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  "id": "soft-forks-and-hard-forks",
  "canonical": "https://degreesofsatoshi.com/encyclopedia/soft-forks-and-hard-forks/",
  "collection": "bitcoin",
  "title": "Soft forks and hard forks: how Bitcoin changes its rules without a boss",
  "description": "Compare soft forks and hard forks by which blocks old and new rules accept. Learn why a protocol change does not always produce a lasting chain split.",
  "aliases": [
    "soft fork",
    "hard fork",
    "soft fork vs hard fork",
    "bitcoin fork",
    "what is a hard fork",
    "backwards compatible soft fork",
    "bitcoin cash fork"
  ],
  "dates": {
    "published": "2026-09-23",
    "modified": "2026-10-02",
    "verified": "2026-10-02T18:22:07.965Z",
    "dataAsOf": null
  },
  "authorship": {
    "publisher": "Degrees of Satoshi editorial project",
    "process": "AI-assisted research and drafting with a separate automated source-verification pass; no external expert or named human review is implied."
  },
  "quickAnswer": {
    "text": "A soft fork restricts the set of blocks considered valid, while a hard fork can admit blocks that older rules reject. A persistent chain split occurs when groups continue following incompatible histories; it is not inevitable every time software changes. The compatibility classification says nothing by itself about whether a proposal is desirable, widely adopted or safe to activate.",
    "claimId": "soft-forks-and-hard-forks-quick-answer",
    "sourceIds": [
      "bip123",
      "bip99"
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  "keyFacts": [
    {
      "label": "Soft fork",
      "value": "Some previously valid structures become invalid; nothing invalid becomes valid",
      "sourceIds": [
        "bip123"
      ],
      "id": "soft-fork",
      "claimId": "soft-forks-and-hard-forks-fact-soft-fork"
    },
    {
      "label": "Hard fork",
      "value": "Structures that were invalid under the old rules become valid",
      "sourceIds": [
        "bip123"
      ],
      "id": "hard-fork",
      "claimId": "soft-forks-and-hard-forks-fact-hard-fork"
    },
    {
      "label": "Why soft forks are compatible",
      "value": "To old nodes the new rules look like miner policy, not consensus",
      "sourceIds": [
        "bip99"
      ],
      "id": "why-soft-forks-are-compatible",
      "claimId": "soft-forks-and-hard-forks-fact-why-soft-forks-are-compatible"
    },
    {
      "label": "P2SH (BIP 16)",
      "value": "Dated 3 January 2012; miners signaled with /P2SH/ in the coinbase",
      "sourceIds": [
        "bip16"
      ],
      "id": "p2sh-bip-16",
      "claimId": "soft-forks-and-hard-forks-fact-p2sh-bip-16"
    },
    {
      "label": "SegWit (BIP 141)",
      "value": "Proposed 21 December 2015; activated August 2017",
      "sourceIds": [
        "bip141",
        "segwit-wiki"
      ],
      "id": "segwit-bip-141",
      "claimId": "soft-forks-and-hard-forks-fact-segwit-bip-141"
    },
    {
      "label": "Taproot (BIP 341)",
      "value": "Activated at block 709,632 on 14 November 2021",
      "sourceIds": [
        "bip341"
      ],
      "id": "taproot-bip-341",
      "claimId": "soft-forks-and-hard-forks-fact-taproot-bip-341"
    },
    {
      "label": "Bitcoin Cash",
      "value": "Hard fork activated 1 August 2017 with blocks over 1 MB and replay protection",
      "sourceIds": [
        "uahf-spec"
      ],
      "id": "bitcoin-cash",
      "claimId": "soft-forks-and-hard-forks-fact-bitcoin-cash"
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  ],
  "prerequisites": [],
  "sections": [
    {
      "id": "what-a-fork-is",
      "heading": "A fork is a disagreement about which blocks count",
      "sourceIds": [
        "bip123",
        "bip99"
      ],
      "paragraphs": [
        "Every Bitcoin node, meaning a computer running the software and checking every block for itself, applies a set of rules: how big a block may be, what makes a signature valid, how many new coins a miner may claim. Those are the consensus rules. Nodes that agree on them will agree on the chain. Change the rules on some nodes and not others, and you have a fork: two groups of computers that may no longer accept each other’s blocks.",
        "The word gets used two ways, which causes confusion. A fork can mean a rule change, proposed and deployed on purpose. It can also mean a chain split, where the network temporarily or permanently follows two different histories. Rule changes can cause splits, but so can bugs; BIP 99 calls that an “accidental consensus fork,” and the [consensus incidents dossier](/history/bitcoin-consensus-incidents/) covers the accidental kind. This article is about the deliberate kind, and the two words that describe them.",
        "BIP 123, a 2015 proposal by Eric Lombrozo that sorts Bitcoin proposals by layer, gives the cleanest definitions. It puts consensus changes in the lowest layer, the one where everyone has to agree, and defines both kinds of fork in a sentence each."
