Soft fork
A backwards-compatible update to a protocol's rules, where old nodes can continue operating on the upgraded network without being forced to update.
How it works
A soft fork is a rules update in which the new rules are stricter than the old ones. Blocks produced under the new rules remain valid for old nodes as well, so the network does not split even if not everyone upgrades.
The difference from a hard fork lies in direction. A hard fork widens what is permitted — old nodes stop recognising new blocks and the chain splits in two. A soft fork narrows it, and compatibility survives one way.
The flip side of that compatibility is that old nodes do not fully verify the new rules. They see a block as valid while not understanding part of its contents. This makes a soft fork gentler in consequences but less transparent in substance.
How it looked in Bitcoin
- SegWit, 2017Signatures were moved into a separate part of the block. Old nodes saw such transactions as valid without parsing the new format. The upgrade laid the ground for the Lightning network.
- Taproot, 2021Added a new signature scheme and made complex spending conditions indistinguishable from ordinary transfers. Activated with over 90% of hash power in support.
- The 1 MB block size limitIntroduced in 2010, also as a tightening: before that there was no limit. The later dispute over changing it produced the hard fork that created Bitcoin Cash.
What to watch for
For a coin holder a soft fork usually passes unnoticed: no new coins appear, the wallet keeps working, nothing needs doing. That is precisely what distinguishes it from a hard fork, after which you hold a balance on two networks at once.
The one thing worth checking is whether your wallet supports the new capabilities. An old version will keep sending and receiving but will not let you use what the upgrade added — cheaper fees from a new address format, for instance.