Wiki / Concepts / Hard fork vs soft fork

Hard fork vs soft fork

From the Bitcoin Takeover Wiki · ✓ 5 sources · 0 podcast anchors · built 2026-11-13

The two ways to change Bitcoin's rules: soft forks tighten them and stay backward-compatible; hard forks loosen them and need everyone to upgrade.

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The wiki's angle: every chain in this wiki's Forks category that really forked Bitcoin is, from Bitcoin's point of view, a hard fork that did not win, while Bitcoin itself has upgraded only by soft fork.[3]

Contents

  1. Definitions
  2. Activation methods
  3. Accidents
  4. Chain splits
  5. References

Definitions

A soft fork is a change in which only previously valid blocks or transactions are made invalid; because old nodes still accept the new blocks, it is backward-compatible[1]. A hard fork makes previously invalid blocks or transactions valid, and therefore requires all users to upgrade[2]. When miners coordinate to enforce new rules it is a miner-activated soft fork (MASF); when full nodes do it without miner support it is a user-activated soft fork (UASF)[1].

Activation methods

Bitcoin Takeover's fork table traces how soft forks were activated: a flag day after a coinbase poll for P2SH, IsSuperMajority version counting for BIP-34, 66 and 65, BIP-9 version bits for CSV and SegWit, and Speedy Trial for Taproot[3]. BIP-9 locks in when 1,916 of 2,016 blocks in a retarget period signal, which the table says gave miners an effective veto[3].

Accidents

The Bitcoin Wiki counts two accidental hard forks in Bitcoin Core: the 2013 BerkeleyDB split described in BIP 50, and CVE-2018-17144 in Bitcoin Core 0.15, which caused no chain split and was fixed by a soft fork in 0.16.3[2]. The August 2010 value overflow is covered separately.

Chain splits

When a hard fork is contested, both chains can survive, as with Bitcoin Cash in 2017[4]. The UASF showed that a soft fork can also threaten a split if nodes reject blocks that miners still produce[5].

References

  1. secondaryBitcoin Wiki (en.bitcoin.it), “Softfork” [archived]
  2. secondaryBitcoin Wiki (en.bitcoin.it), “Hardfork” [archived]
  3. btkBitcoin: All the Hard Forks and Soft Forks (BTCTKVR: Forks!) [archived]
  4. btkBitcoin Cash: The Big Block Breakaway (BTCTKVR: Forks!) [archived]
  5. primaryBIP-148 (Mandatory activation of segwit deployment), Shaolin Fry [archived]

Related

UASF (BIP-148)
The 2017 user-activated soft fork that threatened to reject non-SegWit-signalling blocks from 1 August 2017.
Segregated Witness
The 2017 soft fork that moved signatures out of the legacy block, fixed malleability and became the centre of
Replay protection
Making a transaction valid on only one side of a chain split, so that spending coins on one chain cannot be re
Drivechains (BIP-300/301)
Sidechains whose two-way peg is secured by miners (BIP-300) with blind merged mining (BIP-301).
Categories: Concepts
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