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This paper introduces MonadBFT, a novel Byzantine Fault Tolerant (BFT) consensus protocol that advances both performance and robustness. As a HotStuff-family protocol, MonadBFT features linear message and authenticator complexity, where authenticators refer to signatures or partial signatures, on the happy path and is optimistically responsive, operating as quickly as the network allows. One of the central features of MonadBFT is speculative commitment, which provides the prospective state after three message delays, with reverts occurring only in the rare event of provable leader equivocation. This feature is important for two reasons: first, it decreases the latency for users. Second, it makes MonadBFT resistant to a specific form of Maximal Extractable Value (MEV) attack known as tail-forking. Tail-forking occurs when a malicious leader forks away from its predecessor’s block, causing that block to be abandoned and depriving the predecessor of rewards. This allows the malicious leader to reorder, steal, or exploit transactions, thereby exacerbating MEV exploitation. MonadBFT effectively mitigates such vulnerabilities, ensuring fairness and integrity in transaction processing. To our knowledge, no other pipelined leader-based protocol combines all these features. To make reverts of speculatively committed blocks even less likely, we extend our protocol to Accountable MonadBFT: whenever a speculatively committed block is reverted, at least one-sixth of all validators are provably culpable, with evidence enabling penalties such as slashing.