diff --git a/app.py b/app.py
index 1314d58..838a0de 100644
--- a/app.py
+++ b/app.py
@@ -1,6 +1,23 @@
 # app.py — slim orchestrator
+import mimetypes
 import os
 
+
+def register_static_mime_types() -> None:
+    """Force stable JS module MIME types across platforms.
+
+    Some native Windows setups inherit stale/incorrect registry mappings for
+    ``.js``/``.mjs``, which can make Starlette serve ES modules with a non-JS
+    ``Content-Type`` and cause the UI to load but fail on click. Re-register the
+    standard MIME types at startup so static assets are served consistently.
+    """
+
+    mimetypes.add_type("text/javascript", ".js")
+    mimetypes.add_type("application/javascript", ".mjs")
+
+
+register_static_mime_types()
+
 # Windows: force HuggingFace/fastembed to COPY model files instead of symlinking.
 # On a network-share/UNC data dir Windows can't follow HF's symlinks ([WinError
 # 1463]), so the ONNX embedding model fails to load. huggingface_hub reads this
diff --git a/tests/test_app_static_mime.py b/tests/test_app_static_mime.py
new file mode 100644
index 0000000..a7ff476
--- /dev/null
+++ b/tests/test_app_static_mime.py
@@ -0,0 +1,37 @@
+import ast
+import mimetypes
+from pathlib import Path
+
+
+def _load_register_static_mime_types():
+    app_path = Path(__file__).resolve().parents[1] / "app.py"
+    tree = ast.parse(app_path.read_text(encoding="utf-8"), filename=str(app_path))
+    fn = next(node for node in tree.body if isinstance(node, ast.FunctionDef) and node.name == "register_static_mime_types")
+    module = ast.Module(body=[fn], type_ignores=[])
+    ns = {"mimetypes": mimetypes}
+    exec(compile(module, str(app_path), "exec"), ns)
+    return ns["register_static_mime_types"]
+
+
+def test_register_static_mime_types_restores_js_module_types():
+    register_static_mime_types = _load_register_static_mime_types()
+    original_js = mimetypes.types_map.get(".js")
+    original_mjs = mimetypes.types_map.get(".mjs")
+    try:
+        mimetypes.types_map[".js"] = "text/plain"
+        mimetypes.types_map.pop(".mjs", None)
+
+        register_static_mime_types()
+
+        assert mimetypes.types_map[".js"] == "text/javascript"
+        assert mimetypes.types_map[".mjs"] == "application/javascript"
+    finally:
+        if original_js is None:
+            mimetypes.types_map.pop(".js", None)
+        else:
+            mimetypes.types_map[".js"] = original_js
+
+        if original_mjs is None:
+            mimetypes.types_map.pop(".mjs", None)
+        else:
+            mimetypes.types_map[".mjs"] = original_mjs