Crypto glossary
Fraud proof (fault proof)
A fraud proof lets anyone challenge an invalid state root posted by an optimistic rollup, proving on the base chain that the operator made a mistake.
Optimistic rollups post new state roots to Ethereum without proving them, on the assumption that they are correct. A fraud proof, also called a fault proof, is the mechanism that keeps that optimism honest: during a challenge window, any watcher who disagrees with a proposed root can open a dispute. The two parties narrow their disagreement step by step until a single instruction is executed on Ethereum, which decides who was right.
Fraud proofs are what separate a real optimistic rollup from a multisig bridge. For years major rollups ran without live proofs or with only whitelisted challengers; permissionless systems such as the OP Stack fault proofs and Arbitrum's BoLD went live in 2024 and 2025, a key step toward higher maturity stages.
The model needs at least one honest, online and funded challenger, and it is the reason for the roughly seven-day withdrawal delay. Bonds, delay attacks and bugs in the dispute game are active research topics. A fraud proof is reactive and slow, while a validity proof shows correctness upfront.
Example
A watcher node sees a proposed Arbitrum state root that does not match its own execution, posts a bond and starts a dispute that ends with the incorrect proposal being rejected on Ethereum.