FAQ

Common questions about Foundry.

General

What is Foundry?

Foundry is a fast, portable, and modular toolkit for Ethereum development written in Rust. It includes:

• Forge — Build, test, and deploy smart contracts
• Cast — Interact with the chain from the command line
• Anvil — Local Ethereum node for development
• Chisel — Interactive Solidity REPL

How do I install Foundry?

Use foundryup:

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

See Installation for details.

How do I update Foundry?

Run foundryup again:

$ foundryup

To install a specific version:

$ foundryup --version nightly-abc123

How do I get help with a command?

All Foundry tools support --help:

$ forge --help
$ forge test --help
$ cast --help
$ anvil --help
$ chisel --help

See the CLI reference for full command docs.

Forge

How do I run a specific test?

Use --match-test:

$ forge test --match-test test_Transfer

Use --match-contract for a specific contract:

$ forge test --match-contract TokenTest

How do I see console.log output?

Use verbosity level 2 or higher:

$ forge test -vv

Why are my tests slow?

Common causes:

1. Forking without caching — Enable fork caching with --fork-cache
2. Too many fuzz runs — Reduce fuzz.runs in foundry.toml for local development
3. Large contracts — Consider splitting tests into separate files

How do I debug a failing test?

Use the interactive debugger:

$ forge test --debug test_MyTest

Or increase verbosity to see the trace:

$ forge test --match-test test_MyTest -vvvv

See Debugging for more.

How do I fork mainnet in tests?

Set --fork-url or configure it in foundry.toml:

$ forge test --fork-url https://ethereum.reth.rs/rpc

Or in foundry.toml:

[profile.default]
fork_url = "https://ethereum.reth.rs/rpc"

How do I run tests in parallel?

Tests run in parallel by default. To control parallelism:

$ forge test --jobs 4

Set to 1 for sequential execution (useful for debugging):

$ forge test --jobs 1

Cast

How do I call a contract function?

Use cast call for read-only calls:

$ cast call $CONTRACT "balanceOf(address)" $ADDRESS

Use cast send for state-changing transactions:

$ cast send $CONTRACT "transfer(address,uint256)" $TO $AMOUNT --private-key $KEY

How do I decode transaction data?

Use cast 4byte-decode:

$ cast 4byte-decode 0xa9059cbb000000000000000000000000...

How do I convert between units?

Use cast to-wei and cast from-wei:

$ cast to-wei 1 ether
# 1000000000000000000
 
$ cast from-wei 1000000000000000000
# 1.000000000000000000

Anvil

How do I get test ETH?

Anvil pre-funds 10 accounts with 10,000 ETH each. List them:

$ anvil
# Shows accounts and private keys on startup

How do I impersonate an account?

Start Anvil with auto-impersonation:

$ anvil --auto-impersonate

Or impersonate a specific account via RPC:

$ cast rpc anvil_impersonateAccount $ADDRESS

How do I reset the chain state?

Use the RPC method:

$ cast rpc anvil_reset

Or restart Anvil.

How do I mine blocks manually?

Disable auto-mining and mine manually:

$ anvil --no-mining
# In another terminal:
$ cast rpc anvil_mine 10  # Mine 10 blocks

Chisel

How do I access my project's contracts?

Chisel automatically loads your project. Import and use contracts:

➜ import {Counter} from "src/Counter.sol";
➜ Counter c = new Counter();
➜ c.increment();

How do I fork in Chisel?

Start with a fork:

$ chisel --fork-url https://ethereum.reth.rs/rpc

Or fork during a session:

➜ !fork https://ethereum.reth.rs/rpc

How do I save my session?

Use the !save command:

➜ !save my-session

Load it later:

$ chisel load my-session