      ]
    },
    {
      "id": "soft-forks",
      "heading": "A soft fork adds restrictions that older nodes do not enforce",
      "sourceIds": [
        "bip123",
        "bip99",
        "softfork-wiki"
      ],
      "paragraphs": [
        "BIP 123: “In a soft fork, some structures that were valid under the old rules are no longer valid under the new rules. Structures that were invalid under the old rules continue to be invalid under the new rules.” The set of valid blocks shrinks. Because every block that satisfies the new, stricter rules also satisfies the old ones, a node that never upgraded still accepts the chain. It simply does not know why the miners have stopped doing certain things.",
        "Jorge Timón’s BIP 99, from the same year, explains why this is backward compatible in one line: “For old nodes it just looks like the new rules are policy rules rather than consensus rules.” Policy is what a node chooses to do; consensus is what it must do. To an old node, a soft fork looks like miners being picky. BIP 99 also notes the flip side: “A hashrate majority of miners can impose the new rules,” because if most blocks follow them, blocks that break them get orphaned.",
        "One consequence, from the Bitcoin Wiki’s softfork page: a soft fork cannot be undone with another soft fork, because a soft fork can only shrink the set of valid blocks, never grow it back. Reversing one needs a hard fork."
      ]
    },
    {
      "id": "hard-forks",
      "heading": "A hard fork can produce blocks that older nodes reject",
      "sourceIds": [
        "bip123",
        "bip99"
      ],
      "paragraphs": [
        "BIP 123 again: “In a hard fork, structures that were invalid under the old rules become valid under the new rules.” Now the set of valid blocks grows. A node running old software sees a block that breaks a rule it still enforces, rejects it, and keeps following whatever chain obeys the old rules. If some miners produce old-rule blocks and some produce new-rule blocks, the network splits into two coins with a shared past and different futures. BIP 99’s definition ends with the operational fact: “Hardforks require all users to upgrade.”",
        "BIP 99 sorts hard forks by intent. An uncontroversial hard fork has broad agreement and a distant activation height so everyone has time. A “schism” hard fork is a deliberate split, where one group wants a different Bitcoin than the other. And an emergency hard fork is what you do when a bug leaves no better option. The proposal’s deployment advice differs for each. The point is that a hard fork is not automatically bad, but it always demands coordination that a soft fork does not."
      ]
    },
    {
      "id": "three-soft-forks",
      "heading": "Three soft forks that shipped: P2SH in 2012, SegWit in 2017, Taproot in 2021",
      "sourceIds": [
        "bip16",
        "bip141",
        "segwit-wiki",
        "bip341"
      ],
      "paragraphs": [
        "Pay to Script Hash, BIP 16, was written by Gavin Andresen and dated 3 January 2012. It let a payment go to the hash of a script rather than the script itself, which is what makes multisignature addresses practical. Old nodes checked only that the hash matched and did no further validation, so they accepted the new transactions. Deployment was by miner signal: miners put “/P2SH/” in the coinbase of their blocks, and the plan was to count them on 1 February 2012, with 550 tagged blocks in a week, about 55 percent, as the threshold. The final text applies the new rules to blocks timestamped from 1 April 2012.",
        "Segregated Witness, BIP 141, was proposed on 21 December 2015 by Eric Lombrozo, Johnson Lau and Pieter Wuille. It moved signatures out of the part of the transaction that old nodes examine, and replaced the 1 MB block size limit with a block weight limit of 4,000,000 units. The soft-fork trick was that old nodes see the new outputs as “anyone-can-spend,” which they had always treated as valid, so they accept blocks they cannot fully check. Signaling used BIP 9 version bits from 15 November 2016, and after the long fight told in the [block size war dossier](/history/bitcoin-block-size-war/), SegWit activated in August 2017. Our [SegWit explainer](/encyclopedia/segwit-explained/) goes into what changed for users.",
        "Taproot, BIP 341, by Pieter Wuille, Jonas Nick and Anthony Towns, dated 19 January 2020, added Schnorr signatures and a new output type. It used the same anyone-can-spend pattern: non-upgraded nodes treat the new outputs as spendable by anyone and are “strongly encouraged to upgrade.” Miners signaled between April and August 2021 at a 90 percent threshold, 1,815 blocks in a signaling period, and Taproot activated at block 709,632 on 14 November 2021. See [Taproot explained](/encyclopedia/taproot-explained/)."
