From f86f3aae426629460ad535ddba2fa95b32596b9d Mon Sep 17 00:00:00 2001
From: Otto Allmendinger <otto@bitgo.com>
Date: Tue, 18 Nov 2025 10:46:23 +0100
Subject: [PATCH] feat(abstract-utxo): prohibit legacy tx format on testnet

This adds a safeguard to prevent use of the deprecated legacy tx format
on testnet environments. The new ErrorDeprecatedTxFormat class provides
clear messaging to users about using PSBT instead.

BTC-2732

Co-authored-by: llm-git <llm-git@ttll.de>
---
 modules/abstract-utxo/src/abstractUtxoCoin.ts | 10 +++++++++
 modules/abstract-utxo/test/unit/txFormat.ts   | 21 +++++++++++++++++--
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/modules/abstract-utxo/src/abstractUtxoCoin.ts b/modules/abstract-utxo/src/abstractUtxoCoin.ts
index 058079f62d..a0c740aeb2 100644
--- a/modules/abstract-utxo/src/abstractUtxoCoin.ts
+++ b/modules/abstract-utxo/src/abstractUtxoCoin.ts
@@ -97,6 +97,12 @@ export type TxFormat =
   // While this prevents us to fully verify the transaction fee, we have other checks in place to ensure the fee is within bounds.
   | 'psbt-lite';
 
+export class ErrorDeprecatedTxFormat extends Error {
+  constructor(txFormat: TxFormat) {
+    super(`SDK support for txFormat=${txFormat} is deprecated on this environment. Please use psbt instead.`);
+  }
+}
+
 type UtxoCustomSigningFunction<TNumber extends number | bigint> = {
   (params: {
     coin: IBaseCoin;
@@ -979,6 +985,10 @@ export abstract class AbstractUtxoCoin extends BaseCoin {
   getDefaultTxFormat(wallet: Wallet, requestedFormat?: TxFormat): TxFormat | undefined {
     // If format is explicitly requested, use it
     if (requestedFormat !== undefined) {
+      if (isTestnet(this.network) && requestedFormat === 'legacy') {
+        throw new ErrorDeprecatedTxFormat(requestedFormat);
+      }
+
       return requestedFormat;
     }
 
diff --git a/modules/abstract-utxo/test/unit/txFormat.ts b/modules/abstract-utxo/test/unit/txFormat.ts
index 0911d0e93a..f0e79fdc65 100644
--- a/modules/abstract-utxo/test/unit/txFormat.ts
+++ b/modules/abstract-utxo/test/unit/txFormat.ts
@@ -3,7 +3,7 @@ import * as assert from 'assert';
 import * as utxolib from '@bitgo/utxo-lib';
 import { Wallet } from '@bitgo/sdk-core';
 
-import { AbstractUtxoCoin, TxFormat } from '../../src';
+import { AbstractUtxoCoin, ErrorDeprecatedTxFormat, TxFormat } from '../../src';
 
 import { utxoCoins, defaultBitGo } from './util';
 
@@ -151,7 +151,8 @@ describe('txFormat', function () {
 
     // Test explicitly requested formats
     runTest({
-      description: 'should respect explicitly requested legacy format',
+      description: 'should respect explicitly requested legacy format on mainnet',
+      coinFilter: (coin) => utxolib.isMainnet(coin.network),
       expectedTxFormat: 'legacy',
       requestedTxFormat: 'legacy',
     });
@@ -167,5 +168,21 @@ describe('txFormat', function () {
       expectedTxFormat: 'psbt-lite',
       requestedTxFormat: 'psbt-lite',
     });
+
+    // Test that legacy format is prohibited on testnet
+    it('should throw ErrorDeprecatedTxFormat when legacy format is requested on testnet', function () {
+      for (const coin of utxoCoins) {
+        if (!utxolib.isTestnet(coin.network)) {
+          continue;
+        }
+
+        const wallet = createMockWallet(coin, { type: 'hot' });
+        assert.throws(
+          () => getTxFormat(coin, wallet, 'legacy'),
+          ErrorDeprecatedTxFormat,
+          `Expected ErrorDeprecatedTxFormat for ${coin.getChain()}`
+        );
+      }
+    });
   });
 });