diff --git a/modules/abstract-utxo/src/abstractUtxoCoin.ts b/modules/abstract-utxo/src/abstractUtxoCoin.ts
index d06d0032f3..772fb0a3b1 100644
--- a/modules/abstract-utxo/src/abstractUtxoCoin.ts
+++ b/modules/abstract-utxo/src/abstractUtxoCoin.ts
@@ -57,7 +57,7 @@ import {
   v1Sweep,
   V1SweepParams,
 } from './recovery';
-import { isReplayProtectionUnspent } from './transaction/fixedScript/replayProtection';
+import { getReplayProtectionPubkeys, isReplayProtectionUnspent } from './transaction/fixedScript/replayProtection';
 import { supportedCrossChainRecoveries } from './config';
 import {
   assertValidTransactionRecipient,
@@ -81,6 +81,7 @@ import {
   getMainnetCoinName,
   getNetworkFromCoinName,
   isTestnetCoin,
+  isUtxoCoinNameMainnet,
   UtxoCoinName,
   UtxoCoinNameMainnet,
 } from './names';
@@ -143,6 +144,28 @@ type UtxoCustomSigningFunction<TNumber extends number | bigint> = {
 
 const { isChainCode, scriptTypeForChain, outputScripts } = bitgo;
 
+/**
+ * Check if a decoded transaction has at least one taproot key path spend (MuSig2) input.
+ * Works for both utxolib UtxoPsbt and wasm-utxo BitGoPsbt.
+ */
+function hasKeyPathSpendInput<TNumber extends number | bigint>(
+  tx: DecodedTransaction<TNumber>,
+  pubs: string[] | undefined,
+  coinName: UtxoCoinName
+): boolean {
+  if (tx instanceof bitgo.UtxoPsbt) {
+    return bitgo.isTransactionWithKeyPathSpendInput(tx);
+  }
+  if (tx instanceof fixedScriptWallet.BitGoPsbt) {
+    assert(pubs && isTriple(pubs), 'pub triple is required to check for key path spend inputs in wasm-utxo PSBT');
+    const rootWalletKeys = fixedScriptWallet.RootWalletKeys.fromXpubs(pubs);
+    const replayProtection = { publicKeys: getReplayProtectionPubkeys(coinName) };
+    const parsed = tx.parseTransactionWithWalletKeys(rootWalletKeys, replayProtection);
+    return parsed.inputs.some((input) => input.scriptType === 'p2trMusig2KeyPath');
+  }
+  return false;
+}
+
 /**
  * Convert ValidationError to TxIntentMismatchRecipientError with structured data
  *
@@ -379,7 +402,6 @@ export abstract class AbstractUtxoCoin
 
   public altScriptHash?: number;
   public supportAltScriptDestination?: boolean;
-  public defaultSdkBackend: SdkBackend = 'utxolib';
   public readonly amountType: 'number' | 'bigint';
 
   protected constructor(bitgo: BitGoBase, amountType: 'number' | 'bigint' = 'number') {
@@ -387,6 +409,10 @@ export abstract class AbstractUtxoCoin
     this.amountType = amountType;
   }
 
+  get defaultSdkBackend(): SdkBackend {
+    return isUtxoCoinNameMainnet(this.name) ? 'utxolib' : 'wasm-utxo';
+  }
+
   /**
    * @deprecated - will be removed when we drop support for utxolib
    * Use `name` property instead.
@@ -801,20 +827,20 @@ export abstract class AbstractUtxoCoin
   /**
    * Sign a transaction with a custom signing function. Example use case is express external signer
    * @param customSigningFunction custom signing function that returns a single signed transaction
-   * @param signTransactionParams parameters for custom signing function. Includes txPrebuild and pubs (for legacy tx only).
+   * @param signTransactionParams parameters for custom signing function. Includes txPrebuild and pubs
    *
    * @returns signed transaction as hex string
    */
   async signWithCustomSigningFunction<TNumber extends number | bigint>(
     customSigningFunction: UtxoCustomSigningFunction<TNumber>,
-    signTransactionParams: { txPrebuild: TransactionPrebuild<TNumber>; pubs?: string[] }
+    signTransactionParams: { txPrebuild: TransactionPrebuild<TNumber>; pubs: string[] }
   ): Promise<SignedTransaction> {
     const txHex = signTransactionParams.txPrebuild.txHex;
     assert(txHex, 'missing txHex parameter');
 
     const tx = this.decodeTransaction(txHex);
 
