diff --git a/tests/unit/utils/test_union_contract.py b/tests/unit/utils/test_union_contract.py
new file mode 100644
index 0000000..a660881
--- /dev/null
+++ b/tests/unit/utils/test_union_contract.py
@@ -0,0 +1,92 @@
+"""Union-introspection compatibility contract."""
+
+import sys
+import typing
+from typing import Any, Optional, Union, get_args, get_origin
+
+import pytest
+
+from mode.utils.objects import (
+    _remove_optional,
+    is_optional,
+    is_union,
+    remove_optional,
+)
+
+
+def _eval_pep604(expression: str) -> Any:
+    return eval(expression)
+
+
+@pytest.mark.parametrize(
+    "annotation",
+    [Union[str, int], Optional[str], Union[str, list, dict, None]],
+)
+def test_typing_union_is_recognized(annotation):
+    assert is_union(annotation)
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 10), reason="PEP 604 requires Python 3.10"
+)
+def test_pep604_union_is_recognized():
+    annotations = [
+        _eval_pep604("str | int"),
+        _eval_pep604("str | None"),
+        _eval_pep604("str | list | dict | None"),
+        _eval_pep604("list[str] | dict[str, object] | None"),
+    ]
+    assert all(is_union(annotation) for annotation in annotations)
+
+
+@pytest.mark.parametrize(
+    "annotation,expected",
+    [
+        (Optional[str], True),
+        (Union[str, int, None], True),
+        (Union[str, int], False),
+        (str, False),
+    ],
+)
+def test_typing_optional_detection(annotation, expected):
+    assert is_optional(annotation) is expected
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 10), reason="PEP 604 requires Python 3.10"
+)
+def test_pep604_optional_detection():
+    cases = [
+        (_eval_pep604("str | None"), True),
+        (_eval_pep604("str | int | None"), True),
+        (_eval_pep604("str | int"), False),
+        (_eval_pep604("list[str] | dict[str, object] | None"), True),
+    ]
+    for annotation, expected in cases:
+        assert is_optional(annotation) is expected
+
+
+def test_remove_optional_preserves_multi_type_union():
+    result = remove_optional(Union[str, list, dict, None])
+    assert is_union(result)
+    assert set(get_args(result)) == {str, list, dict}
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 10), reason="PEP 604 requires Python 3.10"
+)
+def test_remove_optional_normalizes_pep604_multi_type_union():
+    result = remove_optional(_eval_pep604("str | list | dict | None"))
+    assert get_origin(result) is typing.Union
+    assert set(get_args(result)) == {str, list, dict}
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 10), reason="PEP 604 requires Python 3.10"
+)
+def test_remove_optional_with_origin_retains_all_non_none_members():
+    args, origin = _remove_optional(
+        _eval_pep604("str | list | dict | None"), find_origin=True
+    )
+    assert origin is typing.Union
+    assert set(args) == {str, list, dict}