On August 15, 2015, Mike Hearn released Bitcoin XT version 0.11A, a fork of Bitcoin Core that implemented BIP 101. Gavin Andresen, who had handed the Bitcoin Core lead-maintainer role to Wladimir van der Laan in April 2014, had authored BIP 101 two months earlier and supported the XT release.
BIP 101 proposed raising Bitcoin’s 1 MB block-size limit to 8 MB starting January 11, 2016, and doubling every two years thereafter, reaching 8 GB by 2036. Activation required a 75% supermajority of mined blocks signaling support over a 1,000-block window.
The launch was the public-fork phase of a dispute that had been building since 2013, when block fullness first became a recurring concern. Hearn and Andresen argued that the 1 MB limit — added by Satoshi in July 2010, enforced from that September, as a temporary anti-spam measure — would soon throttle Bitcoin’s ability to confirm transactions, and that a hard fork to a larger limit was the only credible path to scaling.
Their critics — most prominently the Bitcoin Core developers led by Gregory Maxwell, Pieter Wuille, and Wladimir van der Laan — argued that a contentious hard fork would split the network, that scaling could be achieved via off-chain layers and protocol upgrades like Segregated Witness, and that BIP 101’s growth schedule risked centralizing node operation to a small set of well-resourced operators. Maxwell and Wuille had co-founded Blockstream nine months earlier. How this same fight over XT fed the “Blockstream controls Bitcoin” charge leveled against them is traced in the Blockstream-centralization analysis.
Bitcoin XT briefly attracted around 1,000 nodes in late 2015 but never approached the 75% activation threshold. In January 2016 XT dropped BIP 101 for Bitcoin Classic’s one-time 2 MB increase, and Hearn published “The resolution of the Bitcoin experiment”, declaring Bitcoin had failed and selling all his coins. XT continued to exist as a small network, with fewer than 30 nodes left by January 2017, but was effectively dead as a competitor to Bitcoin Core.
The block-size war did not end with XT. Bitcoin Classic launched in February 2016 with a similar 2 MB proposal; Bitcoin Unlimited followed in October 2016 with a flexible miner-driven approach; the New York Agreement in May 2017 attempted a SegWit + 2 MB compromise; and the dispute culminated in the Bitcoin Cash hard fork on August 1, 2017. The phase-by-phase chronology, the three factions, and the post-2017 governance consequences are collated in the block-size war overview (2015–2017); the broader fork-and-altcoin tree (BCH, BSV, and the wider lineage) is in the Bitcoin family-tree analysis.
The XT launch is the chronological start of the public-fork phase of the dispute. Earlier disagreement existed in mailing-list and BitcoinTalk threads, but August 15, 2015 is the date a Bitcoin Core fork was released as production software, with installable binaries and a published activation schedule.
For a reading of the fork attempts from XT through Bitcoin Cash as a fight over who could still call a chain “Bitcoin,” see the “fork wars as not OSS” analysis.
