Crypto glossary
Soft fork
A soft fork is a backward-compatible upgrade to a blockchain's rules that tightens what counts as valid, so nodes that do not upgrade keep working.
A soft fork changes a blockchain's consensus rules in a way that older software still accepts. The new rules only restrict what was allowed before, so blocks produced under them still look valid to nodes that have not updated. As long as most block producers enforce the new rules, the network stays on a single chain.
Bitcoin has delivered most of its major upgrades as soft forks, including SegWit in August 2017 and Taproot in November 2021. Activation usually depends on miners signalling readiness in the blocks they mine, sometimes with a deadline attached, and the debates over how to activate an upgrade can be as heated as those about the features themselves.
Soft forks avoid splitting the network and the coin, which is why conservative communities prefer them. The trade-off is that non-upgraded nodes validate less than they think they do, and if miner support is weak, a temporary chain split is still possible.
Example
SegWit was added as a soft fork: a node running Bitcoin software from 2016 could keep following the chain after August 2017, even though it did not understand the new transaction format.
