Soft Fork vs Hard Fork: How Blockchains Change Their Rules

A blockchain has no head office that can push an update overnight. Thousands of independent computers each enforce the same rulebook, so changing a rule means persuading all of them to change together.

A fork is what happens when those rules change. The soft fork vs hard fork question decides who has to upgrade, what happens to anyone who doesn't, and whether the network could end up as two separate coins.

What a fork means in a blockchain

Every full node checks each new block against the consensus rules: what a valid transaction looks like, how large a block may be, how new coins are issued. A block that breaks those rules is rejected, no matter who produced it. If you want the background on how that works, see our guide to blockchain networks and nodes.

A fork is a change to the rulebook. Developers write the change into new versions of the node software, and the network then has to move from the old rules to the new ones. There are two broad ways to do that, and the difference between a hard fork and soft fork in blockchain terms comes down to compatibility with that old software.

The word also gets used for something more ordinary. Now and then two miners find a block at almost the same moment, and the chain splits for a block or two until one branch grows longer and the other is dropped. That is normal network behaviour, not a change of rules.

Soft fork: tightening the rules

A soft fork makes the rules stricter. Something that used to be allowed becomes invalid, yet every block produced under the new rules still looks valid to nodes running the old software.

That is what backward compatible means here. Nodes that don't upgrade keep following the same chain. They simply stop checking the new restriction for themselves, so they get slightly weaker guarantees than upgraded nodes, but they are not cut off.

The catch is that whoever produces blocks has to enforce the new rule. If most of the hash power on a proof-of-work chain ignores it, rule-breaking blocks can keep appearing and upgraded nodes would reject them. That is why soft forks in Bitcoin have traditionally waited for a large majority of miners to signal they are ready before switching on.

Hard fork: changing the rules outright

A hard fork loosens or rewrites the rules so that some new blocks are invalid under the old software. Old nodes reject those blocks and stay stuck on a chain the rest of the network has left behind.

That makes a hard fork an all-or-nothing upgrade for node operators. Anyone who wants to stay on the main network has to install the new version before the activation point, which is usually a set block height or timestamp.

When nearly everyone upgrades, the old chain withers and nobody notices a split. When a meaningful group refuses, both chains can keep running, and one network becomes two with a shared history.

Hard vs soft fork at a glance

Compatibility is the core difference. A soft fork is backward compatible and a hard fork is not, so old nodes keep working after a soft fork and fall off the network after a hard fork.

The direction of the change differs too. Soft forks restrict what is allowed. Hard forks expand or replace it, for example by raising a block size limit, changing how fees work or swapping the consensus mechanism.

So does the question of who has to act. In a soft fork, block producers must enforce the new rule, but ordinary node operators can upgrade at their own pace. In a hard fork, everyone running a node has to upgrade, including exchanges and wallet providers.

Finally there is the risk of a split. A poorly supported soft fork can cause a brief split, but it does not normally create a new coin. A contested hard fork can leave a permanent second chain with its own token.

Soft fork examples from Bitcoin

Bitcoin has made almost all of its major changes through soft forks, which suits a network that values keeping old software working. Most textbook examples of a soft fork in crypto come from its history. Pay to Script Hash, which let people send coins to complex scripts such as multisignature setups using an ordinary-looking address, activated this way in April 2012.

Segregated Witness, better known as SegWit, is the most famous of them. It moved signature data into a separate part of each transaction, fixed a long-standing bug called transaction malleability and cleared the way for the Lightning Network. It activated at block 481,824 on 24 August 2017.

Getting there showed how political a soft fork can be. Miner signalling sat far below the 95% threshold for months, until a group of users proposed BIP148, a user-activated soft fork that would reject blocks not signalling support. A compromise proposal, BIP91, then brought miners on board.

Taproot followed at block 709,632 on 14 November 2021, adding Schnorr signatures and more flexible scripts. It used a shorter activation process called Speedy Trial, which needed 90% of blocks in a two-week window to signal support.

Hard fork examples, planned and contested

Ethereum takes the opposite approach. Its regular network upgrades, including London in 2021, the Merge in September 2022 and Pectra in May 2025, were all hard forks. The Merge, which moved Ethereum from proof of work to proof of stake, was one of the biggest rule changes a major chain has ever made, and it still went through without a lasting split.

These upgrades don't divide the network because they are planned months ahead, tested on public testnets and shipped by every major client team. The most recent large one, Fusaka, activated on 3 December 2025 and introduced PeerDAS, which lets nodes check data availability without downloading every piece of data. Our explainer on the Fusaka upgrade covers it in more detail.

Contested hard forks look very different. In 2016 an attacker drained more than 3.6 million ETH from a smart contract called The DAO. Ethereum hard forked at block 1,920,000 on 20 July 2016 to move those funds into a contract where investors could withdraw them.

A minority rejected the change on principle, arguing that the chain's history should never be rewritten. They kept the original chain running, and it became Ethereum Classic.

Bitcoin Cash split from Bitcoin on 1 August 2017, with block 478,558 the last block the two chains share. Its supporters wanted bigger blocks rather than SegWit. In November 2018 Bitcoin Cash split again when Bitcoin SV broke away over the same kind of disagreement.

What a chain split means for your coins

When a hard fork leaves two surviving chains, the shared history means your balance exists on both. Anyone holding BTC in a wallet they controlled at the moment of the Bitcoin Cash split ended up with the same amount of BCH.

Whether you can actually reach forked coins depends on who holds your keys. If an exchange had custody, it decides whether to support the new chain and credit customers, and exchanges have taken different positions on different forks. Check their announcements rather than assuming.

Splits also bring replay risk. Without protection, a transaction signed on one chain can be valid on the other, so sending coins on one chain could move them on both. Bitcoin Cash shipped replay protection from the start, while Ethereum and Ethereum Classic only added it months after their split, both using EIP-155.

Forks attract scams as well. Fake wallets and websites promising to help you claim forked coins have been used to steal recovery phrases for years. A genuine fork never requires you to type your seed phrase into a website.

How to tell which kind of fork is coming

Whether you are reading about a hard fork vs soft fork on Bitcoin or Ethereum, most are announced well ahead of time. Ethereum upgrades are published on the Ethereum Foundation blog and in client release notes with exact activation times. Bitcoin proposals go through the public BIP process, and miner signalling can be watched block by block.

A simple test is to ask whether old software will still accept the new blocks. If it will, it is a soft fork and most people can wait. If it won't, it is a hard fork, and node operators need to upgrade before the deadline.

If you just hold coins with a reputable wallet or exchange, a routine upgrade usually needs nothing from you. The moments that deserve attention are when a fork is contested, when a second coin might appear, or when someone is urging you to move funds in a hurry.