This paper introduces MonadBFT, a novel Byzantine Fault Tolerant (BFT) consensus protocol that advances both performance and robustness. MonadBFT is implemented as the consensus protocol in the Monad blockchain. As a HotStuff-family protocol, MonadBFT has linear message complexity in the common case and is optimistically responsive, operating as quickly as the network allows. A central feature of MonadBFT is its tail-forking resistance. In pipelined BFT protocols, when a leader goes offline, the previous proposal is abandoned. Malicious leaders can exploit this tail-forking behavior as a form of Maximal Extractable Value (MEV) attack by deliberately discarding their predecessor's block, depriving that proposer of rewards and enabling transaction reordering, censorship or theft. MonadBFT prevents such tail-forking attacks, preserving both fairness and integrity in transaction execution. Another related feature of MonadBFT is its notion of speculative finality, which enables parties to execute ordered transactions after a single round (i.e., a single view), with reverts occurring only in the rare case of provable leader equivocation. This mechanism reduces user-perceived latency. Additionally, we introduce the leader fault isolation property, which ensures that the protocol can quickly recover from a failure. To our knowledge, no prior pipelined, leader-based BFT consensus protocol combines all of these properties in a single design.

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