diff --git a/src/modeldock/common/platform.py b/src/modeldock/common/platform.py
index 57f3ee8..6542fd4 100644
--- a/src/modeldock/common/platform.py
+++ b/src/modeldock/common/platform.py
@@ -19,22 +19,22 @@ def app_name() -> str:
 
 def user_config_dir() -> Path:
     """Return the per-user config directory for ModelDock."""
-    return Path(platformdirs.user_config_dir(app_name(), roaming=True))
+    return Path(platformdirs.user_config_dir(app_name(), appauthor=False, roaming=True))
 
 
 def user_cache_dir() -> Path:
     """Return the per-user cache directory for ModelDock."""
-    return Path(platformdirs.user_cache_dir(app_name()))
+    return Path(platformdirs.user_cache_dir(app_name(), appauthor=False))
 
 
 def user_data_dir() -> Path:
     """Return the per-user data directory for ModelDock."""
-    return Path(platformdirs.user_data_dir(app_name()))
+    return Path(platformdirs.user_data_dir(app_name(), appauthor=False))
 
 
 def system_config_dir() -> Path:
     """Return the system-wide config directory (may not exist)."""
-    return Path(platformdirs.site_config_dir(app_name()))
+    return Path(platformdirs.site_config_dir(app_name(), appauthor=False))
 
 
 def default_cache_dir() -> Path:
diff --git a/tests/unit/test_platform.py b/tests/unit/test_platform.py
new file mode 100644
index 0000000..fa11c27
--- /dev/null
+++ b/tests/unit/test_platform.py
@@ -0,0 +1,130 @@
+"""Unit tests for common/platform.py, including Windows-specific path handling."""
+
+from __future__ import annotations
+
+import os
+import sys
+from unittest.mock import MagicMock
+from pathlib import Path
+
+import pytest
+
+from modeldock.common import platform as md_platform
+
+# ---------------------------------------------------------------------------
+# Mocked tests — run on every OS, force Windows-style platformdirs output.
+# ---------------------------------------------------------------------------
+
+
+class TestWindowsPathsMocked:
+    """Force platformdirs to return Windows-style paths and check our wrappers."""
+
+    def test_user_config_dir_windows(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        mock_fn = MagicMock(return_value=r"C:\Users\test\AppData\Roaming\modeldock")
+        monkeypatch.setattr(md_platform.platformdirs, "user_config_dir", mock_fn)
+        result = md_platform.user_config_dir()
+        mock_fn.assert_called_once_with("modeldock", appauthor=False, roaming=True)
+        assert isinstance(result, Path)
+        assert result == Path(r"C:\Users\test\AppData\Roaming\modeldock")
+
+    def test_user_cache_dir_windows(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        mock_fn = MagicMock(return_value=r"C:\Users\test\AppData\Local\modeldock\Cache")
+        monkeypatch.setattr(md_platform.platformdirs, "user_cache_dir", mock_fn)
+        result = md_platform.user_cache_dir()
+        mock_fn.assert_called_once_with("modeldock", appauthor=False)
+        assert isinstance(result, Path)
+        assert result == Path(r"C:\Users\test\AppData\Local\modeldock\Cache")
+
+    def test_user_data_dir_windows(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        mock_fn = MagicMock(return_value=r"C:\Users\test\AppData\Local\modeldock")
+        monkeypatch.setattr(md_platform.platformdirs, "user_data_dir", mock_fn)
+        result = md_platform.user_data_dir()
+        mock_fn.assert_called_once_with("modeldock", appauthor=False)
+        assert isinstance(result, Path)
+        assert result == Path(r"C:\Users\test\AppData\Local\modeldock")
+
+    def test_system_config_dir_windows(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        mock_fn = MagicMock(return_value=r"C:\ProgramData\modeldock")
+        monkeypatch.setattr(md_platform.platformdirs, "site_config_dir", mock_fn)
+        result = md_platform.system_config_dir()
+        mock_fn.assert_called_once_with("modeldock", appauthor=False)
+        assert isinstance(result, Path)
+        assert result == Path(r"C:\ProgramData\modeldock")
+
+    def test_default_cache_dir_windows_no_override(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        monkeypatch.delenv("MODELDOCK_CACHE_DIR", raising=False)
+        mock_fn = MagicMock(return_value=r"C:\Users\test\AppData\Local\modeldock")
+        monkeypatch.setattr(md_platform.platformdirs, "user_cache_dir", mock_fn)
+        result = md_platform.default_cache_dir()
+        assert result == Path(r"C:\Users\test\AppData\Local\modeldock") / "models"
+
+    def test_default_cache_dir_windows_env_override(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        monkeypatch.setenv("MODELDOCK_CACHE_DIR", r"D:\ModelDockCache")
+        result = md_platform.default_cache_dir()
+        assert result == Path(r"D:\ModelDockCache")
+
+    def test_is_windows_true(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        monkeypatch.setattr(md_platform.os, "name", "nt")
+        assert md_platform.is_windows() is True
+
+    def test_is_windows_false_on_posix(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        monkeypatch.setattr(md_platform.os, "name", "posix")
+        assert md_platform.is_windows() is False
+
+
+# ---------------------------------------------------------------------------
+# Real tests — only meaningful (and only run) on an actual Windows host,
+# e.g. the Windows job in the CI matrix. Exercises real platformdirs
+# behavior rather than mocks.
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.skipif(sys.platform != "win32", reason="Windows-only path behavior")
+class TestWindowsPathsReal:
+    """Exercise real (unmocked) platformdirs output on an actual Windows host."""
+
+    def test_is_windows_true_on_real_windows(self) -> None:
+        assert md_platform.is_windows() is True
+
+    def test_user_config_dir_under_real_appdata(self) -> None:
+        result = md_platform.user_config_dir()
+        appdata = os.environ.get("APPDATA", "")
+        assert appdata and str(result).lower().startswith(appdata.lower())
+
+    def test_user_config_dir_not_doubled(self) -> None:
+        """Regression test: appauthor must not default to appname
+        (previously produced AppData\\Roaming\\modeldock\\modeldock)."""
+        result = md_platform.user_config_dir()
+        assert str(result).lower().count("modeldock") == 1
+
+    def test_user_cache_dir_under_real_localappdata(self) -> None:
+        result = md_platform.user_cache_dir()
+        localappdata = os.environ["LOCALAPPDATA"]
+        assert str(result).lower().startswith(localappdata.lower())
+
+    def test_user_cache_dir_not_doubled(self) -> None:
+        result = md_platform.user_cache_dir()
+        assert str(result).lower().count("modeldock") == 1
+
+    def test_user_data_dir_not_doubled(self) -> None:
+        """Regression test: appauthor must not default to appname for the
+        data dir either (mirrors the config/cache doubling bug)."""
+        result = md_platform.user_data_dir()
+        assert str(result).lower().count("modeldock") == 1
+
+    def test_paths_use_backslash_separators(self) -> None:
+        # Real Windows paths render with backslashes via str(Path).
+        assert "\\" in str(md_platform.user_config_dir())
+        assert "\\" in str(md_platform.user_cache_dir())
+
+    def test_default_cache_dir_env_override_with_windows_drive_letter(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        monkeypatch.setenv("MODELDOCK_CACHE_DIR", r"E:\Custom\Cache")
+        result = md_platform.default_cache_dir()
+        assert result == Path(r"E:\Custom\Cache")
+        assert result.drive == "E:"