From d1af7690a8048b1a167666c0a254ff165e321769 Mon Sep 17 00:00:00 2001
From: Yufeng He <40085740+he-yufeng@users.noreply.github.com>
Date: Mon, 22 Jun 2026 17:30:23 +0800
Subject: [PATCH] fix(dataset): propagate system message to rejected_messages
 in DPO preprocessing

When a DPO dataset row has both a `system` key and `rejected_messages`, the
recursive preprocess() call only forwarded `{'messages': row['rejected_messages']}`,
dropping the system key. This meant system was prepended to chosen messages but
never to rejected messages, producing asymmetric conversation prefixes and
corrupting the DPO training signal.

Pass `'system': row.get('system')` to the recursive call so both sides receive
the same system prefix. Also adds a regression test.
---
 swift/dataset/preprocessor/core.py    |  2 +-
 tests/general/test_data_preprocess.py | 33 ++++++++++++++++++++++++++-
 2 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/swift/dataset/preprocessor/core.py b/swift/dataset/preprocessor/core.py
index f8822e2fa4..5cc42e4db2 100644
--- a/swift/dataset/preprocessor/core.py
+++ b/swift/dataset/preprocessor/core.py
@@ -514,7 +514,7 @@ def _to_std_key(messages: List[Dict[str, str]], std_key: str, optional_keys: Lis
     def preprocess(self, row: Dict[str, Any]) -> Optional[Dict[str, Any]]:
         if 'rejected_messages' in row:
             row['rejected_messages'] = MessagesPreprocessor.preprocess(
-                self, {'messages': row['rejected_messages']})['messages']
+                self, {'messages': row['rejected_messages'], 'system': row.get('system')})['messages']
         messages = row['messages']
         if self.inner_key is not None:
             messages = messages[self.inner_key]
diff --git a/tests/general/test_data_preprocess.py b/tests/general/test_data_preprocess.py
index a8330fca20..4ba8adff18 100644
--- a/tests/general/test_data_preprocess.py
+++ b/tests/general/test_data_preprocess.py
@@ -1,6 +1,6 @@
 import unittest
 
-from swift.dataset import EncodePreprocessor, PackingDataset, load_dataset
+from swift.dataset import EncodePreprocessor, MessagesPreprocessor, PackingDataset, load_dataset
 from swift.model import get_processor
 from swift.template import get_template
 
@@ -147,5 +147,36 @@ def test_packing_dataset(self):
         self.assertIn('labels', packed[0])
 
 
+class TestMessagesPreprocessor(unittest.TestCase):
+    """Unit tests for MessagesPreprocessor (no model required)."""
+
+    def test_system_propagated_to_rejected_messages(self):
+        """system message must be prepended to rejected_messages, not only to messages.
+
+        Regression test for the bug where the recursive preprocess() call for
+        rejected_messages omitted the 'system' key, causing chosen and rejected
+        to have asymmetric conversation prefixes during DPO training.
+        """
+        row = {
+            'messages': [
+                {'role': 'user', 'content': 'Q'},
+                {'role': 'assistant', 'content': 'good'},
+            ],
+            'rejected_messages': [
+                {'role': 'user', 'content': 'Q'},
+                {'role': 'assistant', 'content': 'bad'},
+            ],
+            'system': 'You are helpful.',
+        }
+        result = MessagesPreprocessor().preprocess(row)
+        self.assertIsNotNone(result)
+        self.assertEqual(result['messages'][0]['role'], 'system',
+                         'system message should be first in chosen messages')
+        self.assertEqual(result['rejected_messages'][0]['role'], 'system',
+                         'system message should also be first in rejected_messages')
+        self.assertEqual(result['messages'][0]['content'], 'You are helpful.')
+        self.assertEqual(result['rejected_messages'][0]['content'], 'You are helpful.')
+
+
 if __name__ == '__main__':
     unittest.main()