-    const isTxWithKeyPathSpendInput = tx instanceof bitgo.UtxoPsbt && bitgo.isTransactionWithKeyPathSpendInput(tx);
+    const isTxWithKeyPathSpendInput = hasKeyPathSpendInput(tx, signTransactionParams.pubs, this.name);
 
     if (!isTxWithKeyPathSpendInput) {
       return await customSigningFunction({ ...signTransactionParams, coin: this });
diff --git a/modules/abstract-utxo/test/unit/customSigner.ts b/modules/abstract-utxo/test/unit/customSigner.ts
index d8ad50e996..9a96469d90 100644
--- a/modules/abstract-utxo/test/unit/customSigner.ts
+++ b/modules/abstract-utxo/test/unit/customSigner.ts
@@ -43,13 +43,14 @@ describe('UTXO Custom Signer Function', function () {
   });
 
   function nocks(txPrebuild: { txHex: string }) {
+    const pubs = rootWalletKey.triple.map((k) => k.neutered().toBase58());
     nock(bgUrl)
       .post(`/api/v2/${wallet.coin()}/wallet/${wallet.id()}/tx/build`)
       .reply(200, { ...txPrebuild, txInfo: {} });
     nock(bgUrl).get(`/api/v2/${wallet.coin()}/public/block/latest`).reply(200, { height: 1000 });
-    nock(bgUrl).persist().get(`/api/v2/${wallet.coin()}/key/${wallet.keyIds()[0]}`).reply(200, { pub: 'pub' });
-    nock(bgUrl).persist().get(`/api/v2/${wallet.coin()}/key/${wallet.keyIds()[1]}`).reply(200, { pub: 'pub' });
-    nock(bgUrl).persist().get(`/api/v2/${wallet.coin()}/key/${wallet.keyIds()[2]}`).reply(200, { pub: 'pub' });
+    nock(bgUrl).persist().get(`/api/v2/${wallet.coin()}/key/${wallet.keyIds()[0]}`).reply(200, { pub: pubs[0] });
+    nock(bgUrl).persist().get(`/api/v2/${wallet.coin()}/key/${wallet.keyIds()[1]}`).reply(200, { pub: pubs[1] });
+    nock(bgUrl).persist().get(`/api/v2/${wallet.coin()}/key/${wallet.keyIds()[2]}`).reply(200, { pub: pubs[2] });
     return nock(bgUrl).post(`/api/v2/${wallet.coin()}/wallet/${wallet.id()}/tx/send`).reply(200, { ok: true });
   }
 
diff --git a/modules/abstract-utxo/test/unit/explainTransaction.ts b/modules/abstract-utxo/test/unit/explainTransaction.ts
index aa5d368534..bcf1c0486a 100644
--- a/modules/abstract-utxo/test/unit/explainTransaction.ts
+++ b/modules/abstract-utxo/test/unit/explainTransaction.ts
@@ -1,18 +1,46 @@
 import assert from 'assert';
 
-import { Triple } from '@bitgo/sdk-core';
+import { common, Triple, Wallet } from '@bitgo/sdk-core';
+import nock = require('nock');
 
 import { bip322Fixtures } from './fixtures/bip322/fixtures';
 import { psbtTxHex } from './fixtures/psbtHexProof';
-import { getUtxoCoin } from './util';
+import { defaultBitGo, getUtxoCoin } from './util';
+
+nock.disableNetConnect();
+
+const bgUrl = common.Environments[defaultBitGo.getEnv()].uri;
 
 describe('Explain Transaction', function () {
+  afterEach(function () {
+    nock.cleanAll();
+  });
+
   describe('Verify paygo output when explaining psbt transaction', function () {
     const coin = getUtxoCoin('tbtc4');
 
