Soft Fork

A soft fork is a backward-compatible change to a blockchain's consensus rules that tightens what counts as a valid block or transaction. Nodes that have not upgraded still accept blocks produced under the new rules, so the network avoids a permanent split. Bitcoin's Segregated Witness upgrade, activated in August 2017, and Taproot, activated in November 2021, were both soft forks.

Why it matters

Soft forks are how bitcoin evolves without forcing every participant to move at once. Because old software keeps working, upgrades can roll out gradually while the network stays whole. The alternative, a hard fork that loosens rules, splits the chain whenever part of the network declines to follow, as happened when Bitcoin Cash separated in August 2017. The preference for soft forks reflects a deeper principle: changes to money should be opt-in and conservative.

How it works

A soft fork makes the new rule set a strict subset of the old one: everything valid after the change was also valid before, but not the reverse. Miners typically signal readiness through version bits in the blocks they mine, a coordination method described in BIP 9, and the new rules lock in once a signaling threshold is reached. As long as a majority of hash power enforces the tighter rules, the upgraded chain remains the longest valid chain for every node, old or new.