calldata-decoder

name	calldata-decoder
description	Decode raw EVM transaction calldata into a human-readable function call. Resolves 4-byte selectors via openchain.xyz and best-effort decodes parameters. Zero dependencies, works from raw hex or a tx hash. Use when an agent needs to understand what an arbitrary `0x…` blob is actually doing.

calldata-decoder

Turn raw EVM transaction input into a function call you can read. Zero deps.

When to use this skill

• A transaction failed and you have the calldata but no idea what it called.
• You're inspecting a multisig proposal or pending tx and the wallet UI just shows 0xa9059cbb….
• A contract is calling out to another contract and you want to know which function.
• An agent needs to verify that a calldata blob it's about to send matches the intended function.

What it does NOT require

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

It uses Node 18+'s built-in fetch and the public openchain.xyz signature database.

Script

decode.mjs

# Raw calldata:
node decode.mjs 0xa9059cbb000000000000000000000000ab5801a7d398351b8be11c439e05c5b3259aec9b0000000000000000000000000000000000000000000000000de0b6b3a7640000

# From stdin:
cat calldata.hex | node decode.mjs

# Directly from a tx hash on any RPC:
node decode.mjs --tx 0x<hash> --rpc https://mainnet.base.org

# JSON output (machine-readable):
node decode.mjs --json 0xa9059cbb...

Environment variable RPC_URL is honored when --rpc is omitted. Default RPC is Base mainnet.

Output

selector: 0xa9059cbb    (64 bytes of params)

  transfer(address,uint256)
    [0] address    = 0xab5801a7d398351b8be11c439e05c5b3259aec9b
    [1] uint256    = 1000000000000000000

When multiple signatures share a selector (selector collisions are real), all candidates are listed. The agent picks based on context (the to address, surrounding logs, etc.).

When the selector isn't in the openchain database, the script prints the raw 32-byte parameter words so you still have something to work with.

Supported parameter types

• address, bool, bytes1..32
• uint8..256, int8..256 (signed values handled with two's complement)
• string and dynamic bytes (decoded from the tail section)
• Dynamic arrays and tuples: shown as offset + raw tail dump (best-effort)

For perfect ABI decoding of complex nested types, hand off to viem/ethers. This skill is the fast 90% case.

Why this skill

0xa9059cbb… is opaque; transfer(0xab…, 1e18) is not. Every agent that touches transactions eventually needs to translate one to the other, and the answer should not require pulling in a 100kB ABI parser.