      ]
    },
    {
      "id": "bitcoin-cash",
      "heading": "One hard fork that split the chain: Bitcoin Cash, 1 August 2017",
      "sourceIds": [
        "uahf-spec"
      ],
      "paragraphs": [
        "The clearest example of a hard fork is the one that created Bitcoin Cash. Its technical specification, called the User-Activated Hard Fork or UAHF, set activation by median block time at the Unix timestamp 1501590000, which is 1 August 2017 at 12:20 UTC. The first block after activation had to be larger than 1,000,000 bytes, the very thing Bitcoin’s rules forbade, and the new chain accepted blocks of up to 8 MB. That single rule guaranteed that Bitcoin nodes would reject the fork block and that the two chains could never rejoin.",
        "The specification also added replay protection: transactions on the new chain had to be signed with a flag, SIGHASH_FORKID, that Bitcoin nodes do not recognize, so a transaction meant for one chain could not be copied onto the other. Anyone holding bitcoin at the fork held equal amounts on both chains afterwards. Our article on [the Bitcoin Cash fork](/encyclopedia/bitcoin-cash-fork-2017/) tells the story, and the block size war dossier explains the dispute that led there."
      ]
    },
    {
      "id": "who-decides",
      "heading": "Who decides? Miners signal, but nodes and users choose",
      "sourceIds": [
        "bip99",
        "bip16"
      ],
      "paragraphs": [
        "A soft fork is often described as “activated by miners,” and miner signaling has been the usual mechanism since BIP 16. But signaling is a coordination tool, not a vote on whether the change is good. A rule that miners enforce and users reject leaves miners producing blocks nobody values. BIP 99 has a name for that, a “unilateral softfork,” and lists it among the things to avoid. The real gate is whether the people running nodes and holding coins upgrade and keep following the chain.",
        "That is why the process starts with a written proposal rather than a code push. Every fork discussed here began as a [Bitcoin Improvement Proposal](/encyclopedia/bitcoin-improvement-proposals/), argued over in public, and adopted only once enough of the network chose to run it."
      ]
    },
    {
      "id": "reading-the-evidence",
      "heading": "Test compatibility with a block that separates the two rule sets",
      "sourceIds": [
        "bip123",
        "bip99"
      ],
      "paragraphs": [
        "Imagine that old rules accept blocks A and B, while a proposed restriction accepts only A. A is valid under both sets of rules; an older node may still accept B even though an upgraded node rejects it. This is the central compatibility issue in a soft fork, not a claim that old software enforces the new restriction.",
        "Now imagine a change that permits a block C which the old rules reject. An old node cannot simply accept C because more miners produce it. Whether this incompatibility becomes a lasting split also depends on whether participants continue maintaining both histories. Use these validity questions before describing the political outcome of a fork."
