EVM: decode a transaction's calldata (and encode it back)

⚠️ DANGEROUS: use at your own risk. Code that handles keys and signs transactions should always be read and audited by you before you trust it with real funds. Used incorrectly, it can destroy your money.

Task: you have a transaction and want its function arguments. abi-decode is offline and has no signature database: you give it the signature. Part of the EVM contract recipes.

Fixtures are raw Ethereum mainnet JSON-RPC responses in fixtures/evm-mainnet/; the request behind each file is in PROVENANCE.md. Live curl lines are UNTESTED here: CI never touches the network.

01a-erc20-transfer-tx.json is eth_getTransactionByHash for a USDC transfer. Pull input with jq:

jq -r .result.input recipes/fixtures/evm-mainnet/01a-erc20-transfer-tx.json | abi-decode --sig 'transfer(address,uint256)'

The whole response works too. A wrong signature is no_match with the selector (exit 0; exit 2 is malformed input); it never guesses:

abi-decode --sig 'approve(address,uint256)' < recipes/fixtures/evm-mainnet/01a-erc20-transfer-tx.json

Selector unknown? See evm-unknown-selector.md.

Encode: arguments → calldata

abi-encode is the inverse. The Solidity ABI spec example baz(uint32,bool) with 69, true gives selector 0xcdcd77c0:

abi-encode 'baz(uint32,bool)' 69 true | jq -c '{signature,selector}'

Re-encoding the decoded arguments of 01a round-trips:

abi-encode --raw 'transfer(address,uint256)' 0x264bd8291fae1d75db2c5f573b07faa6715997b5 5050510000 | abi-decode --sig 'transfer(address,uint256)'

Live (UNTESTED here): curl -s -X POST -H 'Content-Type: application/json' --data '{"jsonrpc":"2.0","id":1,"method":"eth_getTransactionByHash","params":["0x…"]}' "$RPC" | jq -r .result.input | abi-decode --sig 'transfer(address,uint256)'