A soft fork tightens Bitcoin's rules so old nodes keep working; a hard fork loosens them so old nodes reject the new chain. Learn the difference.
Soft fork: stricter rules
New rules only forbid things that used to be allowed. Blocks valid under the new rules are still valid to old nodes, so the network stays together even if some nodes don't upgrade.
Examples: Pay to script hash (P2SH) (2012), CLTV (2015), CSV (2016), Segregated Witness (SegWit) (2017), Taproot (2021).
Hard fork: looser rules
New rules allow something old nodes reject, like bigger blocks. Old nodes see those blocks as invalid and stay on the old chain. If both sides keep going, you get two coins.
Bitcoin hasn't had a planned hard fork in its main history. In 2017, Bitcoin Cash split off through a hard fork to allow bigger blocks.
How soft forks activate
| Method | How it works |
|---|---|
| Miner signalling (BIP 9) | Activates when enough blocks in a period signal readiness |
| Speedy Trial | Short signalling window with a delayed activation (used for Taproot) |
| User-activated (UASF) | Nodes enforce from a date regardless of miner signals (BIP 148 in 2017) |
Signalling isn't voting: it coordinates timing so miners are ready to follow the new rules.
Bitcoin's soft forks
| Year | Upgrade | What it added |
|---|---|---|
| 2012 | P2SH (BIP 16) | Pay to script hash |
| 2015 | CLTV (BIP 65) | Absolute timelocks |
| 2016 | CSV (BIPs 68, 112, 113) | Relative timelocks |
| 2017 | SegWit (BIP 141) | Witness data, weight limit |
| 2021 | Taproot (BIPs 340–342) | Schnorr, Taproot, Tapscript |
Key takeaways
- Soft forks tighten rules and stay compatible with old nodes.
- Hard forks loosen rules and can split the chain.
- SegWit and Taproot were soft forks.
Common questions
Is a hard fork always bad?
Not inherently, but it needs everyone to upgrade, so in Bitcoin it's seen as risky and is avoided.
Do I need to upgrade for a soft fork?
Your old node keeps working, but to enforce the new rules yourself, you upgrade.
Is a chain split always a hard fork?
Splits come from rule differences. A hard fork that not everyone adopts creates a split; a soft fork usually doesn't.
What was Bitcoin Cash?
A 2017 hard fork that split from Bitcoin to allow larger blocks; it is a separate coin.