@@ -1,4 +1,5 @@
 """ Tests for the internal type cache in CPython. """
+import collections.abc
 import unittest
 import dis
 from test import support
@@ -108,6 +109,25 @@ class HolderSub(Holder):
             Holder.set_value()
             HolderSub.value
 
+    def test_abc_register_invalidates_subclass_versions(self):
+        class Parent:
+            pass
+
+        class Child(Parent):
+            pass
+
+        type_assign_version(Parent)
+        type_assign_version(Child)
+        parent_version = type_get_version(Parent)
+        child_version = type_get_version(Child)
+        if parent_version == 0 or child_version == 0:
+            self.skipTest("Could not assign valid type versions")
+
+        collections.abc.Mapping.register(Parent)
+
+        self.assertEqual(type_get_version(Parent), 0)
+        self.assertEqual(type_get_version(Child), 0)
+
 @support.cpython_only
 @requires_specialization
 class TypeCacheWithSpecializationTests(unittest.TestCase):

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-05-23-09-55-50.gh-issue-148450.2MEVqH.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-05-23-09-55-50.gh-issue-148450.2MEVqH.rst
@@ -0,0 +1 @@
+Fix ``abc.register()`` so it invalidates type version tags for registered classes.
@@ -5398,6 +5398,15 @@ void
 _PyType_SetFlagsRecursive(PyTypeObject *self, unsigned long mask, unsigned long flags)
 {
     BEGIN_TYPE_LOCK();
+    /* Ideally, changing flags and invalidating the old version tag would
+       happen in one step. For this backport, keep it simple and invalidate
+       first while holding TYPE_LOCK. Immutable types are skipped because
+       set_flags_recursive() does not modify them. */
+    if (!PyType_HasFeature(self, Py_TPFLAGS_IMMUTABLETYPE) &&
+        (self->tp_flags & mask) != flags)
+    {
+        type_modified_unlocked(self);
+    }
     set_flags_recursive(self, mask, flags);
     END_TYPE_LOCK();
 }
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Fix ``abc.register()`` so it invalidates type version tags for registered classes.