Hard Fork

Hard forks change the consensus rules in ways old software rejects. The 2017 block-size dispute produced Bitcoin Cash via hard fork. Hard forks force every participant to choose: upgrade, or be left on a chain the rest of the network considers wrong. Bitcoin's culture is correspondingly hostile to hard forks — they're a last resort, not a regular tool.

A hard fork can ride on top of either looser rules (bigger blocks, new opcodes that old software treats as failures) or different rules entirely. When the chain splits, both halves have valid history up to the split block, but every block after is canonical only on its own side. Wallets need to know which chain a transaction lives on; "replay protection" is the technique a forking chain uses to ensure transactions on one chain aren't accidentally valid on the other.

The reason Bitcoin treats hard forks as a last resort isn't ideology — it's that a hard fork imposes a coordination cost on every user, exchange, custodian, miner, and library author simultaneously. If a soft fork can deliver the same effect with backward compatibility, it almost always wins on those grounds alone.