Wiki / Forks / Bitcoin XT

Bitcoin XT

From the Bitcoin Takeover Wiki · ✓ 6 sources · 1 podcast anchors · built 2026-11-13

Mike Hearn's fork of Bitcoin Core that shipped BIP-101 in August 2015 and opened the public, client-versus-client phase of the block size war.

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The wiki's angle: XT is where the war became a contest between competing binaries rather than mailing-list threads. Fork Wars dates that public phase from the XT release in August 2015.[6][1]

Contents

  1. Origins
  2. BIP-101
  3. Release and reaction
  4. Collapse
  5. Heard on Bitcoin Takeover
  6. References

Origins

Bitcoin XT was created by Mike Hearn in 2014, originally to carry alternative P2P rules; its December 2014 0.10 release included his BIP 64 UTXO-lookup extension[1]. After Gavin Andresen stepped down as Bitcoin Core maintainer, he and Hearn organised XT around changes that lacked consensus in Core[1].

BIP-101

BIP-101 set the maximum block size to 8,000,000 bytes at 11 January 2016, doubling every 63,072,000 seconds until 6 January 2036, after which the limit would be 8,192,000,000 bytes[2]. It would activate once 750 of 1,000 consecutive blocks carried version 0x20000007, followed by a two-week grace period[2]. The BIP says the 8 MB starting size was chosen after feedback from miners on bandwidth-constrained networks, in particular those behind the Great Firewall of China[2].

Release and reaction

BIP-101 was merged into XT on 6 August 2015 and released on 15 August 2015[1]. Adam Back criticised the 75% threshold as too low[1]. Hearn later wrote that by running XT “miners could cast a vote for changing the limit” and that the release “pushed powerful emotional buttons” in a small number of people[3].

Collapse

The 75% threshold was never reached[1]. In January 2016 XT replaced BIP-101 with the one-time 2 MB increase used by Bitcoin Classic, and by January 2017 fewer than 30 XT nodes remained[1]. Andresen withdrew BIP-101 from the BIPs repository in February 2016[4]. From August 2017 XT shipped as a Bitcoin Cash client[1].

Heard on Bitcoin Takeover

On S17 E39 (August 2026), Kurt Wuckert Jr. recalled: “we saw the DDoS attacks against the Bitcoin XT nodes.”[5]

References

  1. secondaryBitcoin Wiki (en.bitcoin.it), “Bitcoin XT” [archived]
  2. primaryBIP-101 (Increase maximum block size), Gavin Andresen [archived]
  3. primaryMike Hearn, “The resolution of the Bitcoin experiment”, 14 Jan 2016 (archived copy) [archived]
  4. primarybitcoin/bips pull request #329, “Withdraw BIP 101 proposal” (Gavin Andresen, Feb 2016) [archived]
  5. episodeS17 E39: Kurt Wuckert Jr. on the DDoS attacks on XT nodes▶ listen at 48:20timestamp transcript-derived, pending a listen [archived]
  6. btkFork Wars: the block size war timeline (Bitcoin Takeover) [archived]

Related

Mike Hearn
Former Google engineer, bitcoinj author and co-creator of Bitcoin XT, who declared Bitcoin a failed experiment
Gavin Andresen
Satoshi Nakamoto's successor as Bitcoin's lead developer, author of BIP-101 and BIP-109, and later a casualty
Bitcoin Classic
The early-2016 client that proposed a one-time 2 MB hard fork (BIP-109) after Bitcoin XT stalled.
Block size war
The 2015–2017 fight over Bitcoin's 1 MB limit that produced the first durable chain split.
Categories: Forks · Fork lineage
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