Bitcoin XT

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Bitcoin XT
The identifying marks of an XT client
Original authorsGavin Andresen
Mike Hearn
Bitcoin XT is a fork of Bitcoin Core that aims to make transactions reliable, inexpensive, and accessible. It achieved significant notoriety and support after prematurely adopting BIP 101, making it an important proponent of the block size limit controversy.[1]

After the inauguration of Wladimir van der Laan to the position of Core maintainer, Bitcoin XT was organized[when?] by Gavin Andresen and Mike Hearn to address controversial ideas lacking the consensus required to be implemented in Bitcoin Core.[2]

Due to implementing BIP 0101 in version 0.11.0A, it is now incompatible with the Bitcoin consensus protocol, and when 75% of the last 1000 blocks are observed to have particular version bits set, it ceases enforcing the 1 MB block size limit and also adds a few new rules. Since it is incompatible with the Bitcoin protocol, it is technically an altcoin, but due to an unusually large economic acceptance it may potentially become a "new Bitcoin" some day.

Mission statement

The XT mission statement defines what the project believes is important: commitment to these principles are what differentiates it from Bitcoin Core.

  • Scaling the network up to handle user demand and spam is important, even if that means the network becomes centralised along the way. The idea of a global system used by ordinary people is what motivated many people to support this.
  • XT proponents believe unconfirmed transactions are important. Many merchants want or need to accept payments within seconds rather than minutes or hours. XT accepts this fact and does what it can to minimise the risk, then "helps" sellers judge what remains. It is committed to support only "first seen" policies, and will not adopt changes that make unconfirmed transactions riskier.
  • Decision making is quick and clear. Decisions are made according to a leadership hierarchy. The XT software encodes decisions that follow the above principles: people who disagree are welcome to use different software, or patch ours. We do not consider writing principled software to be centralising and do not refuse to select reasonable defaults.

You can read more about the code changes in Bitcoin XT.

Block size hard fork

In 2010, a block size limit was introduced into Bitcoin by Satoshi Nakamoto. He added it as a safety measure to prevent miners from spamming large blocks and meant for it to be removed once secure lightweight wallets were developed; however, the spam problem has only gotten worse, and secure lightweight wallets never implemented.

There has been much community debate on this topic. You can read analysis and explanations for why we think raising the block size limit is important here:


Miners that side with Bitcoin XT will produce blocks with a new version number.[3] This indicates to the rest of the network that they support XT.[3] When 75% of the last 1000 blocks are new-version blocks, these miners will automatically abandon Bitcoin and begin mining on a new Bitcoin XT blockchain.[3] 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.[3]

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.

Because there is no end date for the miner vote, it is almost certain that if a single mining pool continues advocating XT for the next ten years, it will eventually be deployed inadvertently.[1]

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.

See Also