From 09ae59a85fe6ef2efe001ab471df48f1f62bcbc7 Mon Sep 17 00:00:00 2001
From: Yufeng He <40085740+he-yufeng@users.noreply.github.com>
Date: Mon, 22 Jun 2026 17:33:32 +0800
Subject: [PATCH 1/2] fix(template): preserve non-string prefix elements in
 _replace_system

_replace_system replaced {{SYSTEM}} in string prefix elements but filtered out
non-string elements entirely.  Token-ID lists such as ['bos_token_id'] are
valid prefix components used by ziya, bluelm, emu3_chat and other templates
(prefix=[['bos_token_id'], '{{SYSTEM}}']).  When no system message is given,
those elements were silently dropped, causing every training/inference sequence
for those templates to start without the expected BOS token.

Change the filter comprehension to a conditional expression so that
non-strings pass through unchanged.
---
 swift/template/template_meta.py     |  2 +-
 tests/general/test_template_meta.py | 24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 tests/general/test_template_meta.py

diff --git a/swift/template/template_meta.py b/swift/template/template_meta.py
index 7a8b6dcb3b..7e7a5e171c 100644
--- a/swift/template/template_meta.py
+++ b/swift/template/template_meta.py
@@ -69,7 +69,7 @@ def _has_system(prefix_or_prompt: Prompt) -> bool:
 
     @staticmethod
     def _replace_system(prefix: Prompt) -> Prompt:
-        return [p.replace('{{SYSTEM}}', '') for p in prefix if isinstance(p, str)]
+        return [p.replace('{{SYSTEM}}', '') if isinstance(p, str) else p for p in prefix]
 
     def _check_template_meta(self):
         # check
diff --git a/tests/general/test_template_meta.py b/tests/general/test_template_meta.py
new file mode 100644
index 0000000000..37dad66878
--- /dev/null
+++ b/tests/general/test_template_meta.py
@@ -0,0 +1,24 @@
+# Copyright (c) ModelScope Contributors. All rights reserved.
+from swift.template import TemplateMeta
+
+
+def test_replace_system_preserves_non_string_elements():
+    """_replace_system must not drop list elements like ['bos_token_id'].
+
+    Templates such as ziya, bluelm and emu3_chat use
+    ``prefix=[['bos_token_id'], '{{SYSTEM}}']``.  When no system message is
+    provided the prefix is produced by _replace_system, which should keep every
+    non-string element intact and only strip the placeholder from strings.
+    """
+    meta = TemplateMeta(
+        template_type='_test_replace_system_bug',
+        prefix=[['bos_token_id'], '{{SYSTEM}}'],
+        prompt=['{{QUERY}}'],
+        chat_sep=['\n'],
+    )
+    # __post_init__ moves prefix to system_prefix and builds a no-system prefix
+    # via _replace_system.  The list element must survive.
+    assert any(isinstance(p, list) for p in meta.prefix), (
+        f'_replace_system dropped the bos_token_id list; '
+        f'meta.prefix={meta.prefix!r}'
+    )

From 3ec09580d78d3f6f7274224c6bb6bea38fed0554 Mon Sep 17 00:00:00 2001
From: Yufeng He <40085740+he-yufeng@users.noreply.github.com>
Date: Wed, 24 Jun 2026 11:07:32 +0800
Subject: [PATCH 2/2] style: format the template-meta regression test to
 satisfy yapf

---
 tests/general/test_template_meta.py | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/tests/general/test_template_meta.py b/tests/general/test_template_meta.py
index 37dad66878..58d8757e29 100644
--- a/tests/general/test_template_meta.py
+++ b/tests/general/test_template_meta.py
@@ -18,7 +18,5 @@ def test_replace_system_preserves_non_string_elements():
     )
     # __post_init__ moves prefix to system_prefix and builds a no-system prefix
     # via _replace_system.  The list element must survive.
-    assert any(isinstance(p, list) for p in meta.prefix), (
-        f'_replace_system dropped the bos_token_id list; '
-        f'meta.prefix={meta.prefix!r}'
-    )
+    assert any(isinstance(p, list) for p in meta.prefix), (f'_replace_system dropped the bos_token_id list; '
+                                                           f'meta.prefix={meta.prefix!r}')