diff --git a/apps/worker/video_processing/unified_render_service.py b/apps/worker/video_processing/unified_render_service.py
index c97e6f55d..c17e965ae 100755
--- a/apps/worker/video_processing/unified_render_service.py
+++ b/apps/worker/video_processing/unified_render_service.py
@@ -47,13 +47,12 @@ from video_processing.reverse_engine import ReverseConfig, ReverseEngine
 from video_processing.speed_engine import SpeedEngine
 from video_processing.sticker_engine import StickerEngine
 from video_processing.subtitle_generator import generate_ass_from_timeline
-
-from packages.domain.ass_subtitle_builder import build_ass_content
 from video_processing.transition_engine import TransitionEngine
 from video_processing.trim_engine import TrimConfig, TrimEngine, extract_trim_from_clip_config
 from video_processing.tts_engine import TtsEngine
 from video_processing.watermark_engine import WatermarkConfig, WatermarkEngine
 
+from packages.domain.ass_subtitle_builder import build_ass_content
 from packages.domain.render_layer_utils import LAYER_Z_INDEX as _IMPORTED_LAYER_Z_INDEX
 from packages.domain.render_layer_utils import clip_adjusted_duration as _clip_adjusted_duration_pure
 from packages.domain.render_layer_utils import clip_effective_duration as _clip_effective_duration_pure
@@ -133,7 +132,6 @@ _PIP_SCALE = 0.25  # PiP 占主画面的比例
 # ── 统一渲染引擎 ─────────────────────────────────────────────────────────────
 
 
-
 def _overlay_title_on_ass(
     ass_path: Path,
     *,
diff --git a/tests/unit/test_overlay_title_on_ass.py b/tests/unit/test_overlay_title_on_ass.py
index 9a67e85f6..f085fe4c8 100644
--- a/tests/unit/test_overlay_title_on_ass.py
+++ b/tests/unit/test_overlay_title_on_ass.py
@@ -7,10 +7,11 @@
 4. ASR 失败但有标题时，降级生成标题 ASS
 """
 
-import pytest
 from pathlib import Path
 from unittest.mock import MagicMock, patch
 
+import pytest
+
 from apps.worker.video_processing.unified_render_service import _overlay_title_on_ass
 
 
@@ -122,7 +123,7 @@ class TestMaybeGenerateAssWithTitle:
     def _make_service(self, tmp_path, plan_config, asr_service=None):
         """创建简化的 UnifiedRenderService 实例用于测试"""
         from apps.worker.video_processing.unified_render_service import UnifiedRenderService
-        
+
         service = object.__new__(UnifiedRenderService)
         service.plan = MagicMock()
         service.plan.id = "test_plan_001"
@@ -138,14 +139,21 @@ class TestMaybeGenerateAssWithTitle:
     def test_asr_path_with_title_overlays_title(self, tmp_path):
         """ASR 路径 + 有标题 → 标题叠加到 ASS 文件"""
         plan_config = {
-            "title": {"text": "测试标题", "enabled": True, "position": "top", "size": 48, "font": "思源黑体", "color": "#ffffff"},
+            "title": {
+                "text": "测试标题",
+                "enabled": True,
+                "position": "top",
+                "size": 48,
+                "font": "思源黑体",
+                "color": "#ffffff",
+            },
             "subtitle": {"enabled": True, "auto_generated": True},
         }
-        
+
         # Mock ASR service
         mock_asr = MagicMock()
         service = self._make_service(tmp_path, plan_config, asr_service=mock_asr)
-        
+
         # Mock _generate_asr_subtitles to return a timeline with segments
         mock_timeline = MagicMock()
         mock_segment = MagicMock()
@@ -154,10 +162,10 @@ class TestMaybeGenerateAssWithTitle:
         mock_segment.text = "ASR识别的文字"
         mock_timeline.segments = [mock_segment]
         mock_timeline.segment_count = 1
-        
-        with patch.object(service, '_generate_asr_subtitles', return_value=mock_timeline):
+
+        with patch.object(service, "_generate_asr_subtitles", return_value=mock_timeline):
             result = service._maybe_generate_ass(video_duration=10.0)
-        
+
         assert result is not None, "应生成 ASS 文件"
         content = result.read_text(encoding="utf-8")
         assert "测试标题" in content, "标题应出现在 ASS 文件中"
@@ -169,17 +177,17 @@ class TestMaybeGenerateAssWithTitle:
             "title": {"text": "仅标题", "enabled": True, "position": "top", "size": 48},
             "subtitle": {"enabled": True, "auto_generated": True},
         }
-        
+
         mock_asr = MagicMock()
         service = self._make_service(tmp_path, plan_config, asr_service=mock_asr)
-        
+
         # Mock ASR returns empty timeline
         mock_timeline = MagicMock()
         mock_timeline.segments = []
-        
-        with patch.object(service, '_generate_asr_subtitles', return_value=mock_timeline):
+
+        with patch.object(service, "_generate_asr_subtitles", return_value=mock_timeline):
             result = service._maybe_generate_ass(video_duration=10.0)
-        
+
         assert result is not None, "有标题时应生成 ASS 文件"
         content = result.read_text(encoding="utf-8")
         assert "仅标题" in content, "标题应出现在 ASS 文件中"
@@ -190,14 +198,14 @@ class TestMaybeGenerateAssWithTitle:
             "title": {"text": "降级标题", "enabled": True, "position": "top", "size": 48},
             "subtitle": {"enabled": True, "auto_generated": True},
         }
-        
+
         mock_asr = MagicMock()
         service = self._make_service(tmp_path, plan_config, asr_service=mock_asr)
-        
+
         # Mock ASR raises exception
-        with patch.object(service, '_generate_asr_subtitles', side_effect=RuntimeError("ASR error")):
+        with patch.object(service, "_generate_asr_subtitles", side_effect=RuntimeError("ASR error")):
             result = service._maybe_generate_ass(video_duration=10.0)
-        
+
         assert result is not None, "ASR 失败但有标题时应生成 ASS 文件"
         content = result.read_text(encoding="utf-8")
         assert "降级标题" in content, "标题应出现在降级 ASS 文件中"