Invariant testing in Solidity focuses on proving that certain properties of a smart contract always hold true, no matter what inputs or transactions occur. These tests run random sequences of actions to check if core rules — like balance consistency, ownership logic, or access permissions — ever break. By doing this, developers can find deep logical bugs or regressions that normal unit tests might miss. It’s one of the most reliable QA methods to ensure contract integrity and security over time.

Snapshots capture the contract’s state at a moment in time. They help debug failed transactions and compare pre- and post-execution states.