diff --git a/src/pyrecest/filters/measurement_scoring.py b/src/pyrecest/filters/measurement_scoring.py
index 6d0ea0321..a4e1cf676 100644
--- a/src/pyrecest/filters/measurement_scoring.py
+++ b/src/pyrecest/filters/measurement_scoring.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from collections.abc import Sequence
 from dataclasses import dataclass
 from typing import Any
 
@@ -25,11 +26,19 @@ class MeasurementScore:
     predicted_measurements: Any | None
     innovation_covariance: Any | None
     residual: Any | None
-    active_measurement_indices: list[int]
+    active_measurement_indices: Sequence[int]
     measurement_weights: Any | None = None
     quadratic_form: float | None = None
     skipped_reason: str | None = None
 
+    def __post_init__(self) -> None:
+        # Own caller-provided sequences so this frozen result cannot be mutated indirectly.
+        object.__setattr__(
+            self,
+            "active_measurement_indices",
+            tuple(self.active_measurement_indices),
+        )
+
     @property
     def is_active(self) -> bool:
         """Return whether the score contains at least one active measurement."""
diff --git a/tests/filters/test_measurement_scoring.py b/tests/filters/test_measurement_scoring.py
index bfd63d276..7c750c45c 100644
--- a/tests/filters/test_measurement_scoring.py
+++ b/tests/filters/test_measurement_scoring.py
@@ -15,3 +15,13 @@ def _score(active_measurement_indices):
 def test_measurement_score_is_active_handles_numpy_index_arrays():
     assert _score(np.array([0, 2])).is_active
     assert not _score(np.array([], dtype=int)).is_active
+
+
+def test_measurement_score_owns_active_measurement_indices():
+    active_measurement_indices = []
+    score = _score(active_measurement_indices)
+
+    active_measurement_indices.append(0)
+
+    assert score.active_measurement_indices == ()
+    assert not score.is_active