diff --git a/my/bumble/android.py b/my/bumble/android.py
index 3a159da..54a0441 100644
--- a/my/bumble/android.py
+++ b/my/bumble/android.py
@@ -54,9 +54,10 @@ class Message(_BaseMessage):
 
 import json
 from typing import Union
-from ..core import Res, assert_never
+from ..core import Res
 import sqlite3
 from ..core.sqlite import sqlite_connect_immutable, select
+from my.core.compat import assert_never
 
 EntitiesRes = Res[Union[Person, _Message]]
 
diff --git a/my/codeforces.py b/my/codeforces.py
index 7b37ec6..f2d150a 100644
--- a/my/codeforces.py
+++ b/my/codeforces.py
@@ -6,7 +6,6 @@ from pathlib import Path
 from typing import Dict, Iterator, Sequence
 
 from my.core import get_files, Res, datetime_aware
-from my.core.common import assert_never
 
 from my.config import codeforces as config  # type: ignore[attr-defined]
 
@@ -73,7 +72,7 @@ class Parser:
                 # these contain only contests the user participated in
                 yield from self._parse_competitions(path)
             else:
-                raise RuntimeError("shouldn't happen")  # TODO switch to compat.assert_never
+                raise RuntimeError(f"shouldn't happen: {path.name}")
 
 
 def data() -> Iterator[Res[Competition]]:
diff --git a/my/core/__init__.py b/my/core/__init__.py
index d753760..c79e36e 100644
--- a/my/core/__init__.py
+++ b/my/core/__init__.py
@@ -4,7 +4,7 @@ from .common import Json
 from .common import warn_if_empty
 from .common import stat, Stats
 from .common import datetime_naive, datetime_aware
-from .common import assert_never
+from .compat import assert_never
 
 from .cfg import make_config
 from .error import Res, unwrap
@@ -26,7 +26,7 @@ __all__ = [
     'warn_if_empty',
     'stat', 'Stats',
     'datetime_aware', 'datetime_naive',
-    'assert_never',
+    'assert_never',  # TODO maybe deprecate from use in my.core? will be in stdlib soon
 
     'make_config',
 
@@ -34,7 +34,7 @@ __all__ = [
 
     'Res', 'unwrap',
 
-    'dataclass', 'Path',
+    'dataclass', 'Path',  # TODO deprecate these from use in my.core
 ]
 
 
diff --git a/my/core/common.py b/my/core/common.py
index 98dbacb..9874bed 100644
--- a/my/core/common.py
+++ b/my/core/common.py
@@ -27,7 +27,10 @@ from typing import (
     get_origin,
 )
 import warnings
+
 from . import warnings as core_warnings
+from . import compat
+from .compat import deprecated
 
 # some helper functions
 PathIsh = Union[Path, str]
@@ -633,11 +636,6 @@ class DummyExecutor(Executor):
         self._shutdown = True
 
 
-# see https://hakibenita.com/python-mypy-exhaustive-checking#exhaustiveness-checking
-def assert_never(value: NoReturn) -> NoReturn:
-    assert False, f'Unhandled value: {value} ({type(value).__name__})'
-
-
 def _check_all_hashable(fun):
     # TODO ok, take callable?
     hints = get_type_hints(fun)
@@ -693,6 +691,13 @@ def unique_everseen(
 
 
 ## legacy imports, keeping them here for backwards compatibility
+## hiding behind TYPE_CHECKING so it works in runtime
+## in principle, warnings.deprecated decorator should cooperate with mypy, but doesn't look like it works atm?
+## perhaps it doesn't work when it's used from typing_extensions
+if not TYPE_CHECKING:
+    assert_never = deprecated('use my.core.compat.assert_never instead')(compat.assert_never)
+
+# TODO wrap in deprecated decorator as well?
 from functools import cached_property as cproperty
 from typing import Literal
 from .cachew import mcachew
diff --git a/my/core/compat.py b/my/core/compat.py
index e984695..2c1687d 100644
--- a/my/core/compat.py
+++ b/my/core/compat.py
@@ -121,3 +121,15 @@ if sys.version_info[:2] >= (3, 10):
     from types import NoneType
 else:
     NoneType = type(None)
+
+
+if sys.version_info[:2] >= (3, 13):
+    from warnings import deprecated
+else:
+    from typing_extensions import deprecated
+
+
+if sys.version_info[:2] >= (3, 11):
+    from typing import assert_never
+else:
+    from typing_extensions import assert_never
diff --git a/my/core/sqlite.py b/my/core/sqlite.py
index e04f6fc..2580e15 100644
--- a/my/core/sqlite.py
+++ b/my/core/sqlite.py
@@ -9,7 +9,8 @@ from tempfile import TemporaryDirectory
 from typing import Tuple, Any, Iterator, Callable, Optional, Union, Literal
 
 
-from .common import PathIsh, assert_never
+from .common import PathIsh
+from .compat import assert_never
 
 
 def sqlite_connect_immutable(db: PathIsh) -> sqlite3.Connection:
diff --git a/my/experimental/destructive_parsing.py b/my/experimental/destructive_parsing.py
index 3fc739c..05c5920 100644
--- a/my/experimental/destructive_parsing.py
+++ b/my/experimental/destructive_parsing.py
@@ -1,8 +1,7 @@
 from dataclasses import dataclass
 from typing import Any, Iterator, List, Tuple
 
-from my.core import assert_never
-from my.core.compat import NoneType
+from my.core.compat import NoneType, assert_never
 
 
 # TODO Popper? not sure
diff --git a/my/fbmessenger/android.py b/my/fbmessenger/android.py
index 8a4bf4c..bc06114 100644
--- a/my/fbmessenger/android.py
+++ b/my/fbmessenger/android.py
@@ -10,8 +10,9 @@ from pathlib import Path
 import sqlite3
 from typing import Iterator, Sequence, Optional, Dict, Union, List
 
-from my.core import get_files, Paths, datetime_aware, Res, assert_never, LazyLogger, make_config
+from my.core import get_files, Paths, datetime_aware, Res, LazyLogger, make_config
 from my.core.common import unique_everseen
+from my.core.compat import assert_never
 from my.core.error import echain
 from my.core.sqlite import sqlite_connection
 
diff --git a/my/tinder/android.py b/my/tinder/android.py
index 56ee1cb..d9b256b 100644
--- a/my/tinder/android.py
+++ b/my/tinder/android.py
@@ -11,8 +11,9 @@ from pathlib import Path
 import sqlite3
 from typing import Sequence, Iterator, Union, Dict, List, Mapping
 
-from my.core import Paths, get_files, Res, assert_never, stat, Stats, datetime_aware, make_logger
+from my.core import Paths, get_files, Res, stat, Stats, datetime_aware, make_logger
 from my.core.common import unique_everseen
+from my.core.compat import assert_never
 from my.core.error import echain
 from my.core.sqlite import sqlite_connection
 import my.config
diff --git a/setup.py b/setup.py
index ab96616..cf4b79f 100644
--- a/setup.py
+++ b/setup.py
@@ -5,6 +5,7 @@ from setuptools import setup, find_namespace_packages # type: ignore
 
 INSTALL_REQUIRES = [
     'pytz',           # even though it's not needed by the core, it's so common anyway...
+    'typing-extensions',  # one of the most common pypi packages, ok to depend for core
     'appdirs',        # very common, and makes it portable
     'more-itertools', # it's just too useful and very common anyway
     'decorator'     , # less pain in writing correct decorators. very mature and stable, so worth keeping in core