improvement: tx size includes IsValid field, Haskell omits it

[solidsnakedev]

Summary

calculateFeeIteratively serializes the transaction using a 4-element CBOR array (body, witnessSet, isValid, auxiliaryData). The Cardano node computes the fee-relevant size using toCBORForSizeComputation, which omits IsValid and produces a 3-element array.

This results in a tx size ~1 byte larger than the node calculates, causing a fee over-estimate of ~44 lovelace (1 byte × minFeeCoefficient).

Haskell Reference

-- eras/alonzo/impl/src/Cardano/Ledger/Alonzo/Tx.hs
toCBORForSizeComputation AlonzoTx {atBody, atWits, atAuxData} =
  encodeListLen 3        -- 3 elements, no IsValid
    <> encCBOR atBody
    <> encCBOR atWits
    <> encodeNullStrictMaybe encCBOR atAuxData

Our Code

// TxBuilderImpl.ts — calculateFeeIteratively
const transaction = new Transaction.Transaction({
  body,
  witnessSet: fakeWitnessSet,
  isValid: true,     // included in CBOR, node does not count this
  auxiliaryData: ...
})
const size = yield* calculateTransactionSize(transaction)

Impact

~44 lovelace over-charge per transaction. Safe (node accepts fees above minimum) but imprecise.

Fix

Serialize only [body, witnessSet, auxiliaryData] (3-element array) for size computation, or subtract the isValid CBOR overhead (~2 bytes) from the measured size.

[danbaruka]

Hi @solidsnakedev,
I can take this issue.