What Is a Hard Fork?
A hard fork is a protocol change that introduces new rules incompatible with older validation standards.
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A hard fork is a protocol upgrade that makes a blockchain’s rules incompatible with older versions. Blocks produced under the new rules are rejected by nodes that have not upgraded. Every participant therefore has to update their software; if a group refuses, the chain can split permanently in two.
How does a hard fork work?
Each node checks incoming blocks against its own rules. A hard fork makes something valid that was previously invalid, such as raising the block size limit or adding a new transaction type. Upgraded nodes accept these blocks while old nodes reject them. From a set block height onward, the two groups follow different chains.
If the whole network adopts the change, the old chain is simply abandoned and a single chain continues. Most of Ethereum’s regular network upgrades happen this way, as planned and uncontested hard forks.
Planned upgrades vs. contentious splits
Planned upgrade: Developers and the ecosystem agree in advance and the software activates on a set date. Ethereum’s London (2021), Merge (2022) and Dencun (2024) upgrades are examples; no new coin is created.
Contentious split: Part of the community rejects the change and continues under the old rules. Two separate chains and two separate coins result. At the moment of the split, balances on the old chain usually exist on both chains.
Well-known hard forks
Bitcoin Cash (2017): Supporters of larger blocks split from Bitcoin and launched a chain that allows bigger blocks. At the split, BTC holders received the same amount of BCH.
Ethereum Classic (2016): To recover funds drained in The DAO exploit, the Ethereum community hard-forked to rewrite chain history. Those who rejected the change kept the original chain, known today as Ethereum Classic.
Hard fork vs. soft fork
A soft fork tightens the rules and is backward compatible; old nodes keep accepting new blocks. A hard fork changes the rules, requires the whole network to upgrade and can lead to a permanent split. Which approach is chosen depends on the technical nature of the change and on community support.
Risks for users
- Replay attacks: If two chains share the same transaction format, a transaction signed on one can be valid on the other. Split chains usually add protection; confirm it exists before transacting.
- Exchange support: Exchanges may pause deposits and withdrawals during a split and decide independently whether to credit the new coin.
- Scams: Sites asking you to “enter your private key to claim your fork coin” are the most common trap. Never enter your private key or recovery phrase on any website.