Bitcoin XT is a fork of Bitcoin Core, the reference client for the bitcoin network created by Gavin Andresen in 2012. In mid-2015, the concept achieved significant attention within the bitcoin community amid a contentious debate among core developers over increasing the block size cap. Originally designed to introduce alternative P2P rules, it later gained significant notoriety and support after its early adoption of BIP 101 in 2015, giving it importance in the block size limit controversy. The current reference implementation for bitcoin contains a computational bottleneck bottleneck. This averages to a daily maximum of around 300,000 transactions. After Andresen's resignation from the position of Bitcoin Core maintainer, he and Mike Hearn organized Bitcoin XT to address several controversial ideas lacking the consensus required to be implemented in Bitcoin Core.
Bitcoin XT is an implementation of a full node client network Bitcoin Cash (BCH) and Bitcoin (BTC). XT corresponds to the original vision of Bitcoin: simple, reliable, low-cost transactions for everyone in the world. Initially, Bitcoin XT was a simple set of patches on top of Bitcoin Core, now Bitcoin XT is an independently developed client of the Bitcoin network. In order for everyone to be able to verify that the downloaded files are consistent with the source code, the Deterministic Builds method is used.
It was proposed that the block size increase to eight megabytes, and from then onwards to automatically increase it exponentially, doubling every two years. The proposal did not gain the necessary support to go into effect on the Bitcoin network by early 2016, the earliest possible switchover date. Its use has been in steady decline from March 2016 onwards.
On June 10, 2014 Mike Hearn published a Bitcoin Improvement Proposal (BIP 64), calling for the addition of "a small P2P protocol extension that performs UTXO lookups given a set of outpoints." On December 27, 2014 Hearn released version 0.10 of the client, with the BIP 64 changes. It was intended to support queries for his Lighthouse crowdfunding platform project.
On June 22, 2015, Gavin Andresen published BIP 101 calling for an increase in the maximum block size. The changes would activate a fork allowing eight MB blocks (doubling in size every two years) once 75% of a stretch of 1,000 mined blocks is achieved after the beginning of 2016. The new maximum transaction rate under XT would have been 24 transactions per second.
BIP 101 was implemented in Bitcoin XT on August 6, 2015 and released on August 15. If 75% of the last 1000 blocks were observed to have particular version bits set, it would have ceased enforcing the 1 MB block size limit and also have added a few new rules. However, this threshold was never achieved and no new XT blocks are being created today. On August 15, 2015 version 0.11A was released to the public. Bip 101 was reverted and the 2-MB block size bump of Bitcoin Classic was applied instead.
In January 2016, BIP 101 was removed from Bitcoin XT and replaced with the one-time block size increase to 2 MB present in Bitcoin Classic. In the year following this change, adoption of Bitcoin XT decreased dramatically, with fewer than 30 nodes remaining by January 2017. Later attempts by other codebases to increase the block size to 2 MB including Bitcoin Classic and SegWit2x have also failed. According bitcoin statistics website coin.dance the software peaked at more than 1,000 nodes in August 2015 before declining to a second peak in January 2016 of around 670 nodes. From March 2016 the program was seen a consistent decline in its user base to less than 30 instances.
The August 2015 release of XT received widespread media coverage. The Guardian wrote that "bitcoin is facing civil war". Reason magazine wrote that "Bitcoin XT represents a technical and philosophical divergence." Wired wrote that "Bitcoin XT exposes the extremely social — extremely democratic — underpinnings of the open source idea, an approach that makes open source so much more powerful than technology controlled by any one person or organization." Developer Adam Back was critical of the 75% activation threshold being too low and that some of the changes were insecure.
In the event, the XT hard fork stalled. On 11 January 2016, the earliest possible date for the transition to take effect, only approximately 10% of Bitcoin miners were using the XT protocol to sign blocks, whereas 75% would have been required to make the new protocol the de facto standard. The software lacked the support of some of China's biggest mining pools. The Bitcoin community response to the software has been attributed to the adherence to gradual change based on consensus.
The commitment to follow the principles listed below is what sets us apart from Bitcoin Core. They are trying to match the original vision of Satoshi Nakamoto, as it was this vision that brought the bitcoin community together. The XT mission statement define what we consider important:
- It is important for us to follow the scaling of the network in line with the growing demands of users, even if this means that some technical details will change. This is what Satoshi himself wanted, and the idea of a global system that can be used by everyone in the world is what motivated many of us to join the network.
- XT provides people with all the information they need, even if its use requires decision-making involving some risk. For example:
- We believe that unconfirmed transactions are necessary. After all, many sellers need to accept payments within seconds, not minutes or hours. The Bitcoin XT client accepts this fact and does everything possible to reduce the risk, and then helps sellers make their own decision. We are committed to the first-seen rule. At the same time, we are not going to make changes that will make unconfirmed transactions more risky.
- Easy wallets are needed. Most users cannot or simply will not run a full node. Most of the world's population does not have their own computer: such people use the Internet through smartphones. They will have to sacrifice some security in order to participate in the network, so that the Bitcoin XT client supports any compromises that wallet developers can offer.
- Decision - making by developers is a fast and transparent process. Decisions are made according to the hierarchy in the project guide. Bitcoin XT embodies in the code solutions that comply with the above principles, and people who do not support changes can use another client, or patch ours. Thus, the principle of decentralization remains.
- The user community XT friendly, pragmatic, and cares about the developers of third-party applications. We care about user experience in all areas. We value professionalism both technically and in communication.
If you mining with XT, you will get blocks with a special number in the "version number"block. For the rest of the network, this will mean that you support increasing the block size. As soon as 75% of the last thousand units are with this special number, the network will automatically decide that mining of the increased units can be started.
Soft constraints: XT supports setting the maximum block size to mine. If you apply this setting, your node will not mine blocks greater than this limit, but it will accept large blocks from others. This will begin after a waiting period of two weeks in hopes the economy in this time may force anyone who hasn't switched yet to do so.
Users and miners running full Bitcoin nodes will reject the XT blockchain starting with the first block that is larger than one megabyte in size, and thus be unaffected provided it fails to achieve economic consensus. 
Users and merchants
If insufficient mining hash power runs XT to reach supermajority then nothing will happen. If enough does, XT users will follow a new blockchain and cease to be using and trading bitcoins.
Later in January 2016, frustrated of his proposal being massively outvoted, Mike Hearn made a media stunt declaring on various US national and international press agencies that "Bitcoin has failed". Max Keiser defined this episode a "whining ragequit" (beginning of E855 on RT) and other members of the community pointed to an extensive account of manipulations http://gettopical.com/bitcoin/e289a9a5189008d994e80666e9038089 (unfortunately today many twitter screenshots are broken).
- Disagreement over Bitcoin’s future threatens to fork the currency
- bips/bip-0064.mediawiki at master · bitcoin/bips · GitHub
- Bitcoin XT debate overshadowing growth opportunities
- Bitcoin splits: Will it break, or be better than ever?
- Scalability Debate Continues As Bitcoin XT Proposal Stalls, CoinDesk, 11 January 2016, accessed 18 May 2016.
- BitcoinXT on GIthub
- Why is Bitcoin forking? — Faith and future