Multiple contracts in one file

Severity: Info ID: multi-contract-file

Flags source files that declare more than one top-level contract, interface, or library.

What it does

Reports each top-level contract, interface, or library definition (after the first) in a file that contains more than one such declaration.

Why is this bad?

Keeping one contract per file improves discoverability (grep, IDE jump-to-file), simplifies import paths, and avoids unintentional bytecode bloat from artifacts that bundle unrelated contracts.

Example

Bad

solidity

// File: Token.sol
contract TokenA { /* ... */ }
contract TokenB { /* ... */ }

Good

solidity

// File: TokenA.sol
contract TokenA { /* ... */ }

// File: TokenB.sol
contract TokenB { /* ... */ }

Configuration

Set multi_contract_file_exceptions under [lint.lint_specific] in foundry.toml to allow multiple interfaces, libraries, or abstract contracts in one file. Regular contracts cannot be exempted.

toml

[lint.lint_specific]
multi_contract_file_exceptions = ["interface", "library", "abstract_contract"]