Miner-Activated Soft Fork (MASF)

A Miner-Activated Soft Fork (MASF) is a backward-compatible protocol upgrade that switches on after a supermajority of miners signal their support by setting a bit in the blocks they produce. It is the historically dominant activation method and the mechanism formalized by BIP9 version bits, where activation locks in once a threshold—classically 1,916 of 2,016 blocks, or 95%—signals within a single difficulty period.

How Signaling Works

Each candidate soft fork is assigned a version bit. While the deployment is in its signaling window, miners set that bit in their block headers to indicate readiness. If enough blocks signal during a retarget period, the rule change moves to a locked-in state and then becomes active and enforced. If the timeout passes without reaching the threshold, the deployment fails and the bit is freed for reuse.

Strengths and Trade-offs

MASF is coordinated and low-drama when miners broadly agree, because signaling gives the ecosystem a clear, on-chain readiness measurement before rules turn on. Its weakness is that it effectively grants a minority of hashpower a veto: if even a small fraction declines to signal, activation can stall indefinitely, exactly the deadlock that gave rise to user-activated alternatives. Because miners are signaling readiness rather than approval, MASF works best for changes the technical community already considers safe.

MASF is the counterpart to the User-Activated Soft Fork, and the supermajority figure it relies on is the deployment's activation threshold. Both approaches aim to upgrade Bitcoin without forcing a contentious split.