EVM Calldata Decoder

Calldata is the input data sent with an Ethereum transaction when calling a smart contract function. It's ABI-encoded: the first 4 bytes are the function selector (keccak256 hash of the function signature, first 4 bytes), followed by the ABI-encoded arguments. Understanding calldata is essential for debugging transactions and building EVM tooling.

A function selector is the first 4 bytes of the keccak256 hash of the function signature. For `transfer(address,uint256)`, the selector is `0xa9059cbb`. Ethereum uses selectors to route calls to the correct function in a contract. You can look up known selectors at 4byte.directory.

After the 4-byte selector, arguments are ABI-encoded as 32-byte slots. Static types (uint256, address, bool, bytes32) are right-padded or left-padded to 32 bytes. Dynamic types (string, bytes, arrays) have an offset pointer in the static section pointing to the dynamic data location. Addresses are 20 bytes left-padded to 32 bytes.

Raw calldata without a selector appears in: fallback function calls, Ethereum transfer (empty calldata), some proxy patterns, and direct contract construction. In these cases, decode from offset 0 rather than offset 4.

Get the transaction's `input` field from a block explorer (Etherscan, Basescan). Paste it here or use `cast calldata-decode` from Foundry. Knowing the function signature (from the contract ABI or 4byte.directory) is required for full decoding. The 4-byte selector alone tells you which function was called.