diff --git a/tests/test_fitstats_rust_required.py b/tests/test_fitstats_rust_required.py
new file mode 100644
index 000000000..9ec2a9580
--- /dev/null
+++ b/tests/test_fitstats_rust_required.py
@@ -0,0 +1,38 @@
+"""Fail-first production-ownership contracts for fit-statistics arithmetic."""
+
+from __future__ import annotations
+
+import numpy as np
+import pytest
+
+import fast_mlsirm.fitstats as fitstats
+
+
+def test_chi2_survival_requires_compiled_rust_core(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Public chi-square tail probability must not select Python fallback arithmetic."""
+    monkeypatch.setattr(fitstats, "_core_module", lambda: None)
+
+    with pytest.raises(RuntimeError, match=r"fit statistics require the compiled Rust core"):
+        fitstats.chi2_sf(5.0, 2.0)
+
+
+def test_bh_decision_requires_compiled_rust_core(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Public BH decisions must fail closed rather than recompute in NumPy."""
+    monkeypatch.setattr(fitstats, "_core_module", lambda: None)
+
+    with pytest.raises(RuntimeError, match=r"fit statistics require the compiled Rust core"):
+        fitstats.benjamini_hochberg(np.array([0.001, 0.02, 0.4], dtype=np.float64), q=0.05)
+
+
+class _IncompleteCore:
+    """Compiled-core sentinel missing the migrated fit-statistics entrypoints."""
+
+
+def test_incompatible_core_fails_with_same_capability_error(monkeypatch: pytest.MonkeyPatch) -> None:
+    """An old/incompatible extension must not reactivate Python numerical ownership."""
+    monkeypatch.setattr(fitstats, "_core_module", lambda: _IncompleteCore())
+
+    with pytest.raises(RuntimeError, match=r"fit statistics require the compiled Rust core"):
+        fitstats.chi2_sf(3.0, 1.0)
+    with pytest.raises(RuntimeError, match=r"fit statistics require the compiled Rust core"):
+        fitstats.benjamini_hochberg(np.array([0.01, 0.2], dtype=np.float64))