Cast Skill

Cast is a Swiss Army knife for interacting with Ethereum applications from the command line. It is part of the Foundry toolkit.

Overview

Cast provides commands for:

Encoding/decoding ABI data
Querying blockchain state (accounts, blocks, transactions)
Sending transactions
ENS lookups
Data conversions (hex, wei, bytes32, etc.)
Contract interaction and inspection
Wallet management

Command Categories

Cast commands are organized into the following categories. Each category has a dedicated reference file for detailed command information:

Category	Reference	Description
ABI	`references/abi-commands.md`	Encode/decode ABI data
4byte	`references/4byte-commands.md`	Signature lookups via openchain.xyz
Account	`references/account-commands.md`	Balances, nonces, proxy info
Block	`references/block-commands.md`	Block information
Chain	`references/chain-commands.md`	Chain ID, client version
Code	`references/code-commands.md`	Bytecode inspection
Conversion	`references/conversion-commands.md`	hex, wei, bytes32, RLP, UTF8
ENS	`references/ens-commands.md`	ENS lookups
Transaction	`references/transaction-commands.md`	Send, call, receipts
Crypto	`references/crypto-commands.md`	Keccak, EIP-191 hashing
Utility	`references/utility-commands.md`	Storage, CREATE2, selectors
Wallet	`references/wallet-commands.md`	Wallet management

Common Options

Most Cast commands support:

--rpc-url <URL> - RPC endpoint
-e, --etherscan-api-key <KEY> - Etherscan API key
--chain <CHAIN> - Chain name or ID
-h, --help - Help

Quick Examples

# Query balance
cast balance 0x... --rpc-url https://eth.llamarpc.com

# Call contract
cast call 0x... "balanceOf(address)" 0x... --rpc-url https://eth.llamarpc.com

# Send transaction
cast send 0x... "transfer(address,uint256)" 0x... 1000000 --private-key $PK

# Encode calldata
cast calldata "transfer(address,uint256)" 0x... 1000000

# Convert wei to ether
cast from-wei 1000000000000000000

# Get function selector
cast sig "transfer(address,uint256)"

Installation

curl -L https://foundry.paradigm.xyz | bash
foundryup

Documentation

Full docs: https://getfoundry.sh/cast/reference/cast

Troubleshooting

Cast crashes with "Attempted to create a NULL object" on macOS

If cast call, cast send, or other network commands crash with:

The application panicked (crashed).
Message:  Attempted to create a NULL object.
Location: .../system-configuration-0.6.1/src/dynamic_store.rs:154

Cause: Sandboxed environments (like Claude Code's sandbox) block access to macOS System Configuration framework. Foundry's HTTP client tries to read system proxy settings and crashes when this access is denied.

Solutions:

Disable sandbox for network calls - Run cast network commands with sandbox disabled

Use curl with JSON-RPC - Direct RPC calls work in sandbox:

# Get function selector (works in sandbox)
SELECTOR=$(cast sig "balanceOf(address)")

# Encode the argument
DATA="${SELECTOR}000000000000000000000000<address-without-0x>"

# Make the call via curl
curl -s -X POST https://eth.llamarpc.com \
  -H "Content-Type: application/json" \
  --data "{\"jsonrpc\":\"2.0\",\"method\":\"eth_call\",\"params\":[{\"to\":\"0x...\",\"data\":\"$DATA\"},\"latest\"],\"id\":1}"

Offline commands work fine - These don't need network access:
- cast sig - Get function selector
- cast calldata / cast abi-encode - Encode calldata
- cast abi-decode / cast calldata-decode - Decode data
- cast from-wei / cast to-wei - Unit conversions
- cast keccak - Hash data