diff --git a/tests/test_linking_fixed_anchor_rust_ownership.py b/tests/test_linking_fixed_anchor_rust_ownership.py
new file mode 100644
index 000000000..e22b090e5
--- /dev/null
+++ b/tests/test_linking_fixed_anchor_rust_ownership.py
@@ -0,0 +1,52 @@
+"""Fail-first ownership contract for fixed-anchor parameter linking."""
+
+from __future__ import annotations
+
+import numpy as np
+
+import fast_mlsirm._core as core
+from fast_mlsirm.linking import link_fixed_item_parameters
+from fast_mlsirm.types import MLSIRMParams
+
+
+def _params(*, theta: list[float], slopes: list[float], intercepts: list[float]) -> MLSIRMParams:
+    """Build a tiny one-dimensional parameter set for delegation evidence."""
+    return MLSIRMParams(
+        theta=np.asarray(theta, dtype=np.float64)[:, None],
+        alpha=np.log(np.asarray(slopes, dtype=np.float64)),
+        b=np.asarray(intercepts, dtype=np.float64),
+        xi=np.zeros((len(theta), 1), dtype=np.float64),
+        zeta=np.zeros((len(slopes), 1), dtype=np.float64),
+        tau=-30.0,
+    )
+
+
+def test_fixed_anchor_linking_delegates_transformation_to_rust(monkeypatch) -> None:
+    """Linked parameters and affine evidence must come from the Rust owner."""
+    source = _params(theta=[-0.5, 0.5], slopes=[1.0, 1.5], intercepts=[-0.2, 0.7])
+    target = _params(theta=[-0.4, 0.6], slopes=[0.9, 1.4], intercepts=[-0.1, 0.6])
+    anchors = np.array([0, 1], dtype=np.int64)
+    factors = np.array([0, 0], dtype=np.int64)
+    calls: list[tuple[tuple[object, ...], dict[str, object]]] = []
+
+    def fake_link(*args, **kwargs):
+        calls.append((args, kwargs))
+        return {
+            "theta": [[9.0], [8.0]],
+            "alpha": [0.125, 0.25],
+            "b": [-0.75, 1.25],
+            "scale": [1.75],
+            "shift": [-0.375],
+        }
+
+    monkeypatch.setattr(core, "link_fixed_item_parameters", fake_link, raising=False)
+
+    linked, evidence = link_fixed_item_parameters(source, target, anchors, factors)
+
+    assert len(calls) == 1
+    assert np.array_equal(linked.theta, np.array([[9.0], [8.0]], dtype=np.float64))
+    assert np.array_equal(linked.alpha, np.array([0.125, 0.25], dtype=np.float64))
+    assert np.array_equal(linked.b, np.array([-0.75, 1.25], dtype=np.float64))
+    assert np.array_equal(evidence["scale"], np.array([1.75], dtype=np.float64))
+    assert np.array_equal(evidence["shift"], np.array([-0.375], dtype=np.float64))
+    assert np.array_equal(evidence["anchor_items"], anchors)