Soft Fork
network · advanced
A consensus rule change that tightens what is valid. Old nodes still accept new blocks; they just don't enforce the new rule.
SegWit (2017) and Taproot (2021) were soft forks. They added new rules that older software treats as opaque; new software enforces them. Because old nodes still see the chain as valid, soft forks can be deployed without forcing every user to upgrade — though wider node adoption strengthens enforcement.
Soft forks work by tightening: a rule that was previously allowed becomes disallowed. Old nodes still accept tightened blocks because the tightened rules are a subset of the old rules. The classic mechanism for adding new functionality without breaking old software is "anyone can spend" — a new output type is reserved for the soft fork, defined by upgraded nodes as having specific spending conditions, and treated by old nodes as freely spendable. Old nodes therefore never object.
Activation is the political layer. BIP-9 (miner version signaling), BIP-8 (lock-in via flag day or signaling), and the "user-activated soft fork" (UASF) approach all coordinate when a soft fork's new rules become enforced. Taproot used Speedy Trial, a fast version of BIP-8 with a 3-month signaling window; it activated in 9 weeks.