diff --git a/packages/shared/ai_service.py b/packages/shared/ai_service.py
index 32014d23d..c4a02a0fa 100755
--- a/packages/shared/ai_service.py
+++ b/packages/shared/ai_service.py
@@ -323,9 +323,16 @@ def _call_ai_cover_service(
         }
 
     if cover_type == "manual" and frame_time is not None:
+        svg_placeholder = (
+            "data:image/svg+xml,"
+            "<svg xmlns='http://www.w3.org/2000/svg' width='1080' height='1920'>"
+            "<rect width='1080' height='1920' fill='#1a1a2e'/>"
+            "<text x='540' y='960' text-anchor='middle' fill='#e0e0e0' font-size='48' font-family='sans-serif'>手动选帧</text>"
+            "</svg>"
+        )
         return {
             "type": "manual",
-            "image_url": f"/api/v1/assets/placeholder/cover?time={frame_time}",
+            "image_url": svg_placeholder,
             "frame_time": frame_time,
         }
 
@@ -368,12 +375,20 @@ def _call_ai_cover_service(
             except Exception as e:
                 logger.exception("MediaKit 抽帧失败，降级到 stub: %s", str(e))
 
-    # 降级：stub 行为
+    # 降级：stub 行为 - 返回 SVG data URI 占位图
     logger.info("使用 stub 封面: plan_id=%s", plan_id)
     time.sleep(0.3)
+    svg_placeholder = (
+        "data:image/svg+xml,"
+        "<svg xmlns='http://www.w3.org/2000/svg' width='1080' height='1920'>"
+        "<rect width='1080' height='1920' fill='#1a1a2e'/>"
+        "<text x='540' y='920' text-anchor='middle' fill='#e0e0e0' font-size='48' font-family='sans-serif'>封面生成中</text>"
+        "<text x='540' y='1000' text-anchor='middle' fill='#888888' font-size='32' font-family='sans-serif'>请配置 MediaKit API Key</text>"
+        "</svg>"
+    )
     return {
         "type": "ai_frame",
-        "image_url": f"/api/v1/assets/placeholder/cover?plan={plan_id}",
+        "image_url": svg_placeholder,
         "frame_time": round(random.uniform(1.0, 10.0), 1),
         "confidence": round(random.uniform(0.80, 0.98), 2),
     }
diff --git a/tests/unit/test_mediakit_cover.py b/tests/unit/test_mediakit_cover.py
index 49864e136..22193e67e 100755
--- a/tests/unit/test_mediakit_cover.py
+++ b/tests/unit/test_mediakit_cover.py
@@ -185,7 +185,7 @@ class TestAICoverService:
 
         # 应该降级到 stub
         assert result["type"] == "ai_frame"
-        assert "placeholder" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
 
     def test_call_ai_cover_without_video_url_fallback(self):
         """没有视频 URL 时使用 stub."""
@@ -199,7 +199,7 @@ class TestAICoverService:
         )
 
         assert result["type"] == "ai_frame"
-        assert "placeholder" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
 
     def test_call_ai_cover_upload_type(self):
         """upload 类型直接返回."""
@@ -247,7 +247,7 @@ class TestAICoverService:
         )
 
         assert result["type"] == "ai_frame"
-        assert "placeholder" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
 
     @patch("packages.shared.mediakit_client.get_mediakit_client")
     def test_call_ai_cover_empty_frames_fallback(self, mock_get_client):
@@ -267,7 +267,7 @@ class TestAICoverService:
         )
 
         assert result["type"] == "ai_frame"
-        assert "placeholder" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
 
 
 class TestGenerateCover:
diff --git a/tests/unit/test_shared_ai_service.py b/tests/unit/test_shared_ai_service.py
index f46002dbf..7603a90bb 100755
--- a/tests/unit/test_shared_ai_service.py
+++ b/tests/unit/test_shared_ai_service.py
@@ -431,7 +431,8 @@ class TestAiCoverService:
             result = _call_ai_cover_service("plan1", ["a1"], "manual", frame_time=5.5)
         assert result["type"] == "manual"
         assert result["frame_time"] == 5.5
-        assert "5.5" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
+        assert "手动选帧" in result["image_url"]
 
     def test_cover_type_ai_frame(self):
         with patch("shared.ai_service.time.sleep"):
@@ -440,7 +441,8 @@ class TestAiCoverService:
         assert result["type"] == "ai_frame"
         assert result["frame_time"] == 5.0
         assert result["confidence"] == 0.9
-        assert "plan1" in result["image_url"]
+        assert result["image_url"].startswith("data:image/svg+xml,")
+        assert "封面生成中" in result["image_url"]
 
     def test_cover_type_ai_regenerate(self):
         with patch("shared.ai_service.time.sleep"):
@@ -451,3 +453,20 @@ class TestAiCoverService:
         with patch("shared.ai_service.time.sleep"):
             result = _call_ai_cover_service("plan1", ["a1"], "ai_frame")
         assert 1.0 <= result["frame_time"] <= 10.0
+
+    def test_stub_returns_svg_data_uri(self):
+        """stub 降级返回 SVG data URI，不含任何后端 API 路径."""
+        with patch("shared.ai_service.time.sleep"):
+            result = _call_ai_cover_service("plan1", ["a1"], "ai_frame")
+        assert result["image_url"].startswith("data:image/svg+xml,")
+        assert "/api/v1/" not in result["image_url"]
+        assert "1080" in result["image_url"]
+        assert "1920" in result["image_url"]
+
+    def test_manual_stub_returns_svg_data_uri(self):
+        """manual 模式返回 SVG data URI 占位图."""
+        with patch("shared.ai_service.time.sleep"):
+            result = _call_ai_cover_service("plan1", ["a1"], "manual", frame_time=3.0)
+        assert result["image_url"].startswith("data:image/svg+xml,")
+        assert "/api/v1/" not in result["image_url"]
+        assert "手动选帧" in result["image_url"]