diff --git a/src/detectmatelibrary/detectors/value_range_detector.py b/src/detectmatelibrary/detectors/value_range_detector.py
index 7f61cc8..444663e 100644
--- a/src/detectmatelibrary/detectors/value_range_detector.py
+++ b/src/detectmatelibrary/detectors/value_range_detector.py
@@ -28,19 +28,28 @@ def __init__(
         self.config: ValueRangeDetectorConfig  # type narrowing for IDE
 
     def add_value(self, tracker: SingleStabilityTracker, value: Any) -> None:
-        """Add a new value to the tracker (range semantics)."""
+        """Add a new value to the tracker (range semantics).
+
+        Only the extremes are stored: ``unique_set`` holds at most two elements
+        ({min, max}), so storage is O(1) per variable instead of O(distinct
+        values). ``change_series`` and the min/max are byte-identical to storing
+        every value. The classifier's RANDOM branch can no longer fire for range
+        variables (size <= 2 never equals the change_series length); monotonic
+        counters stay excluded as UNSTABLE via their all-True series.
+        """
         try:
             value = float(value)
             value = int(value) if value.is_integer() else value
         except ValueError:
             return
-        if len(tracker.unique_set) > 0:
+        if tracker.unique_set:
             min_ = min(tracker.unique_set)
             max_ = max(tracker.unique_set)
             tracker.change_series.append(value < min_ or value > max_)
+            tracker.unique_set = {min(min_, value), max(max_, value)}
         else:
             tracker.change_series.append(True)
-        tracker.unique_set.add(value)
+            tracker.unique_set = {value}
 
     def _event_data_kwargs(self) -> Optional[Dict[str, Any]]:
         return self._stability_kwargs()
diff --git a/tests/test_detectors/test_value_range_detector.py b/tests/test_detectors/test_value_range_detector.py
index 3ce7df8..e16379f 100644
--- a/tests/test_detectors/test_value_range_detector.py
+++ b/tests/test_detectors/test_value_range_detector.py
@@ -98,6 +98,21 @@ def test_add_value_updates_tracker(self):
         assert tracker.unique_set == {5, 7}
         assert list(tracker.change_series) == [True, True]
 
+    def test_add_value_stores_only_min_max(self):
+        """unique_set stays bounded to {min, max} regardless of how many
+        distinct values are seen; change detection still tracks the true
+        range."""
+        detector = ValueRangeDetector()
+        tracker = SingleStabilityTracker()
+
+        for v in [50, 10, 30, 90, 20, 70]:  # min=10, max=90
+            detector.add_value(tracker, v)
+
+        assert tracker.unique_set == {10, 90}
+        # 50 (first, True), 10 (new min, True), 90 (new max, True) extend range;
+        # 30, 20, 70 fall inside -> False
+        assert list(tracker.change_series) == [True, True, False, True, False, False]
+
 
 class TestValueRangeDetectorTraining:
     """Test ValueRangeDetector training functionality."""