diff --git a/deep_select/interface.py b/deep_select/interface.py
index 06081f2..7e3d146 100644
--- a/deep_select/interface.py
+++ b/deep_select/interface.py
@@ -55,6 +55,15 @@ def topk(
                     The output tensors may not be contiguous, when topk * sizeof(input.dtype or indices_dtype) is not a multiple of 32 Bytes
     """
 
+    if begin is not None:
+        raise ValueError("`begin` is not supported now")
+    if hint is not None:
+        raise ValueError("`hint` is not supported now")
+    if output_idx is not None and output_idx.dtype != indices_type:
+        raise TypeError(
+            f"`output_idx` dtype ({output_idx.dtype}) must match `indices_type` ({indices_type})"
+        )
+
     N = input.shape[0]
 
     def get_empty_and_aligned_tensor(dim0: int, dim1: int, device: torch.device, dtype: torch.dtype):
@@ -70,11 +79,7 @@ def get_empty_and_aligned_tensor(dim0: int, dim1: int, device: torch.device, dty
     output_val = get_empty_and_aligned_tensor(N, topk, device=input.device, dtype=input.dtype) if return_value else None
     if output_idx is None:
         output_idx = get_empty_and_aligned_tensor(N, topk, device=input.device, dtype=indices_type)
-    else:
-        assert output_idx.dtype == indices_type
 
-    assert begin is None, "`begin` is not supported now"
-    assert hint is None, "`hint` is not supported now"
     backend_args = (
         input,
         topk,
diff --git a/tests/test_interface_validation.py b/tests/test_interface_validation.py
new file mode 100644
index 0000000..c4c9946
--- /dev/null
+++ b/tests/test_interface_validation.py
@@ -0,0 +1,93 @@
+"""CPU-only tests for Python interface validation.
+
+Run with ``python -O`` to ensure public validation does not depend on removable
+assertions. The CUDA extension is replaced with a tiny fake, but real PyTorch
+tensors exercise the public function's argument handling.
+"""
+
+from __future__ import annotations
+
+import importlib
+import sys
+import types
+import unittest
+from pathlib import Path
+
+import torch
+
+
+class InterfaceValidationTest(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls) -> None:
+        cls.backend_calls = 0
+        module_names = (
+            "deep_select",
+            "deep_select.deep_select_cuda",
+            "deep_select.interface",
+        )
+        cls.saved_modules = {name: sys.modules.get(name) for name in module_names}
+        backend = types.ModuleType("deep_select.deep_select_cuda")
+        backend.get_alignment_requirement = lambda: (16, 32)
+
+        def topk(*_args: object) -> None:
+            cls.backend_calls += 1
+
+        backend.topk = topk
+        package = types.ModuleType("deep_select")
+        package.__path__ = [str(Path(__file__).resolve().parents[1] / "deep_select")]
+        sys.modules["deep_select"] = package
+        sys.modules["deep_select.deep_select_cuda"] = backend
+        cls.interface = importlib.import_module("deep_select.interface")
+
+    @classmethod
+    def tearDownClass(cls) -> None:
+        for name, module in cls.saved_modules.items():
+            if module is None:
+                sys.modules.pop(name, None)
+            else:
+                sys.modules[name] = module
+
+    def setUp(self) -> None:
+        self.__class__.backend_calls = 0
+        self.input = torch.empty((1, 32), dtype=torch.float32)
+
+    def assert_rejected_before_backend(
+        self, error_type: type[Exception], **kwargs: object
+    ) -> None:
+        with self.assertRaises(error_type):
+            self.interface.topk(self.input, 4, return_value=False, **kwargs)
+        self.assertEqual(self.backend_calls, 0)
+
+    def test_begin_is_rejected_explicitly(self) -> None:
+        self.assert_rejected_before_backend(
+            ValueError, begin=torch.empty(1, dtype=torch.int32)
+        )
+
+    def test_hint_is_rejected_explicitly(self) -> None:
+        self.assert_rejected_before_backend(
+            ValueError, hint=torch.empty(1, dtype=torch.int32)
+        )
+
+    def test_preallocated_output_dtype_must_match(self) -> None:
+        self.assert_rejected_before_backend(
+            TypeError,
+            output_idx=torch.empty((1, 4), dtype=torch.int32),
+            indices_type=torch.int64,
+        )
+
+    def test_valid_preallocated_output_reaches_backend(self) -> None:
+        output_idx = torch.empty((1, 4), dtype=torch.int64)
+        output_value, returned_idx = self.interface.topk(
+            self.input,
+            4,
+            return_value=False,
+            output_idx=output_idx,
+            indices_type=torch.int64,
+        )
+        self.assertIsNone(output_value)
+        self.assertIs(returned_idx, output_idx)
+        self.assertEqual(self.backend_calls, 1)
+
+
+if __name__ == "__main__":
+    unittest.main()