldata

The encoded input data of a transaction or contract call — the bytes that tell the EVM which function to invoke and what arguments to pass. Calldata starts with a 4-byte function selector (the first 4 bytes of the Keccak-256 hash of the function signature), followed by ABI-encoded arguments. Reading and decoding calldata is essential for understanding what a transaction actually does — block explorers like Etherscan auto-decode it when the contract source is verified, and tools like 4byte.directory let you look up unknown function selectors.