A hard fork is a backward-incompatible change to a blockchain’s rules that requires nodes, miners, and other participants to upgrade to remain on the same network. If part of the community refuses to adopt the new rules, the blockchain can split into two separate chains, each continuing independently. A well-known example is the split between Bitcoin and Bitcoin Cash in 2017.

A soft fork is a backward-compatible upgrade where the new rules are stricter than the old ones, allowing non-upgraded nodes to still recognize the chain as valid even if they do not fully enforce the new functionality. Major Bitcoin upgrades such as SegWit in 2017 and Taproot in 2021 were implemented as soft forks.

Forks are a fundamental part of how blockchains evolve. Because decentralized networks have no central authority controlling upgrades, protocol changes must be coordinated across miners, developers, exchanges, wallet providers, and node operators. Whether a change is implemented as a hard fork or soft fork affects how disruptive the upgrade is and whether the blockchain risks splitting into competing networks.

What Is a Hard Fork?

A hard fork is a blockchain protocol upgrade that changes the network’s rules in a way that is incompatible with older versions of the software. Nodes running the old rules will reject blocks created under the new rules, while upgraded nodes will accept them. If both groups continue operating, the blockchain can permanently split into two independent networks. Key characteristics of a hard fork include:

  • Backward incompatibility: Nodes that do not upgrade cannot validate blocks produced under the new rules.
  • Network-wide coordination: Miners, nodes, exchanges, wallets, and other participants generally need to upgrade together to remain on the same chain.
  • Potential chain splits: If consensus is not reached, the network can divide into two separate blockchains, each with its own history, community, and token.
  • Broader protocol changes: Hard forks can introduce major functionality changes that may not be possible through soft forks alone.

Notable examples include the 2017 split between Bitcoin and Bitcoin Cash, as well as the 2016 division between Ethereum and Ethereum Classic following the DAO hack.

What Is a Soft Fork?

A soft fork is a blockchain protocol upgrade that changes the network’s rules while remaining backward compatible. Blocks created under the new rules are still recognized as valid by older nodes, even if those nodes do not fully understand or enforce the new functionality themselves.

Key characteristics of a soft fork include:

  • Backward compatibility: Non-upgraded nodes continue recognizing the blockchain as valid, allowing the network to remain unified during upgrades.
  • Gradual adoption: Miners, nodes, wallets, and exchanges can often upgrade over time rather than requiring simultaneous coordination across the entire network.
  • Lower risk of chain splits: Because older nodes do not reject blocks created under the new rules, soft forks are generally less disruptive than hard forks.
  • More limited upgrade scope: Soft forks typically add restrictions, efficiency improvements, or optional new features rather than fundamentally changing the network’s core rules.

Bitcoin has historically favored soft forks to minimize coordination risk and avoid splitting the network. Major Bitcoin soft forks include Pay to Script Hash (P2SH) in 2012, Segregated Witness (SegWit) in 2017, and Taproot in 2021.

Hard Fork vs. Soft Fork: What Are the Differences?

Hard forks and soft forks are both ways to upgrade blockchain protocols, but they differ in compatibility, coordination needs, and network risk.

  1. Compatibility: Hard forks are not backward-compatible. Nodes running old software will reject blocks created under the new rules. Soft forks are backward-compatible, meaning older nodes can still recognize upgraded blocks as valid, even if they do not enforce the new rules themselves.
  2. Coordination Requirements: Hard forks usually require broad coordination across miners, validators, node operators, exchanges, wallets, and the wider community. Without enough agreement, the blockchain may split. Soft forks can often activate more gradually, as long as enough mining power or validators enforce the upgraded rules.
  3. Risk Profile: Hard forks carry higher risks, including chain splits, duplicate tokens, replay attacks, and community fragmentation. Soft forks are usually less disruptive, but they are more limited because they must remain compatible with older nodes.
  4. Typical Use Cases: Hard forks are often used for major protocol changes, such as block size increases, new consensus rules, or controversial governance decisions. Soft forks are more common for incremental upgrades, including efficiency improvements, new signature systems, or enhanced scripting features.

