diff --git a/src/pyrecest/__init__.py b/src/pyrecest/__init__.py
index 9df2b5bfa..97fef7162 100644
--- a/src/pyrecest/__init__.py
+++ b/src/pyrecest/__init__.py
@@ -91,8 +91,26 @@ def squeeze(x, axis=None):
     backend.squeeze = squeeze
 
 
+def _patch_set_diag_arraylike_facade() -> None:
+    """Make public set_diag accept array-like matrix inputs."""
+
+    import pyrecest.backend as backend  # pylint: disable=import-outside-toplevel
+
+    original_set_diag = backend.set_diag
+
+    def set_diag(x, new_diag):
+        if not backend.is_array(x):
+            x = backend.array(x)
+        return original_set_diag(x, new_diag)
+
+    set_diag.__name__ = getattr(original_set_diag, "__name__", "set_diag")
+    set_diag.__doc__ = getattr(original_set_diag, "__doc__", None)
+    backend.set_diag = set_diag
+
+
 _patch_shared_numpy_copy_facade()
 _patch_shared_numpy_squeeze_facade()
+_patch_set_diag_arraylike_facade()
 
 
 def _patch_pytorch_comparison_facade() -> None:
diff --git a/tests/test_backend_set_diag_contract.py b/tests/test_backend_set_diag_contract.py
new file mode 100644
index 000000000..266810524
--- /dev/null
+++ b/tests/test_backend_set_diag_contract.py
@@ -0,0 +1,14 @@
+import pyrecest.backend as backend
+
+
+def _to_python(value):
+    value = backend.to_numpy(value)
+    if hasattr(value, "tolist"):
+        return value.tolist()
+    return value
+
+
+def test_set_diag_accepts_array_like_matrix_inputs():
+    result = backend.set_diag([[1, 2], [3, 4]], [9, 8])
+
+    assert _to_python(result) == [[9, 2], [3, 8]]