How to Decode ABI-Encoded Calldata and Event Logs

What Is ABI-Encoded Data?

When you call a smart contract function, your wallet encodes the function name and arguments into a hex string called calldata. It starts with a 4-byte function selector followed by 32-byte encoded arguments.

0xa9059cbb
  0000000000000000000000005B38Da6a701c568545dCfcB03FcB875f56beddC4
  0000000000000000000000000000000000000000000000000de0b6b3a7640000

→ transfer(address, uint256)
  to: 0x5B38Da6a701c568545dCfcB03FcB875f56beddC4
  amount: 1000000000000000000

Event logs work similarly — they contain indexed topics (used for filtering) and ABI-encoded data fields. Without the ABI, these are just opaque hex blobs.

The Hard Way: Decoding by Hand

The traditional approach to decoding calldata or event logs requires development tooling:

• →Extract the 4-byte selector and look it up in a signature database
• →Use ethers.js Interface.decodeFunctionData() or web3.js to parse the arguments
• →For events, match topic[0] against known event signatures and decode the data fields
• →Handle edge cases: proxy contracts, custom errors, packed encoding
• →Write and run a script every time you need to inspect a transaction

The Easy Way: ABI Page Builder

ABI Page Builder includes two built-in decoders that work instantly in your browser:

When Would You Use This?

• Debugging a failed transaction — see exactly which function was called and with what arguments
• Auditing contract interactions — verify that a multisig payload matches the intended action
• Investigating on-chain activity — decode MEV bot transactions or suspicious contract calls
• Verifying governance proposals — confirm that encoded execution data matches the proposal description
• Inspecting event history — decode raw logs to understand what happened in a transaction

Manual Decoding vs ABI Page Builder