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:

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

2. 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}"
   

3. 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