diff --git a/stdlib/builtins.pyi b/stdlib/builtins.pyi
index 5695b17ca36d..303afce20888 100644
--- a/stdlib/builtins.pyi
+++ b/stdlib/builtins.pyi
@@ -629,7 +629,7 @@ class str(Sequence[str]):
     @overload
     def __add__(self, value: str, /) -> str: ...  # type: ignore[misc]
     # Incompatible with Sequence.__contains__
-    def __contains__(self, key: str, /) -> bool: ...  # type: ignore[override]
+    def __contains__(self, key: str, /) -> TypeGuard[str]: ...  # type: ignore[override]
     def __eq__(self, value: object, /) -> bool: ...
     def __ge__(self, value: str, /) -> bool: ...
     @overload
@@ -1162,7 +1162,7 @@ class list(MutableSequence[_T]):
     def __mul__(self, value: SupportsIndex, /) -> list[_T]: ...
     def __rmul__(self, value: SupportsIndex, /) -> list[_T]: ...
     def __imul__(self, value: SupportsIndex, /) -> Self: ...
-    def __contains__(self, key: object, /) -> bool: ...
+    def __contains__(self, key: object, /) -> TypeGuard[_T]: ...
     def __reversed__(self) -> Iterator[_T]: ...
     def __gt__(self, value: list[_T], /) -> bool: ...
     def __ge__(self, value: list[_T], /) -> bool: ...
@@ -1276,7 +1276,7 @@ class set(MutableSet[_T]):
     def union(self, *s: Iterable[_S]) -> set[_T | _S]: ...
     def update(self, *s: Iterable[_T]) -> None: ...
     def __len__(self) -> int: ...
-    def __contains__(self, o: object, /) -> bool: ...
+    def __contains__(self, o: object, /) -> TypeGuard[_T]: ...
     def __iter__(self) -> Iterator[_T]: ...
     def __and__(self, value: AbstractSet[object], /) -> set[_T]: ...
     def __iand__(self, value: AbstractSet[object], /) -> Self: ...
@@ -1309,7 +1309,7 @@ class frozenset(AbstractSet[_T_co]):
     def symmetric_difference(self, s: Iterable[_T_co], /) -> frozenset[_T_co]: ...
     def union(self, *s: Iterable[_S]) -> frozenset[_T_co | _S]: ...
     def __len__(self) -> int: ...
-    def __contains__(self, o: object, /) -> bool: ...
+    def __contains__(self, o: object, /) -> TypeGuard[_T_co]: ...
     def __iter__(self) -> Iterator[_T_co]: ...
     def __and__(self, value: AbstractSet[_T_co], /) -> frozenset[_T_co]: ...
     def __or__(self, value: AbstractSet[_S], /) -> frozenset[_T_co | _S]: ...
diff --git a/stdlib/unittest/mock.pyi b/stdlib/unittest/mock.pyi
index ef51d721297a..a6721b5dca4c 100644
--- a/stdlib/unittest/mock.pyi
+++ b/stdlib/unittest/mock.pyi
@@ -4,7 +4,7 @@ from collections.abc import Awaitable, Callable, Coroutine, Iterable, Mapping, S
 from contextlib import _GeneratorContextManager
 from types import TracebackType
 from typing import Any, ClassVar, Final, Generic, Literal, TypeVar, overload, type_check_only
-from typing_extensions import ParamSpec, Self, TypeAlias, disjoint_base
+from typing_extensions import ParamSpec, Self, TypeAlias, TypeGuard, disjoint_base
 
 _T = TypeVar("_T")
 _TT = TypeVar("_TT", bound=type[Any])
@@ -132,7 +132,7 @@ else:
 call: _Call
 
 class _CallList(list[_Call]):
-    def __contains__(self, value: Any) -> bool: ...
+    def __contains__(self, value: Any) -> TypeGuard[_Call]: ...
 
 class Base:
     def __init__(self, *args: Any, **kwargs: Any) -> None: ...