feat(abstract-utxo): add extractTransaction option for signing

Add `extractTransaction` option to control whether finalized PSBTs are
extracted to legacy transaction format. When false, returns the
finalized PSBT, preserving derivation info useful for testing.

Default behavior (true) maintains backward compatibility by extracting
to legacy format.

Issue: BTC-2768

Co-authored-by: llm-git <llm-git@ttll.de>

Author: OttoAllmendinger

modules/abstract-utxo/src/abstractUtxoCoin.ts

@@ -338,6 +338,11 @@ type UtxoBaseSignTransactionOptions<TNumber extends number | bigint = number> =
    */
   returnLegacyFormat?: boolean;
   wallet?: UtxoWallet;
+  /**
+   * When true (default), extract finalized PSBT to legacy transaction format.
+   * When false, return finalized PSBT. Useful for testing to keep transactions in PSBT format.
+   */
+  extractTransaction?: boolean;
 };
 
 export type SignTransactionOptions<TNumber extends number | bigint = number> = UtxoBaseSignTransactionOptions<TNumber> &

modules/abstract-utxo/src/transaction/fixedScript/signTransaction.ts

@@ -67,6 +67,8 @@ export async function signTransaction<
     allowNonSegwitSigningWithoutPrevTx: boolean;
     pubs: string[] | undefined;
     cosignerPub: string | undefined;
+    /** When true (default), extract finalized PSBT to legacy transaction format. When false, return finalized PSBT. */
+    extractTransaction?: boolean;
   }
 ): Promise<
   utxolib.bitgo.UtxoPsbt | utxolib.bitgo.UtxoTransaction<bigint | number> | fixedScriptWallet.BitGoPsbt | Buffer
@@ -77,6 +79,8 @@ export async function signTransaction<
     isLastSignature = params.isLastSignature;
   }
 
+  const { extractTransaction = true } = params;
+
   if (tx instanceof bitgo.UtxoPsbt) {
     const signedPsbt = await signPsbtWithMusig2ParticipantUtxolib(
       coin as Musig2Participant<utxolib.bitgo.UtxoPsbt>,
@@ -88,8 +92,12 @@ export async function signTransaction<
       }
     );
     if (isLastSignature) {
-      signedPsbt.finalizeAllInputs();
-      return signedPsbt.extractTransaction();
+      if (extractTransaction) {
+        signedPsbt.finalizeAllInputs();
+        return signedPsbt.extractTransaction();
+      }
+      // Return signed PSBT without finalizing to preserve derivation info
+      return signedPsbt;
     }
     return signedPsbt;
   } else if (tx instanceof fixedScriptWallet.BitGoPsbt) {
@@ -110,8 +118,12 @@ export async function signTransaction<
       }
     );
     if (isLastSignature) {
-      signedPsbt.finalizeAllInputs();
-      return Buffer.from(signedPsbt.extractTransaction().toBytes());
+      if (extractTransaction) {
+        signedPsbt.finalizeAllInputs();
+        return Buffer.from(signedPsbt.extractTransaction().toBytes());
+      }
+      // Return finalized PSBT without extracting to legacy format
+      return signedPsbt;
     }
     return signedPsbt;
   }

modules/abstract-utxo/src/transaction/signTransaction.ts

@@ -78,6 +78,7 @@ export async function signTransaction<TNumber extends number | bigint>(
       allowNonSegwitSigningWithoutPrevTx: params.allowNonSegwitSigningWithoutPrevTx ?? false,
       pubs: params.pubs,
       cosignerPub: params.cosignerPub,
+      extractTransaction: params.extractTransaction,
     });
 
     // Convert half-signed PSBT to legacy format when the caller explicitly requested txFormat: 'legacy'

modules/abstract-utxo/test/unit/transaction.ts

@@ -5,7 +5,7 @@ import * as _ from 'lodash';
 import * as utxolib from '@bitgo/utxo-lib';
 import nock = require('nock');
 import { BIP32Interface, bitgo, testutil } from '@bitgo/utxo-lib';
-import { address as wasmAddress } from '@bitgo/wasm-utxo';
+import { address as wasmAddress, fixedScriptWallet } from '@bitgo/wasm-utxo';
 import {
   common,
   FullySignedTransaction,
@@ -108,6 +108,7 @@ function run<TNumber extends number | bigint = number>(
         prv: signer.toBase58(),
         pubs: walletKeys.triple.map((k) => k.neutered().toBase58()),
         cosignerPub: cosigner.neutered().toBase58(),
+        extractTransaction: false,
       } as WalletSignTransactionOptions;
     }
 
@@ -223,7 +224,7 @@ function run<TNumber extends number | bigint = number>(
 
       function toTransactionStagesObj(stages: TransactionStages): TransactionObjStages {
         return _.mapValues(stages, (v) =>
-          v === undefined || v instanceof utxolib.bitgo.UtxoPsbt
+          v === undefined || v instanceof utxolib.bitgo.UtxoPsbt || v instanceof fixedScriptWallet.BitGoPsbt
             ? undefined
             : v instanceof utxolib.bitgo.UtxoTransaction
             ? transactionToObj<TNumber>(v)
@@ -266,7 +267,7 @@ function run<TNumber extends number | bigint = number>(
       signedBy: BIP32Interface[],
       sign: 'halfsigned' | 'fullsigned'
     ) {
-      if (txFormat === 'psbt' && sign === 'halfsigned') {
+      if (txFormat === 'psbt') {
         testPsbtValidSignatures(tx, signedBy);
         return;
       }
@@ -398,6 +399,8 @@ function run<TNumber extends number | bigint = number>(
         const txHex =
           stageTx instanceof utxolib.bitgo.UtxoPsbt || stageTx instanceof utxolib.bitgo.UtxoTransaction
             ? stageTx.toBuffer().toString('hex')
+            : stageTx instanceof fixedScriptWallet.BitGoPsbt
+            ? Buffer.from(stageTx.serialize()).toString('hex')
             : stageTx.txHex;
 
         const pubs = walletKeys.triple.map((k) => k.neutered().toBase58()) as Triple<string>;