Soft fork
A soft fork is a backwards-compatible update to Bitcoin's consensus rules, where new rules are stricter than old ones, letting non-upgraded nodes continue following the chain.
In a soft fork existing rules are tightened: what was valid before may now be invalid; what was invalid remains invalid. Old nodes still accept the new blocks (they see for instance the new witness data but still judge the transaction as valid). SegWit (2017) and Taproot (2021) are iconic soft forks. Activation usually via miner signalling (BIP 9/8) or a 'speedy trial' with deadline.
Example
Taproot activation in November 2021: nodes of v0.21+ got new rules for Taproot spends. Old nodes (v0.20) accept Taproot transactions as 'anyone-can-spend' — technically valid for them, but new rules prevent abuse.
Frequently asked questions
Soft fork or hard fork — which is safer?
Soft fork. No chain split required, slow steady adoption, existing nodes keep working. Hard forks create two chains if not everyone upgrades.
Can a soft fork be cancelled?
Technically yes, but practically very hard once miners and users follow the new rules. An 'un-fork' would itself be a new fork — almost never done in Bitcoin.
Related terms
Further reading
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