Use the withdraw-zero trick

A zero-value withdrawal runs a reward script once for the transaction. Hydra supports this pattern by creating a zero-balance reward account during ledger validation. See CIP-112 for the motivation for transaction-level validation.

Run this example from the Hydra repository root in a development shell with cabal, cardano-cli, curl, jq, and websocat. It uses demo keys and an offline head.

Rewarding script

Use dummyRewardingScript, which accepts rewarding and certifying contexts. Start GHCi:

cabal repl hydra-plutus:lib:hydra-plutus

Export the script, then exit GHCi. Cabal runs this session in hydra-plutus/, so ../rewarding.plutus writes to the repository root:

import Prelude
import Hydra.Contract.Dummy
import Hydra.Cardano.Api
let script = PlutusScript dummyRewardingScript
writeFileTextEnvelope (File "../rewarding.plutus") Nothing script
:quit

Back in the shell, derive the address from the exported script. This avoids hard-coding a hash that changes when the script is recompiled:

STAKE_ADDR=$(cardano-cli conway stake-address build \
  --stake-script-file rewarding.plutus --testnet-magic 42)

Withdraw zero transaction

Create an offline head containing 100 ADA owned by Alice:

Setup with offline head

cat > utxo.json <<EOF
{
  "0000000000000000000000000000000000000000000000000000000000000000#0": {
    "address": "addr_test1vp5cxztpc6hep9ds7fjgmle3l225tk8ske3rmwr9adu0m6qchmx5z",
    "value": {
      "lovelace": 100000000
    }
  }
}
EOF

cabal run hydra-node:exe:hydra-node -- \
  --offline-head-seed 0001 \
  --initial-utxo utxo.json \
  --ledger-protocol-parameters hydra-cluster/config/protocol-parameters.json \
  --hydra-signing-key demo/alice.sk \
  --persistence-dir tmp-withdraw-zero

Keep the node running in that terminal. In another terminal at the repository root, set STAKE_ADDR as above. Offline mode deposits the supplied UTxO after opening the head. Wait for CommitFinalized before spending it:

websocat "ws://127.0.0.1:4001?history=yes"

Stop the monitor with Ctrl-C once the event appears.

Use a fresh tmp-withdraw-zero/ directory for each run. Build, sign, and submit the transaction once; it spends the initial output:

Withdraw zero transaction

cardano-cli conway transaction build-raw \
  --tx-in 0000000000000000000000000000000000000000000000000000000000000000#0 \
  --tx-in-collateral 0000000000000000000000000000000000000000000000000000000000000000#0 \
  --tx-out addr_test1vp5cxztpc6hep9ds7fjgmle3l225tk8ske3rmwr9adu0m6qchmx5z+100000000 \
  --withdrawal "$STAKE_ADDR+0" \
  --withdrawal-script-file rewarding.plutus \
  --withdrawal-redeemer-value "{}" \
  --withdrawal-execution-units "(10000000000, 14000000)" \
  --protocol-params-file hydra-cluster/config/protocol-parameters.json \
  --fee 0 \
  --out-file tx.json

cardano-cli conway transaction sign \
  --tx-body-file tx.json \
  --signing-key-file hydra-cluster/config/credentials/alice-funds.sk \
  --out-file tx-signed.json

curl --fail-with-body -sS http://127.0.0.1:4001/transaction \
  -H 'Content-Type: application/json' \
  --data-binary @tx-signed.json

An HTTP 200 response confirms inclusion in a snapshot. A 202 response means the transaction is still pending; check the snapshot or WebSocket events.

Registration is not required

The reward account does not need a registration certificate for this pattern. Hydra creates the account for each zero-value withdrawal during validation; it does not maintain persistent staking or reward-account state between transactions.