Starknet expresses its block-validity rules through the Starknet Operating System, or SNOS, a Cairo program. Given an initial state and transactions, this program checks the execution and produces the resulting state. A proof of that computation lets Ethereum verify the corresponding state transition without replaying every application instruction. The precise program matters: proving an arbitrary calculation would not establish that a block followed Starknet's rules. Proofs therefore connect a particular execution specification to the state accepted by the settlement contract.

Starknet uses account contracts to validate and execute a user's requests. The account can implement its own authorization scheme instead of relying exclusively on a single fixed signature format. This permits features such as multiple signers, but also places responsibility on the account code. Starknet's documentation highlights validation requirements because an incorrectly implemented execution entry point can bypass intended checks. A wallet's interface and backup method should consequently be evaluated alongside the actual account contract, its deployment, and any upgrade controls.

The transaction API distinguishes execution status from finality status. A transaction may succeed on Layer 2 before the corresponding state is accepted on Layer 1. It can also be included with a reverted execution, in which case fees and nonce changes can still occur even though application changes are rolled back. These distinctions explain apparent contradictions such as a paid fee accompanying a failed action. Cairo proofs establish correct execution of rules; they do not automatically make application inputs secret or certify that the application is economically safe.