A soft fork changes a blockchain's rules in a backward-compatible way. Old nodes can still validate new blocks. They may not understand the new rules, but they accept the blocks as valid. That means a soft fork requires only a majority of miners or validators to adopt it. Nodes that do not upgrade keep working. The chain does not split. Soft forks are the preferred way to upgrade a blockchain because they cause less disruption.

Bitcoin's SegWit upgrade in 2017 was a soft fork. It changed how transaction data is stored, which increased block capacity without changing the block size limit. Old nodes saw the blocks as valid, even though they could not process the new witness data. Not every soft fork is smooth. If a significant minority of miners refuses to adopt the new rules, they can still mine blocks that the upgraded nodes reject. That can cause confusion and temporary chain splits. Soft forks are less contentious than hard forks, but they are not risk-free. They require coordination and clear communication. The alternative is a hard fork, which forces everyone to upgrade or split the chain. Soft forks are the gentler path.

Soft fork characteristics

• Backward-compatible — old nodes still validate
• Majority hash rate required — miners must adopt
• No chain split — the network stays unified
• Example — Bitcoin SegWit in 2017

Soft forks are the standard upgrade mechanism for Bitcoin. They are conservative by design.