diff --git a/backend/config/config_manager.py b/backend/config/config_manager.py
index 16883284a..54bbedec5 100644
--- a/backend/config/config_manager.py
+++ b/backend/config/config_manager.py
@@ -139,12 +139,16 @@ class Config:
         self.__dict__.update(entries)
 
     @functools.cached_property
-    def has_structure_path_b(self) -> bool:
-        # Structure A (roms/{platform}) takes priority: if the top-level roms
-        # folder exists, never claim Structure B even if some platform dirs
-        # happen to contain a roms sub-folder.
+    def has_structure_path_a(self) -> bool:
+        # Structure A ({roms_folder}/{platform}) takes priority: if the top-level roms
+        # folder exists, claim Structure A even if some platform dirs happen to
+        # contain a {roms_folder} sub-folder.
         roms_path = os.path.join(LIBRARY_BASE_PATH, self.ROMS_FOLDER_NAME)
-        if os.path.isdir(roms_path):
+        return os.path.isdir(roms_path)
+
+    @functools.cached_property
+    def has_structure_path_b(self) -> bool:
+        if self.has_structure_path_a:
             return False
 
         pattern = os.path.join(
@@ -153,6 +157,7 @@ class Config:
         for match in glob.iglob(pattern):
             if os.path.isdir(match):
                 return True
+
         return False
 
 
diff --git a/backend/handler/filesystem/platforms_handler.py b/backend/handler/filesystem/platforms_handler.py
index 8d1c4f5f2..7e204e4d7 100644
--- a/backend/handler/filesystem/platforms_handler.py
+++ b/backend/handler/filesystem/platforms_handler.py
@@ -33,9 +33,9 @@ class FSPlatformsHandler(FSHandler):
     def detect_library_structure(self) -> LibraryStructure | None:
         """Detects the library structure type.
 
-        Structure A (roms/{platform}) takes priority over Structure B
-        ({platform}/roms) so that existing libraries are not broken when a
-        stray {platform}/roms directory happens to exist alongside them.
+        Structure A ({roms_folder}/{platform}) takes priority over Structure B
+        ({platform}/{roms_folder}) so that existing libraries are not broken when a
+        stray {platform}/{roms_folder} directory happens to exist alongside them.
 
         Returns:
             "LibraryStructure.A" for Structure A (roms/{platform}) when the
@@ -47,8 +47,7 @@ class FSPlatformsHandler(FSHandler):
         """
         cnfg = cm.get_config()
 
-        roms_path = os.path.join(LIBRARY_BASE_PATH, cnfg.ROMS_FOLDER_NAME)
-        if os.path.exists(roms_path):
+        if cnfg.has_structure_path_a:
             return LibraryStructure.A
 
         if cnfg.has_structure_path_b:
@@ -59,7 +58,10 @@ class FSPlatformsHandler(FSHandler):
     def get_platforms_directory(self) -> str:
         cnfg = cm.get_config()
 
-        # Fallback to config hint when detection is inconclusive
+        # Fallback to config hint when detection is inconclusive: default to
+        # Structure A (roms/{platform}) so a malformed library fails loudly
+        # (FolderStructureNotMatchException) rather than treating the bare
+        # library root as a flat list of platforms.
         return "" if cnfg.has_structure_path_b else cnfg.ROMS_FOLDER_NAME
 