      ]
    }
  ],
  "faq": [
    {
      "question": "Is a soft fork always safer than a hard fork?",
      "answer": "Not automatically. A soft fork narrows the set of valid blocks, which gives it a particular compatibility relationship with old rules. Older nodes do not enforce the added restriction. The change’s desirability, implementation quality and activation risks must be assessed separately.",
      "sourceIds": [
        "bip99",
        "bip123"
      ]
    },
    {
      "question": "Do I need to do anything when Bitcoin soft-forks?",
      "answer": "Whether to update depends on the change and the software you use. Blocks satisfying the new restrictions can also satisfy old rules, but an older node does not independently enforce those restrictions. Wallet support for new output types is a separate compatibility question; read the relevant implementation and deployment guidance.",
      "sourceIds": [
        "bip141",
        "bip341"
      ]
    },
    {
      "question": "Has Bitcoin ever had a hard fork?",
      "answer": "The rule changes covered here that the Bitcoin chain adopted, P2SH, SegWit and Taproot, were all soft forks. Bitcoin Cash was a hard fork that activated on 1 August 2017 and produced a separate chain and coin rather than changing Bitcoin itself.",
      "sourceIds": [
        "bip16",
        "bip141",
        "bip341",
        "uahf-spec"
      ]
    },
    {
      "question": "What is a chain split?",
      "answer": "A chain split is when parts of the network follow different histories. A hard fork can produce a persistent split when incompatible rules continue to be used, as in the Bitcoin Cash case. Splits can also happen by accident when different software disagrees about validity; BIP 99 calls these accidental consensus forks, and the site’s consensus incidents dossier documents Bitcoin’s.",
      "sourceIds": [
        "bip99",
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    }
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      "checkedAt": "2026-10-02T15:04:18.083097+00:00",
      "contentSha256": "eeb42f0e7c097ce7bd4513691197534544046b9df152a7f38afa744cf01f5545",
      "version": "927b6de9915c9262615a6399de51b200f81e5aa4",
      "locator": "Defines softfork and hardfork, explains that to old nodes a soft fork looks like policy, and classifies accidental, unilateral, schism, uncontroversial and emergency forks.",
      "recordId": "source-4b78632ad482cf32"
    },
    {
      "id": "softfork-wiki",
      "label": "Softfork",
      "publisher": "Bitcoin Wiki",
      "url": "https://en.bitcoin.it/wiki/Softfork",
      "note": "Explains that a softfork only makes previously valid blocks invalid, that old clients therefore accept them, and that a softfork cannot be reversed without a hardfork.",
      "checkedAt": "2026-10-02T14:48:54.139048+00:00",
      "contentSha256": "40d7b754742158e577f656419890b7372e8961ad5c7cec33e55b19c6a06e7131",
      "locator": "Explains that a softfork only makes previously valid blocks invalid, that old clients therefore accept them, and that a softfork cannot be reversed without a hardfork.",
      "recordId": "source-693e6a1d23ddac79",
      "version": null
    },
    {
      "id": "bip16",
      "label": "BIP 16: Pay to Script Hash",
      "publisher": "bitcoin/bips repository, GitHub",
      "author": "Gavin Andresen",
      "issued": "2012-01-03",
      "url": "https://raw.githubusercontent.com/bitcoin/bips/927b6de9915c9262615a6399de51b200f81e5aa4/bip-0016.mediawiki",
      "note": "Gives the backwards-compatibility reasoning, the /P2SH/ coinbase signal, the 1 February 2012 count with a 550-block (about 55%) threshold, and the 1 April 2012 enforcement timestamp.",
      "checkedAt": "2026-10-02T15:04:13.407672+00:00",
      "contentSha256": "9f89c0731bd718ae084802c897c95eb6f85ed2730d01fbc3233ee963b4679feb",
      "version": "927b6de9915c9262615a6399de51b200f81e5aa4",
      "locator": "Gives the backwards-compatibility reasoning, the /P2SH/ coinbase signal, the 1 February 2012 count with a 550-block (about 55%) threshold, and the 1 April 2012 enforcement timestamp.",
      "recordId": "source-ca859c1b51d0e152"
    },
    {
      "id": "bip141",
      "label": "BIP 141: Segregated Witness (Consensus layer)",
      "publisher": "bitcoin/bips repository, GitHub",
      "author": "Eric Lombrozo, Johnson Lau and Pieter Wuille",
      "issued": "2015-12-21",
      "url": "https://raw.githubusercontent.com/bitcoin/bips/927b6de9915c9262615a6399de51b200f81e5aa4/bip-0141.mediawiki",
      "note": "States that old nodes treat witness programs as anyone-can-spend, sets the 4,000,000 weight-unit limit, and gives the BIP 9 signaling window from 15 November 2016.",
