The Hydra version that has already been published in the Hydra and Interhead Hydra papers requires all participants to be online at all times. According to this IOHK blog post and this presentation the so-called tail protocol is supposed to make Hydra usable for low-availability clients such as mobile phones.

Beyond a few mentions I couldn't find any explanation on how the Hydra tail protocol will actually work. Do we have an approximate idea how this protocol is going to work or any similar protocols from other blockchains?

  • Charles Hoskinson mentioned Plasma here in the context of Hydra so maybe the tail protocol will work similarly to Plasma.
    – user142
    Mar 25 at 9:31

1 Answer 1


We will have a much better sense of the Hydra Tail protocol once the research paper(s) come out. For now, we can infer its functionality based on some of the design patterns of projects currently building on the Hydra family. For example, SingularityNET's HyperCycle sidechain whitepaper discusses CSL, Plutus, and Hydra integrations, including some talk of Hydra Tail.

In short, Hydra Head is a protocol for validator rings - synchronous, permissioned, high-speed execution environments with CSL snapshots. This is a good way for certain sidechains to interface with Cardano, though others like Hypercycle require that some consensus agents may be partially or completely offline (semi-synchronous or asynchronous). Additional protocols - Hydra Tail - are necessary to account for such agents.

Do note however: the authors of the Hypercycle whitepaper mention that the Hydra Tail protocol is "not yet publicly well specified", and further state the intention "to create a custom version of the Hydra tail protocol that suits Hypercycle’s needs, incorporating mechanisms and ideas beyond those that have previous occurred in the Cardano sphere."

TBE when Hydra Tail paper comes out

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