@@ -1,4 +1,5 @@
 """ Tests for the internal type cache in CPython. """
+import collections.abc
 import dis
 import unittest
 import warnings
@@ -114,6 +115,25 @@ 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
 class TypeCacheWithSpecializationTests(unittest.TestCase):
     def tearDown(self):

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-04-15-15-48-04.gh-issue-148450.2MEVqH.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-04-15-15-48-04.gh-issue-148450.2MEVqH.rst
@@ -0,0 +1 @@
+Fix ``abc.register()`` so it invalidates type version tags for registered classes.
@@ -6066,6 +6066,15 @@ 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. In 3.14, invalidate first while holding TYPE_LOCK so readers
+       cannot assign a fresh tag from stale flags. Immutable types are skipped by
+       set_flags_recursive(). */
+    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.