What Simplex BFT is
Simplex BFT is Tempo's consensus algorithm, provided by the open-source Commonware library. It delivers deterministic sub-second finality: once a block is marked final — in well under a second — its transactions are permanently guaranteed and cannot be re-orged.
This is different from probabilistic chains where finality strengthens over time. On Tempo, finality is a binary event, which is what a payment rail needs: a merchant can treat a confirmed transfer as settled.
Safety over liveness
Simplex BFT prioritises safety over liveness. It requires a 2f+1 quorum to progress, and will halt rather than finalise conflicting blocks if more than one-third of validators are unavailable.
In practice this is a deliberate trade-off: Tempo would rather stop than ever produce two conflicting final histories. For money movement, a brief halt is recoverable; a double-spend or re-org is not.
VRF leader election
Block proposers are chosen using VRF leader election — a Verifiable Random Function embedded in Commonware's Simplex variants — which picks proposers unpredictably without sacrificing safety or responsiveness. The same VRF also supplies post-facto execution randomness.
The Tempo × Commonware partnership was announced on 8 November 2025, with a stated goal of sub-250ms finality for a globally distributed payments network. Note that a separate, planned sub-blocks mechanism (which would let validators reserve blockspace across blocks they don't propose) is currently disabled — its enabling flag was removed — and is slated to return in a later release.