Soft Fork

A soft fork is a change to a blockchain’s rules that is backward compatible, meaning nodes that have not upgraded their software can still participate in the network without being kicked off. While a hard fork creates a permanent split if not everyone upgrades, a soft fork tightens the existing rules in a way that old nodes can still accept new blocks — they just cannot produce certain types of blocks themselves. The result is that soft forks can be deployed more gradually and with less coordination risk than hard forks.

The key is that a soft fork makes the rules more restrictive, not more permissive. If the old rules allowed blocks up to 2 megabytes and the new rules restrict them to 1 megabyte, an old node will still accept the new 1-megabyte blocks because they technically comply with the old rules. However, if an old node tries to produce a 1.5-megabyte block, the upgraded nodes will reject it. Bitcoin’s Segregated Witness (SegWit) upgrade in 2017 is the most famous soft fork.

Example: Imagine a city that originally allowed cars of any size on all roads. A soft fork would be like a new rule saying only cars under a certain width can use the main highway. Old narrow cars still qualify. Only very wide vehicles get excluded. The transition happens gradually rather than all at once.