What is Miner Activated Soft Fork (MASF)?
Crypto Glossary Definition
A soft fork activated because miners signaled (voted) for it.
Why Miner Activated Soft Fork (MASF) Matters
A MASF relies on miners signaling support (usually via a threshold like 95% of mined blocks) before the new rules formally activate, which puts miners in a de facto gatekeeping role over protocol upgrades. This is part of why the alternative UASF model — where node operators rather than miners drive activation — became an important precedent during Bitcoin's SegWit rollout.
Miner Activated Soft Fork (MASF) in Practice
During Bitcoin's rollout of the SegWit upgrade, the proposed activation mechanism required miners to signal their readiness by setting a specific bit in the blocks they mined, with the rule stating that once 95% of blocks within a given period carried that signal, the new consensus rules would formally activate network-wide — a textbook Miner Activated Soft Fork. For months, signaling stalled well below that threshold, as a meaningful share of large mining pools held back their support, whether over technical disagreements or economic incentives tied to competing scaling proposals. Frustration grew within parts of the community who wanted the upgrade to proceed regardless of miner cooperation, since under a MASF model, miners effectively hold veto power over which rule changes get adopted even though they don't control the software everyone else runs. This standoff became a direct catalyst for the User Activated Soft Fork movement, where node operators and businesses instead committed to enforcing the new rules on a set date regardless of miner signaling, creating economic pressure that ultimately pushed enough miners to signal support before the UASF deadline arrived. The episode is frequently cited as a clear real-world illustration of the difference between MASF and UASF: one model treats miners as the deciding authority, the other treats the economic majority of node-running users as the true source of consensus.
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