What Is a Soft Fork vs a Hard Fork?

Bitcoin has changed its rules many times since 2009 without ever needing a single company to push an update to everyone’s phone. How that actually happens splits into two very different categories: soft forks and hard forks.

A soft fork is a rule change that tightens Bitcoin’s existing rules — it makes something that used to be valid now invalid, but anything valid under the new rules is still valid under the old rules too. Nodes that haven’t upgraded can still verify blocks created under the new rules as valid. SegWit and Taproot were both soft forks.

A hard fork is a rule change that loosens or changes the rules in a way that’s not backward-compatible — blocks valid under the new rules would be rejected as invalid by nodes still running the old software. Every node must upgrade, or the network permanently splits into two separate chains.

That splitting risk is exactly what happened in 2017, when a disagreement over how to scale Bitcoin’s transaction capacity led to Bitcoin Cash splitting off via a hard fork. Anyone holding Bitcoin at the moment of the split ended up holding an equivalent balance on both resulting chains.

Why does Bitcoin’s development culture lean so heavily toward soft forks when possible? Because a soft fork can be activated gradually, with nodes upgrading at their own pace. A hard fork requires near-universal, coordinated agreement to avoid a permanent split — a much higher bar to clear with no central authority who can mandate it.

Both mechanisms exist for good reasons, and neither is inherently better — soft forks favor cautious, gradual, backward-compatible evolution, while hard forks are sometimes the only option when a change genuinely can’t be made compatible with the old rules.

Curious about the actual process for proposing and coordinating one of these changes? Continue learning in the Bitcoin Academy.