Fork

Hard forks vs. soft forks

The two main categories:

• Soft fork — backward-compatible. New rules are stricter than old rules, so non-upgraded nodes still recognize new blocks as valid. The chain doesn't split; old nodes just don't enforce the new rules. Examples: Bitcoin's SegWit and Taproot activations.
• Hard fork — backward-incompatible. Non-upgraded nodes reject the new blocks, potentially producing two separate chains. Examples: Ethereum/Ethereum Classic split, Bitcoin/Bitcoin Cash split.

In a hard fork, if both forks have ongoing community and developer support, two separate cryptocurrencies result. If one fork has minimal support, the other becomes the dominant chain and the smaller fork either dies or persists as a niche asset.

Famous forks

• Ethereum/Ethereum Classic (July 2016). Following the DAO hack, Ethereum forked to reverse the stolen funds. Ethereum Classic continued the unforked chain. Ethereum proper is the dominant chain by economic activity; ETC persists as a smaller alternative.
• Bitcoin/Bitcoin Cash (August 2017). Bitcoin Cash forked to increase block size. Subsequent further forks (Bitcoin SV) split off from Bitcoin Cash itself.
• Bitcoin Gold (October 2017). Forked Bitcoin to use a GPU-mineable algorithm instead of ASIC-dominated SHA-256.
• Ethereum's "Merge" (September 2022). Switched from proof-of-work to proof-of-stake. Some former Ethereum miners maintained EthereumPoW (ETHW), which has minimal economic activity.

Routine hard forks

Most hard forks aren't contentious — they're scheduled protocol upgrades with broad community support. Ethereum has hard-forked multiple times (Frontier → Homestead → Byzantium → Constantinople → Istanbul → Berlin → London → Paris → Shanghai → Cancun → Pectra) without producing meaningful chain splits because everyone upgrades together.

These routine forks are sometimes called "upgrades" rather than "forks" in casual conversation, even though they're technically hard forks under the consensus definition.

What happens to assets

When a chain hard-forks:

• Holders typically receive equivalent balances on both chains. A user holding 10 BTC at the Bitcoin Cash fork received 10 BTC on the original chain plus 10 BCH on the new chain.
• Each chain is then independent. Subsequent transactions on one chain don't affect the other.
• Replay attacks are possible if both chains share transaction formats. Most modern forks include "replay protection" — chain identifiers preventing transactions from being valid on both chains.

For exchanges and custodians, supporting forks is operationally complex. Some support both forks; some only support the dominant chain; users with funds on the unsupported chain can lose access.

Why forks happen

Several reasons chains fork:

• Protocol upgrades — most common cause; new features require code changes.
• Disagreements — community splits over direction. The Bitcoin block-size debate produced multiple forks.
• Emergency response — Ethereum forked to reverse the DAO hack, prioritizing user fund recovery over strict immutability.
• Failed upgrades — sometimes a planned soft fork doesn't activate cleanly and produces an unplanned split.

Forks and immutability

The Ethereum DAO fork remains controversial because it violates a strict reading of "immutability." The community decided to retroactively change the chain's history to undo a theft.

Defenders argue this was an extraordinary case warranting extraordinary measures; the fork preserved overall trust in Ethereum more than it harmed it.

Critics argue any precedent for reversing transactions undermines the value proposition. Ethereum Classic specifically exists as the chain where this didn't happen.

The bigger picture: even Bitcoin's "immutability" is enforced socially, not just technically. If a sufficient supermajority of stakeholders agreed to fork Bitcoin, they could. The fact that they don't is a social/economic property, not a pure technical one.

Soft forks in practice

Soft forks are typically used for:

• Adding new features that older nodes can ignore (SegWit transactions look "legal but unspendable" to old Bitcoin nodes).
• Tightening rules (a stricter version of an existing requirement).
• Bug fixes that don't change the fundamental protocol.

Activation usually requires miner signaling and supermajority support. Some Bitcoin soft forks (BIP 9, BIP 8 activation) had complex coordination dynamics.

In smart-contract platforms

Ethereum and other smart-contract platforms fork at the protocol level the way Bitcoin does, but they also see "forks" at the application layer:

• Forking a DEX — copying the open-source code and deploying as a competitor. SushiSwap forked Uniswap; many others have done the same.
• Forking a chain itself — copying the chain's state and code and running it separately. The "based forks" of various L1s have produced derivative chains (Avalanche subnets, Optimism's Bedrock-derived chains, etc.).

Application-level forking is one of crypto's distinguishing features. Open-source code plus on-chain state means competitors can stand up an alternative quickly. The 2020 SushiSwap "vampire attack" — incentivizing Uniswap LPs to migrate to a forked DEX — was an early demonstration of this dynamic.

What forks mean for users

For typical users:

• Routine upgrades — minor inconvenience; software updates, possibly briefly downgraded experience around the fork.
• Contentious forks — opportunity to claim "forked" assets (sometimes meaningful, often not). Need to manage replay risk if not handled by the protocol.
• Application forks — typically irrelevant; the original protocol continues working normally.

The high-stakes contentious forks of crypto's earlier years are increasingly rare. Both Bitcoin and Ethereum have settled into more stable governance with broadly supported upgrades and fewer dramatic splits.