What Is Soft Fork?

In a soft fork, the new rules are a subset of the old rules. Upgraded nodes accept only blocks that follow the new rules, while nodes that have not upgraded also see these blocks as valid, because the blocks do not violate their rules either. This allows the network to adopt the change without splitting into two separate chains. For the change to work safely, a large majority of block producers, miners in Bitcoin, must enforce the new rules.

The SegWit (2017) and Taproot (2021) upgrades in Bitcoin were rolled out as soft forks. In Bitcoin, such changes are mostly activated by miners signaling support in their blocks; when enough support exceeds the threshold within a certain period, the new rules take effect. In a hard fork, on the other hand, because the rules are expanded or changed, nodes that do not upgrade reject the new blocks and a permanent chain split can occur.

Soft forks are advantageous for preserving the integrity of the network, but they are not free from criticism. Nodes that do not upgrade cannot fully verify the new features and must trust the upgraded majority on this point. In addition, activation methods can lead to serious debates within the community over the say of miners and users.