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Known issues & limitations

Please be aware of the following limitations before running hydra-node on the Cardano --mainnet; as an incredibly technical project, Hydra in its current form requires an elevated level of understanding of the underlying infrastructure. Without the expertise required to operate a hydra-node, users may put their funds at risk if they are unfamiliar with the implementation and usage process.

Head protocol


The current transaction size on mainnet is limited to 16KB, a limitation which has the following consequences:

  • The protocol can only handle a maximum number of participants in a Head (see cost of collectcom transaction). Upon startup, the hydra-node will inform you of the current configured maximum when trying to configure too many peers.

It's currently possible to be denied access to funds by other protocol participants at various stages in a Hydra Head because of the complexity or size of the UTxO being committed or created while the Head is open:

  • The head cannot be finalized if holding more than ~60 assets (see cost of fanout transaction for latest numbers), although it can be closed
  • Tokens minted and not burnt in an open head will prevent it from being finalized
  • If one or more participants commit UTxO too large to be processed together in a CollectCom or Abort transaction, the Head will be stuck in the initialising stage
  • Committing reference scripts from L1 to a Head is problematic and the hydra-node will prevent this should a client try to do so. Note that a Commit transaction could perfectly be crafted outside of the hydra-node and would therefore put the Head in an uncloseable state
  • Using reference scripts on the L2 is non problematic as they will be committed back on the L1 along with all the other UTxO

There are couple of items in the roadmap around reducing the risk of losing funds in a Hydra Head:


Authentication & Authorization

The messages exchanged through the Hydra Network layer between participants are authenticated: Each message is signed using the Hydra signing key of the emitting party which is identified by the corresponding verification key. When a message with an unknown or incorrect signature is received, it is dropped and a notification is logged.

Messages are however not encrypted and therefore, if confidentiality is required, some external mechanism needs to be put in place to prevent other parties from observing the messages exchanged within a Head.

Fault tolerance

The Hydra Head protocol guarantees safety of all (honest) participants' funds, but does not in itself guarantee liveness, hence all parties involved in a Hydra Head must be online and reactive for the protocol to make progress.

This implies that, should one or several participants' hydra-node become permanently unreachable from the others, through a crash, or a network partition, no more transactions can happen in the Head and it must be closed. However, the Hydra network layer is tolerant to transient disconnections and (non-Byzantine) crashes.


Independently from the Head protocol itself, the way the hydra-node is implemented has the following consequences:

  • There is a hard-coded limit on mainnet network where you can only commit up to 100 ADA into the Hydra head. This is only a safety precaution and is going to be increased as we gain more experience in running Hydra heads on the mainnet.


  • TUI crashes when user tries to post a new transaction without any UTXO remaining.

  • Recipient addresses to send money to in the TUI are inferred from the current UTXO set. If a party does not commit a UTXO or consumes all its UTXO in a Head, it won't be able to send or receive anything anymore.