+    const pubs: Triple<string> = [
+      'xpub661MyMwAqRbcFaKvNBFdV6HY7ibXxFSbL7rDjY1cVM8s3pGPTNKfjTu8SmatNZ7AcZQehSqcEnC7vezMoprQvhqQUszLhuY4G8ruv6PGEr7',
+      'xpub661MyMwAqRbcGkAVVQVHrEYQA4hfbDW9Rpn35b6sXA9TSBd5Qzjwz7F6Weje57kBVeVfimfJjXutwUDBSMz5yRwsWik9gNyxrdvSaJbjgi6',
+      'xpub661MyMwAqRbcGCCL3GYNbvKs1t5k5yeKZcV5smto9T5Z17zkcgRF4X9uzDfPxMHHedwF4JcJ6kpg8M2NWHEFC5LMSv1t3nMMm1GC9PcVmq5',
+    ];
+
+    const keyIds = ['key-user', 'key-backup', 'key-bitgo'];
+
+    function nockKeyFetch(): void {
+      keyIds.forEach((id, i) => {
+        nock(bgUrl).get(`/api/v2/${coin.getChain()}/key/${id}`).reply(200, { pub: pubs[i] });
+      });
+    }
+
     it('should detect and verify paygo address proof in PSBT', async function () {
-      // Call explainTransaction
-      await coin.explainTransaction(psbtTxHex);
+      nockKeyFetch();
+      const wallet = new Wallet(defaultBitGo, coin, {
+        id: 'mock-wallet-id',
+        coin: coin.getChain(),
+        keys: keyIds,
+      });
+      await coin.explainTransaction(psbtTxHex, wallet);
     });
   });
 
diff --git a/modules/abstract-utxo/test/unit/wallet.ts b/modules/abstract-utxo/test/unit/wallet.ts
index 6835b108f8..451873cf4d 100644
--- a/modules/abstract-utxo/test/unit/wallet.ts
+++ b/modules/abstract-utxo/test/unit/wallet.ts
@@ -52,11 +52,11 @@ describe('manage unspents', function () {
     );
 
     nocks.push(
-      ...psbts.map((psbt) =>
+      ...psbts.map(() =>
         nock(bgUrl)
           .post(
             `/api/v2/${wallet.coin()}/wallet/${wallet.id()}/tx/send`,
-            _.matches({ txHex: psbt.signAllInputsHD(rootWalletKey.user).toHex() })
+            _.matches({ type: 'consolidate', bulk: true })
           )
           .reply(200)
       )
@@ -92,10 +92,7 @@ describe('manage unspents', function () {
 
     nocks.push(
       nock(bgUrl)
-        .post(
-          `/api/v2/${wallet.coin()}/wallet/${wallet.id()}/tx/send`,
-          _.matches({ txHex: psbt.signAllInputsHD(rootWalletKey.user).toHex() })
-        )
+        .post(`/api/v2/${wallet.coin()}/wallet/${wallet.id()}/tx/send`, _.matches({ type: 'consolidate' }))
         .reply(200)
     );
 
diff --git a/modules/sdk-core/src/bitgo/baseCoin/iBaseCoin.ts b/modules/sdk-core/src/bitgo/baseCoin/iBaseCoin.ts
index cfb424aba3..936d4edd3d 100644
--- a/modules/sdk-core/src/bitgo/baseCoin/iBaseCoin.ts
+++ b/modules/sdk-core/src/bitgo/baseCoin/iBaseCoin.ts
@@ -613,7 +613,7 @@ export interface IBaseCoin {
   presignTransaction(params: PresignTransactionOptions): Promise<PresignTransactionOptions>;
   signWithCustomSigningFunction?(
     customSigningFunction: CustomSigningFunction,
-    signTransactionParams: { txPrebuild: TransactionPrebuild; pubs?: string[] }
+    signTransactionParams: { txPrebuild: TransactionPrebuild; pubs: string[] }
   ): Promise<SignedTransaction>;
   newWalletObject(walletParams: any): IWallet;
   feeEstimate(params: FeeEstimateOptions): Promise<any>;
diff --git a/modules/sdk-core/src/bitgo/wallet/wallet.ts b/modules/sdk-core/src/bitgo/wallet/wallet.ts
index bdbf815115..3786c0c2e1 100644
--- a/modules/sdk-core/src/bitgo/wallet/wallet.ts
+++ b/modules/sdk-core/src/bitgo/wallet/wallet.ts
@@ -2194,7 +2194,11 @@ export class Wallet implements IWallet {
 
     if (_.isFunction(params.customSigningFunction)) {
       if (typeof this.baseCoin.signWithCustomSigningFunction === 'function') {
-        return this.baseCoin.signWithCustomSigningFunction(params.customSigningFunction, signTransactionParams);
+        assert(pubs, 'pubs are required for custom signing');
+        return this.baseCoin.signWithCustomSigningFunction(params.customSigningFunction, {
+          ...signTransactionParams,
+          pubs,
+        });
       }
       const keys = await this.baseCoin.keychains().getKeysForSigning({ wallet: this });
       const signTransactionParamsWithSeed = {