     def get_platform_fs_structure(self, fs_slug: str) -> str:
diff --git a/backend/tests/handler/filesystem/test_platforms_handler.py b/backend/tests/handler/filesystem/test_platforms_handler.py
index 4743b7c30..5a6615c73 100644
--- a/backend/tests/handler/filesystem/test_platforms_handler.py
+++ b/backend/tests/handler/filesystem/test_platforms_handler.py
@@ -101,6 +101,7 @@ class TestFSPlatformsHandler:
         self, handler: FSPlatformsHandler, config
     ):
         """Test get_platforms_directory with Structure B ({platform}/roms)"""
+        config.has_structure_path_a = False
         config.has_structure_path_b = True
         with patch(
             "handler.filesystem.platforms_handler.cm.get_config", return_value=config
@@ -203,7 +204,7 @@ class TestFSPlatformsHandler:
         self, handler: FSPlatformsHandler, config
     ):
         """Test that get_platforms calls list_directories with correct path"""
-        config.has_structure_path_b = False
+        config.has_structure_path_a = True
         with patch(
             "handler.filesystem.platforms_handler.cm.get_config", return_value=config
         ):
@@ -217,6 +218,8 @@ class TestFSPlatformsHandler:
         self, handler: FSPlatformsHandler, config
     ):
         """Test that get_platforms calls list_directories with empty path for normal structure"""
+        config.has_structure_path_a = False
+        config.has_structure_path_b = True
         with patch(
             "handler.filesystem.platforms_handler.cm.get_config", return_value=config
         ):
@@ -315,66 +318,62 @@ class TestFSPlatformsHandler:
         self, handler: FSPlatformsHandler, config
     ):
         """Test detect_library_structure detects Structure A (roms/{platform})"""
-        roms_path = f"{LIBRARY_BASE_PATH}/{config.ROMS_FOLDER_NAME}"
+        config.has_structure_path_a = True
         config.has_structure_path_b = False
 
         with patch(
             "handler.filesystem.platforms_handler.cm.get_config", return_value=config
         ):
-            with patch("os.path.exists") as mock_exists:
-                mock_exists.return_value = True
-
-                result = handler.detect_library_structure()
-                assert result == LibraryStructure.A
-                mock_exists.assert_called_once_with(roms_path)
+            result = handler.detect_library_structure()
+            assert result == LibraryStructure.A
 
     def test_detect_library_structure_structure_b(
         self, handler: FSPlatformsHandler, config
     ):
         """Test detect_library_structure detects Structure B ({platform}/roms)"""
+        config.has_structure_path_a = False
         config.has_structure_path_b = True
 
         with patch(
             "handler.filesystem.platforms_handler.cm.get_config", return_value=config
         ):
-            with patch("os.path.exists", return_value=False):
-                result = handler.detect_library_structure()
-                assert result == LibraryStructure.B
+            result = handler.detect_library_structure()
+            assert result == LibraryStructure.B
 
     def test_detect_library_structure_a_takes_priority_over_b(
         self, handler: FSPlatformsHandler, config
     ):
         """Structure A is reported when the top-level roms folder exists, even
         when Structure B directories are also present."""
+        config.has_structure_path_a = True
         config.has_structure_path_b = True
 
         with patch(
             "handler.filesystem.platforms_handler.cm.get_config", return_value=config
         ):
-            with patch("os.path.exists", return_value=True):
-                result = handler.detect_library_structure()
-                assert result == LibraryStructure.A
+            result = handler.detect_library_structure()
+            assert result == LibraryStructure.A
 
     def test_detect_library_structure_none(self, handler: FSPlatformsHandler, config):
         """Test detect_library_structure returns None when no structure detected"""
+        config.has_structure_path_a = False
         config.has_structure_path_b = False
 
         with patch(
             "handler.filesystem.platforms_handler.cm.get_config", return_value=config
         ):
-            with patch("os.path.exists", return_value=False):
-                result = handler.detect_library_structure()
-                assert result is None
+            result = handler.detect_library_structure()
+            assert result is None
 
     def test_detect_library_structure_empty_library(
         self, handler: FSPlatformsHandler, config
     ):
         """Test detect_library_structure with empty library directory"""
+        config.has_structure_path_a = False
         config.has_structure_path_b = False
 
         with patch(
             "handler.filesystem.platforms_handler.cm.get_config", return_value=config
         ):
-            with patch("os.path.exists", return_value=False):
-                result = handler.detect_library_structure()
-                assert result is None
+            result = handler.detect_library_structure()
+            assert result is None