diff --git a/py/api/images.py b/py/api/images.py
index bde28c1..3335773 100644
--- a/py/api/images.py
+++ b/py/api/images.py
@@ -1049,15 +1049,12 @@ class ImageRoutesMixin:
                     }
                 )
 
-            # Verify this is actually our thumbnails directory
+            # Verify this is actually our thumbnails directory: a real child
+            # of the output root (symlinks resolved), not merely a path that
+            # shares its prefix such as "<root>2/thumbnails".
             try:
-                thumbnails_dir_resolved = thumbnails_dir.resolve()
-                output_path_resolved = output_path.resolve()
-
                 if (
-                    not str(thumbnails_dir_resolved).startswith(
-                        str(output_path_resolved)
-                    )
+                    not _is_within(thumbnails_dir, output_path)
                     or thumbnails_dir.name != "thumbnails"
                 ):
                     self.logger.error(
diff --git a/tests/test_api_images.py b/tests/test_api_images.py
index db0d598..7f879ea 100644
--- a/tests/test_api_images.py
+++ b/tests/test_api_images.py
@@ -1254,6 +1254,22 @@ class TestClearThumbnails(ImageRouteCoverageCase):
         status, _ = await self._json("POST", self.URL)
         self.assertEqual(status, 500)
 
+    async def test_sibling_prefix_directory_is_refused(self):
+        # "<base>/out2/thumbnails" starts with "<base>/out" as a string but is
+        # not inside it; a symlinked thumbnails dir pointing there must be
+        # refused rather than emptied.
+        out = self.comfy_dir / "out"
+        out.mkdir()
+        sibling = make_png(self.comfy_dir / "out2" / "thumbnails" / "a_thumb.png")
+        self._make_symlink(sibling.parent, out / "thumbnails")
+        self.api._find_comfyui_output_dir = lambda: str(out)
+
+        status, data = await self._json("POST", self.URL)
+
+        self.assertEqual(status, 400)
+        self.assertFalse(data["success"])
+        self.assertTrue(sibling.is_file())
+
 
 class TestImagePromptLookup(ImageRouteCoverageCase):