What Happens During a Chain Split?

A chain split can happen when a hard fork becomes contentious and part of the community refuses to adopt the new rules. Both chains share the same transaction history before the fork, but after the split they operate as separate networks with different rules, communities, and native tokens.

Users who held coins before the split usually receive equivalent balances on both chains because the original ledger history is copied. However, each chain’s value, security, adoption, and long-term relevance can diverge significantly.

Notable blockchain splits include:

  • Bitcoin and Bitcoin Cash (August 2017): A disagreement over scaling and block size led to Bitcoin Cash launching as a separate blockchain with larger blocks.
  • Ethereum and Ethereum Classic (July 2016): After the DAO hack, most of the Ethereum community supported a hard fork to reverse the exploit, while a minority continued supporting the original chain.
  • Bitcoin Cash and Bitcoin SV (November 2018): Internal disagreements over scaling direction and protocol philosophy caused another major chain split.

Read More: What Is Bitcoin Cash (BCH) Payments Blockchain and How Does It Work?

What Are Replay Attacks?

Replay attacks are a risk after a blockchain split. Because both chains may initially share the same transaction history, wallet addresses, and cryptographic rules, a transaction signed on one chain may also be valid on the other. This means a user sending coins on one chain could unintentionally move the equivalent coins on the second chain as well.

To reduce this risk, major hard forks often add replay protection. Replay protection makes transactions valid only on the intended network, helping users safely separate assets across the two chains.

Why Does Bitcoin Prefer Soft Forks?

Bitcoin has historically preferred soft forks because they reduce disruption and lower the risk of splitting the network. This reflects Bitcoin’s conservative approach to protocol development, where stability, backward compatibility, and long-term security are prioritized over rapid experimentation.

Key reasons include:

  • Network unity: Soft forks help avoid chain splits and preserve Bitcoin’s network effects.
  • Lower coordination risk: Not every participant needs to upgrade at the same time.
  • More flexible enforcement: If issues appear, a soft fork is usually easier to stop enforcing than reversing a contentious hard fork.
  • Conservative development culture: Bitcoin upgrades are typically small, heavily reviewed, and designed to protect the network’s monetary and security properties.

This is why Bitcoin upgrades tend to be infrequent, carefully reviewed, and widely debated before activation.

What’s the Difference Between a Soft Fork and a Network Upgrade?

A network upgrade is the general term for any change to a blockchain’s rules, features, or technical design. It may improve security, add functions, change transaction formats, or update block validation rules. A soft fork is one specific type of network upgrade that remains backward-compatible, meaning old and new nodes can still stay on the same chain.

The key difference is:

  • Network upgrade: The broad category for blockchain improvements or rule changes.
  • Soft fork: A backward-compatible upgrade that keeps old and new nodes compatible.
  • Hard fork: A non-backward-compatible upgrade that requires participants to update or risk a chain split.

In short, all soft forks are network upgrades, but not all network upgrades are soft forks.

Summary

Forks are one of the main ways blockchains upgrade, evolve, and sometimes split into separate networks. A hard fork introduces non-backward-compatible changes, so participants must upgrade to remain on the same chain. If the community does not agree, a hard fork can create a permanent chain split and a new token.

A soft fork introduces backward-compatible rule changes, allowing older nodes to remain compatible with the upgraded chain. Soft forks usually carry lower coordination risk and help preserve network unity, but they are more limited in scope. Understanding the difference between hard forks and soft forks is important for following blockchain governance, protocol upgrades, and new tokens created after chain splits.

Related Concepts

  1. What Is Hard Fork?
  2. What Is a Node?
  3. What Is a Validator?
  4. What Is a Bitcoin?

Further Reading

  1. What Is Bitcoin Cash (BCH) Payments Blockchain and How Does It Work?
  2. What Are the Best Bitcoin Cash (BCH) Wallets to Use in 2026?
  3. What Is Ethereum Glamsterdam Upgrade in H1 2026 and What Changes Does the Hard Fork Bring?
  4. What Is Dash (DASH), the Bitcoin-Forked Privacy Coin?