ADR 6: Support reference inputs in constraint library

Date: 2022-08-09


koslambrou <>




After the Vasil HF, the Cardano blockchain will support reference inputs by adding a new field in the transaction data type. With reference inputs, transactions can take a look at UTXOs without actually spending them.

Thus, we need to adapt our transaction constraint data type (TxConstraints) to support referencing UTXOs.


  • We will add the data constuctor MustReferenceOutput TxOutRef to the TxConstraints data type.

  • The PlutusV1 on-chain implementation of this new constraint will simply return False. However, cardano-ledger throws a phase-1 validation error if transactions that use the some of the new features (reference inputs, inline datums and reference scripts) try to execute PlutusV1 scripts. See the Babbage era ledger specification. Therefore, the only way to get a phase-2 validation error would be to use this constraint on-chain in a PlutusV1 script, without using any of the new Babbage era features off-chain.

  • The PlutusV2 on-chain implementation of this new constraint will check that the provided TxOutRef is part of the ScriptContext’s reference inputs.


At first glance, we might think that we need two data constructors for reference inputs such as MustReferencePubKeyOutput and MustReferenceScriptOutput in contrast to the existing MustSpendPubKeyOutput and MustSpendScriptOutput constraints. However, we do not need to make the distinction between public key outputs and script outputs because we’re not spending the output, therefore, we don’t need to provide a redeemer nor the actual script as a witness to the transaction input.


This ADR has been addressed in the PRs #640 and #661.