diff --git a/mapper.py b/mapper.py
index 81e8d22..a42a2d6 100644
--- a/mapper.py
+++ b/mapper.py
@@ -92,7 +92,7 @@ class ModelMapper:
         try:
             import folder_paths
 
-            self._folder_files = {}
+            folder_files = {}
             for folder_type in folder_paths.folder_names_and_paths:
                 if folder_type in EXCLUDED_FOLDER_TYPES:
                     continue
@@ -101,13 +101,17 @@ class ModelMapper:
                 except Exception:
                     files = []
                 if files:
-                    self._folder_files[folder_type] = frozenset(files)
+                    folder_files[folder_type] = frozenset(files)
 
-            # Reverse map: filename -> folder_type (last write wins on collision)
-            self._reverse = {}
-            for folder_type, files in self._folder_files.items():
+            # Build reverse map: filename -> folder_type (last write wins on collision)
+            reverse = {}
+            for folder_type, files in folder_files.items():
                 for f in files:
-                    self._reverse[f] = folder_type
+                    reverse[f] = folder_type
+
+            # Assign atomically only on success
+            self._folder_files = folder_files
+            self._reverse = reverse
 
         except Exception:
             logger.warning("ModelMapper: failed to build model map", exc_info=True)
diff --git a/tests/test_model_mapper.py b/tests/test_model_mapper.py
index 0df081a..6bfd339 100644
--- a/tests/test_model_mapper.py
+++ b/tests/test_model_mapper.py
@@ -53,7 +53,7 @@ def test_unknown_filename_returns_none():
     assert m.get_model_type("nonexistent.ckpt") is None
 
 
-def test_extract_models_from_prompt():
+def test_extract_models_from_prompt(monkeypatch):
     m = _make_mapper()
 
     fake_node_cls = MagicMock()
@@ -72,13 +72,13 @@ def test_extract_models_from_prompt():
     }
 
     import nodes as comfy_nodes
-    comfy_nodes.NODE_CLASS_MAPPINGS = {"CheckpointLoaderSimple": fake_node_cls}
+    monkeypatch.setattr(comfy_nodes, "NODE_CLASS_MAPPINGS", {"CheckpointLoaderSimple": fake_node_cls})
     results = m.extract_models_from_prompt(fake_prompt)
 
     assert ("dream.safetensors", "checkpoints") in results
 
 
-def test_extract_models_skips_non_list_inputs():
+def test_extract_models_skips_non_list_inputs(monkeypatch):
     m = _make_mapper()
 
     fake_node_cls = MagicMock()
@@ -90,7 +90,7 @@ def test_extract_models_skips_non_list_inputs():
     fake_prompt = {"1": {"class_type": "CLIPTextEncode", "inputs": {"text": "hello"}}}
 
     import nodes as comfy_nodes
-    comfy_nodes.NODE_CLASS_MAPPINGS = {"CLIPTextEncode": fake_node_cls}
+    monkeypatch.setattr(comfy_nodes, "NODE_CLASS_MAPPINGS", {"CLIPTextEncode": fake_node_cls})
     results = m.extract_models_from_prompt(fake_prompt)
 
     assert results == []