      "checkedAt": "2026-10-02T15:04:13.421045+00:00",
      "contentSha256": "d72409335872a32494f75e7547589abf4b3b23b6beebe6bc6999c7a84b135e2c",
      "version": "927b6de9915c9262615a6399de51b200f81e5aa4",
      "locator": "States that old nodes treat witness programs as anyone-can-spend, sets the 4,000,000 weight-unit limit, and gives the BIP 9 signaling window from 15 November 2016.",
      "recordId": "source-896e868816ad19d1"
    },
    {
      "id": "segwit-wiki",
      "label": "Segregated Witness",
      "publisher": "Bitcoin Wiki",
      "url": "https://en.bitcoin.it/wiki/Segregated_Witness",
      "note": "Records that activation was blocked by miners through 2016 and 2017 and that SegWit activated soon after 1 August 2017.",
      "checkedAt": "2026-10-02T14:48:54.469759+00:00",
      "contentSha256": "43053e9d2e756f61e9679513e0c29b3eb335ee81648b9d3a4919196260f835ac",
      "locator": "Records that activation was blocked by miners through 2016 and 2017 and that SegWit activated soon after 1 August 2017.",
      "recordId": "source-7113c9dfd8442daf",
      "version": null
    },
    {
      "id": "bip341",
      "label": "BIP 341: Taproot: SegWit version 1 spending rules",
      "publisher": "bitcoin/bips repository, GitHub",
      "author": "Pieter Wuille, Jonas Nick and Anthony Towns",
      "issued": "2020-01-19",
      "url": "https://raw.githubusercontent.com/bitcoin/bips/927b6de9915c9262615a6399de51b200f81e5aa4/bip-0341.mediawiki",
      "note": "Gives the soft-fork compatibility note, the April to August 2021 signaling window with a 1,815-block (90%) threshold, and activation at block 709,632 on 14 November 2021.",
      "checkedAt": "2026-10-02T15:04:13.593548+00:00",
      "contentSha256": "463690d4c409587759c0ff6e8b8fc3a2bf6cb2cdda0c06ff193d317e0ed898ec",
      "version": "927b6de9915c9262615a6399de51b200f81e5aa4",
      "locator": "Gives the soft-fork compatibility note, the April to August 2021 signaling window with a 1,815-block (90%) threshold, and activation at block 709,632 on 14 November 2021.",
      "recordId": "source-774443f2ae10accb"
    },
    {
      "id": "uahf-spec",
      "label": "UAHF Technical Specification",
      "publisher": "bitcoincashorg/bitcoincash.org repository, GitHub",
      "url": "https://raw.githubusercontent.com/bitcoincashorg/bitcoincash.org/3e2e6da8c38dab7ba12149d327bc4b259aaad684/spec/uahf-technical-spec.md",
      "note": "Sets activation at median time past 1501590000 (1 August 2017), requires a fork block over 1,000,000 bytes with an 8 MB limit, and mandates SIGHASH_FORKID replay protection.",
      "checkedAt": "2026-10-02T15:04:18.120183+00:00",
      "contentSha256": "459652647bcf38f92305a93b98e24cb1c827fab83186065f6e4b71b2ac11078a",
      "version": "3e2e6da8c38dab7ba12149d327bc4b259aaad684",
      "locator": "Sets activation at median time past 1501590000 (1 August 2017), requires a fork block over 1,000,000 bytes with an 8 MB limit, and mandates SIGHASH_FORKID replay protection.",
      "recordId": "source-565c71046e083d3d"
    }
  ],
  "related": {
    "articles": [
      "bitcoin-improvement-proposals",
      "segwit-explained",
      "taproot-explained",
      "bitcoin-cash-fork-2017",
      "bitcoin-nodes-explained"
    ],
    "dossiers": [
      "bitcoin-block-size-war",
      "bitcoin-consensus-incidents"
    ],
    "wallets": []
  },
  "revisionHistory": [
    {
      "date": "2026-09-23",
      "kind": "published",
      "summary": "Initial Bitcoin encyclopedia entry at this permanent URL."
    },
    {
      "date": "2026-10-02",
      "kind": "correction",
      "summary": "Revised direct answer to preserve source scope and qualifications. Corrected scope or wording: A hard fork causes one deliberately, as Bitcoin Cash did."
    },
    {
      "date": "2026-10-02",
      "kind": "publishing-format",
      "summary": "Added reusable claims, explicit source locators, and matching Markdown and JSON. This publishing change does not itself establish factual verification."
    },
    {
      "date": "2026-10-02",
      "kind": "content-revision",
      "summary": "Added “Test compatibility with a block that separates the two rule sets”, clarified the search description, and corrected overbroad section headings. Independent verification is recorded separately."
    }
  ],
  "citation": "Degrees of Satoshi editorial project. “Soft forks and hard forks: how Bitcoin changes its rules without a boss.” Published 2026-09-23; updated 2026-10-02. https://degreesofsatoshi.com/encyclopedia/soft-forks-and-hard-forks/"
}