diff --git a/docs/changelog.d/974-utility-control-boundary.md b/docs/changelog.d/974-utility-control-boundary.md
new file mode 100644
index 000000000..452ce8a02
--- /dev/null
+++ b/docs/changelog.d/974-utility-control-boundary.md
@@ -0,0 +1,6 @@
+# Selection utility numeric trust boundary
+
+## Fixed
+
+- Hardened classical selection-utility and Taylor-Russell scalar controls so booleans, non-real objects, and non-finite values fail with package-owned validation before compiled Rust discovery.
+- Prevented arbitrary caller-defined `__float__` callbacks from executing during public control marshalling while preserving genuine Python/NumPy real scalar compatibility and keeping all BCG, Naylor-Shine, and Taylor-Russell arithmetic Rust-owned.
diff --git a/python/fast_mlsirm/utility.py b/python/fast_mlsirm/utility.py
index c4152203b..6aa2834e5 100644
--- a/python/fast_mlsirm/utility.py
+++ b/python/fast_mlsirm/utility.py
@@ -36,8 +36,29 @@
 
 from __future__ import annotations
 
+import math
 from dataclasses import dataclass
 
+import numpy as np
+
+
+_TRUSTED_REAL_SCALAR_TYPES = (
+    int,
+    float,
+    np.int8,
+    np.int16,
+    np.int32,
+    np.int64,
+    np.uint8,
+    np.uint16,
+    np.uint32,
+    np.uint64,
+    np.float16,
+    np.float32,
+    np.float64,
+    np.longdouble,
+)
+
 
 @dataclass
 class SelectionUtilityResult:
@@ -46,7 +67,7 @@ class SelectionUtilityResult:
     ``xc`` is the standard-normal predictor cutoff ``Phi^-1(1 - sr)``;
     ``ux`` the selection intensity ``phi(xc)/sr`` (mean standardized
     predictor of those selected); ``pux = rxy * ux`` the Naylor-Shine mean
-    standardized criterion of those selected; ``utility_gain`` the BCG
+    standardized criterion of those selected); ``utility_gain`` the BCG
     gain ``n * period * sdy * pux - cost_total``."""
 
     xc: float
@@ -67,6 +88,16 @@ class TaylorRussellResult:
     q_joint: float
 
 
+def _coerce_finite_real(value: object, *, name: str) -> float:
+    """Return a trusted finite real scalar without invoking caller protocols."""
+    if type(value) not in _TRUSTED_REAL_SCALAR_TYPES:
+        raise ValueError(f"{name} must be a finite real number")
+    marshaled = float(value)
+    if not math.isfinite(marshaled):
+        raise ValueError(f"{name} must be a finite real number")
+    return marshaled
+
+
 def selection_utility(
     n: float,
     sdy: float,
@@ -84,10 +115,22 @@ def selection_utility(
     applicant" but never multiplies by ``n``; we document the actual
     semantics), ``period`` expected tenure (>= 1).
     """
+    marshaled_n = _coerce_finite_real(n, name="n")
+    marshaled_sdy = _coerce_finite_real(sdy, name="sdy")
+    marshaled_rxy = _coerce_finite_real(rxy, name="rxy")
+    marshaled_sr = _coerce_finite_real(sr, name="sr")
+    marshaled_cost_total = _coerce_finite_real(cost_total, name="cost_total")
+    marshaled_period = _coerce_finite_real(period, name="period")
+
     from . import _core
 
     r = _core.selection_utility(
-        float(n), float(sdy), float(rxy), float(sr), float(cost_total), float(period)
+        marshaled_n,
+        marshaled_sdy,
+        marshaled_rxy,
+        marshaled_sr,
+        marshaled_cost_total,
+        marshaled_period,
     )
     return SelectionUtilityResult(
         xc=r["xc"], ux=r["ux"], pux=r["pux"], utility_gain=r["utility_gain"]
@@ -101,9 +144,13 @@ def taylor_russell(rxy: float, sr: float, br: float) -> TaylorRussellResult:
     base rate of success in (0, 1). At ``rxy = 0`` the success ratio
     equals ``br`` (no selection information).
     """
+    marshaled_rxy = _coerce_finite_real(rxy, name="rxy")
+    marshaled_sr = _coerce_finite_real(sr, name="sr")
+    marshaled_br = _coerce_finite_real(br, name="br")
+
     from . import _core
 
