diff --git a/tests/test_dxvk_fps_limit.py b/tests/test_dxvk_fps_limit.py
new file mode 100644
index 0000000..1e859f6
--- /dev/null
+++ b/tests/test_dxvk_fps_limit.py
@@ -0,0 +1,95 @@
+import os
+import tempfile
+import unittest
+
+import dxvk_fps
+
+
+class DxvkFpsLimitTests(unittest.TestCase):
+    def write_config(self, root, text):
+        path = os.path.join(root, "dxvk.conf")
+        with open(path, "w", encoding="utf-8", newline="\n") as handle:
+            handle.write(text)
+        return path
+
+    def read_config(self, path):
+        with open(path, "r", encoding="utf-8") as handle:
+            return handle.read()
+
+    def test_enable_replaces_bundled_commented_value_and_preserves_other_options(self):
+        with tempfile.TemporaryDirectory() as root:
+            path = self.write_config(
+                root,
+                "# DXVK configuration\n"
+                "# d3d9.maxFrameRate = 1000\n"
+                "dxvk.allowFse = False\n",
+            )
+
+            dxvk_fps.apply_dxvk_fps_limit(path, True, 165)
+            updated = self.read_config(path)
+
+            self.assertIn("d3d9.maxFrameRate = 165\n", updated)
+            self.assertNotIn("# d3d9.maxFrameRate = 1000", updated)
+            self.assertIn("dxvk.allowFse = False\n", updated)
+            self.assertEqual(updated.count("d3d9.maxFrameRate"), 1)
+            self.assertEqual(dxvk_fps.read_dxvk_fps_limit(path), (True, 165))
+
+    def test_disable_comments_the_setting(self):
+        with tempfile.TemporaryDirectory() as root:
+            path = self.write_config(
+                root,
+                "d3d9.maxFrameRate = 144\n"
+                "dxvk.numCompilerThreads = 4\n",
+            )
+
+            dxvk_fps.apply_dxvk_fps_limit(path, False, 144)
+            updated = self.read_config(path)
+
+            self.assertIn("# d3d9.maxFrameRate = 144\n", updated)
+            self.assertNotIn("\nd3d9.maxFrameRate = 144\n", "\n" + updated)
+            self.assertEqual(dxvk_fps.read_dxvk_fps_limit(path), (False, 144))
+
+    def test_duplicate_limit_lines_are_collapsed_to_one(self):
+        with tempfile.TemporaryDirectory() as root:
+            path = self.write_config(
+                root,
+                "d3d9.maxFrameRate = 60\n"
+                "# d3d9.maxFrameRate = 120\n"
+                "dxvk.allowFse = False\n",
+            )
+
+            dxvk_fps.apply_dxvk_fps_limit(path, True, 240)
+            updated = self.read_config(path)
+
+            self.assertEqual(updated.count("d3d9.maxFrameRate"), 1)
+            self.assertIn("d3d9.maxFrameRate = 240\n", updated)
+            self.assertIn("dxvk.allowFse = False\n", updated)
+
+    def test_missing_limit_line_is_appended(self):
+        with tempfile.TemporaryDirectory() as root:
+            path = self.write_config(root, "dxvk.allowFse = False\n")
+
+            dxvk_fps.apply_dxvk_fps_limit(path, True, 75)
+            updated = self.read_config(path)
+
+            self.assertIn("dxvk.allowFse = False\n", updated)
+            self.assertTrue(updated.endswith("d3d9.maxFrameRate = 75\n"))
+
+    def test_invalid_values_are_rejected_without_modifying_file(self):
+        with tempfile.TemporaryDirectory() as root:
+            original = "# d3d9.maxFrameRate = 1000\n"
+            path = self.write_config(root, original)
+
+            with self.assertRaises(ValueError):
+                dxvk_fps.apply_dxvk_fps_limit(path, True, 0)
+
+            self.assertEqual(self.read_config(path), original)
+
+    def test_non_windows_detection_uses_fallback(self):
+        if os.name == "nt":
+            self.skipTest("Non-Windows fallback test")
+        self.assertEqual(dxvk_fps.detect_max_refresh_rate(default=77), 77)
+
+
+if __name__ == "__main__":
+    unittest.main()