-    r = _core.taylor_russell(float(rxy), float(sr), float(br))
+    r = _core.taylor_russell(marshaled_rxy, marshaled_sr, marshaled_br)
     return TaylorRussellResult(
         success_ratio=r["success_ratio"],
         base_rate=r["base_rate"],
diff --git a/tests/test_utility_control_boundary.py b/tests/test_utility_control_boundary.py
new file mode 100644
index 000000000..23ce5fa97
--- /dev/null
+++ b/tests/test_utility_control_boundary.py
@@ -0,0 +1,93 @@
+"""Trust-boundary regressions for classical selection utility controls."""
+
+from __future__ import annotations
+
+import math
+
+import numpy as np
+import pytest
+
+from fast_mlsirm.utility import selection_utility, taylor_russell
+
+
+class _HostileFloat:
+    """Arbitrary float-protocol object whose callback must stay unreachable."""
+
+    calls = 0
+
+    @classmethod
+    def reset(cls) -> None:
+        """Reset the callback counter."""
+        cls.calls = 0
+
+    def __float__(self) -> float:
+        type(self).calls += 1
+        raise AssertionError("caller __float__ callback must not execute")
+
+
+class _HostileFloatSubclass(float):
+    """Float subclass whose conversion callback must stay unreachable."""
+
+    calls = 0
+
+    @classmethod
+    def reset(cls) -> None:
+        """Reset the callback counter."""
+        cls.calls = 0
+
+    def __float__(self) -> float:
+        type(self).calls += 1
+        raise AssertionError("caller float-subclass __float__ callback must not execute")
+
+
+_UTILITY_CONTROL_CALLS = (
+    lambda value: selection_utility(value, 1.0, 0.5, 0.5),
+    lambda value: selection_utility(1.0, value, 0.5, 0.5),
+    lambda value: selection_utility(1.0, 1.0, value, 0.5),
+    lambda value: selection_utility(1.0, 1.0, 0.5, value),
+    lambda value: selection_utility(1.0, 1.0, 0.5, 0.5, value, 1.0),
+    lambda value: selection_utility(1.0, 1.0, 0.5, 0.5, 0.0, value),
+    lambda value: taylor_russell(value, 0.5, 0.5),
+    lambda value: taylor_russell(0.4, value, 0.5),
+    lambda value: taylor_russell(0.4, 0.5, value),
+)
+
+
+@pytest.mark.parametrize("call", _UTILITY_CONTROL_CALLS)
+@pytest.mark.parametrize("hostile_type", (_HostileFloat, _HostileFloatSubclass))
+def test_utility_rejects_float_protocol_objects_without_callbacks(
+    call, hostile_type
+) -> None:
+    """Every scalar position rejects hostile conversion without callback execution."""
+    hostile_type.reset()
+
+    with pytest.raises(ValueError, match="must be a finite real number"):
+        call(hostile_type())
+
+    assert hostile_type.calls == 0
+
+
+@pytest.mark.parametrize("call", _UTILITY_CONTROL_CALLS)
+@pytest.mark.parametrize("value", (True, False, math.inf, -math.inf, math.nan))
+def test_utility_rejects_boolean_and_nonfinite_controls(call, value) -> None:
+    """Every scalar position rejects boolean and non-finite control values."""
+    with pytest.raises(ValueError, match="must be a finite real number"):
+        call(value)
+
+
+def test_numpy_real_scalars_preserve_native_results() -> None:
+    """Trusted NumPy real scalars produce the same Rust-owned results as floats."""
+    py_utility = selection_utility(10.0, 2.0, 0.4, 0.5, 1.0, 2.0)
+    np_utility = selection_utility(
+        np.float64(10.0),
+        np.float64(2.0),
+        np.float64(0.4),
+        np.float64(0.5),
+        np.float64(1.0),
+        np.float64(2.0),
+    )
+    assert np_utility == py_utility
+
+    py_tr = taylor_russell(0.4, 0.5, 0.6)
+    np_tr = taylor_russell(np.float64(0.4), np.float64(0.5), np.float64(0.6))
+    assert np_tr == py_tr