diff --git a/clif/backend/matcher.cc b/clif/backend/matcher.cc
index d790e1ae..a0300fe3 100644
--- a/clif/backend/matcher.cc
+++ b/clif/backend/matcher.cc
@@ -39,10 +39,6 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 
-#ifndef PYCLIF_LLVM_VERSION_MAJOR
-#define PYCLIF_LLVM_VERSION_MAJOR LLVM_VERSION_MAJOR
-#endif
-
 #if PYCLIF_LLVM_VERSION_MAJOR > 16
 #include "clang/Sema/EnterExpressionEvaluationContext.h"
 #endif
@@ -2989,20 +2985,20 @@ const clang::FunctionDecl* ClifMatcher::SpecializeFunctionTemplate(
 }
 
 std::string ClifMatcher::TemplateDeductionResult(
-    Sema::TemplateDeductionResult specialized_result) const {
+    PYCLIF_CLANG_TEMPLATEDEDUCTIONRESULT_TYPE specialized_result) const {
   switch (specialized_result) {
-    case Sema::TDK_Invalid:
+    case PYCLIF_CLANG_TEMPLATEDEDUCTIONRESULT_ENUM(Invalid):
       return "The template function declaration was invalid.";
-    case Sema::TDK_InstantiationDepth:
+    case PYCLIF_CLANG_TEMPLATEDEDUCTIONRESULT_ENUM(InstantiationDepth):
       return "Template argument deduction exceeded the maximum template "
              "instantiation depth.";
-    case Sema::TDK_Incomplete:
+    case PYCLIF_CLANG_TEMPLATEDEDUCTIONRESULT_ENUM(Incomplete):
       return "Template argument deduction did not deduce a value for every "
              "template parameter.";
-    case Sema::TDK_Inconsistent:
+    case PYCLIF_CLANG_TEMPLATEDEDUCTIONRESULT_ENUM(Inconsistent):
       return "Template argument deduction produced inconsistent deduced "
              "values.";
-    case Sema::TDK_Underqualified:
+    case PYCLIF_CLANG_TEMPLATEDEDUCTIONRESULT_ENUM(Underqualified):
       return "Template argument deduction failed due to inconsistent "
              "cv-qualifiers.";
     default:
diff --git a/clif/backend/matcher.h b/clif/backend/matcher.h
index d5313d68..802f8a9a 100644
--- a/clif/backend/matcher.h
+++ b/clif/backend/matcher.h
@@ -48,6 +48,10 @@
 #include "clif/protos/ast.pb.h"
 #include "gtest/gtest_prod.h"  // Defines FRIEND_TEST.
 
+#ifndef PYCLIF_LLVM_VERSION_MAJOR
+#define PYCLIF_LLVM_VERSION_MAJOR LLVM_VERSION_MAJOR
+#endif
+
 namespace clif {
 
 // ############################################################################
@@ -177,6 +181,16 @@ enum TypeMatchFlags : unsigned int {
 class ClifError;
 class ClifMatcherTest;
 
+#if PYCLIF_LLVM_VERSION_MAJOR >= 19  // llvm/llvm-project#81398
+#define PYCLIF_CLANG_TEMPLATEDEDUCTIONRESULT_TYPE clang::TemplateDeductionResult
+#define PYCLIF_CLANG_TEMPLATEDEDUCTIONRESULT_ENUM(Name) \
+  clang::TemplateDeductionResult::Name
+#else
+#define PYCLIF_CLANG_TEMPLATEDEDUCTIONRESULT_TYPE \
+  clang::Sema::TemplateDeductionResult
+#define PYCLIF_CLANG_TEMPLATEDEDUCTIONRESULT_ENUM(Name) Sema::TDK_##Name
+#endif
+
 class ClifMatcher {
   friend class ClifMatcherTest;
  public:
@@ -402,7 +416,7 @@ class ClifMatcher {
 
   // Transform template type deduction error codes into error messages.
   std::string TemplateDeductionResult(
-      clang::Sema::TemplateDeductionResult specialized_result) const;
+      PYCLIF_CLANG_TEMPLATEDEDUCTIONRESULT_TYPE specialized_result) const;
 
   // Add the class type as the first parameter to a function.
   void AdjustForNonClassMethods(protos::FuncDecl* clif_func_decl) const;
diff --git a/clif/pybind11/classes.py b/clif/pybind11/classes.py
index 4f4aa7b1..2fb2b61f 100644
--- a/clif/pybind11/classes.py
+++ b/clif/pybind11/classes.py
@@ -26,6 +26,8 @@
 
 I = utils.I
 
+_USE_PYTYPE_TYPE_AS_METACLASS = True
+
 _KNOWN_TYPES_WITH_MULTIPLE_INHERITANCE = {
     '::borg::Config': ['::borg::ConfigArgs']
 }
@@ -100,6 +102,8 @@ def generate_from(
         base.cpp_canonical_type in codegen_info.dynamic_attr_types):
       enable_instance_dict = True
       break
+  if _USE_PYTYPE_TYPE_AS_METACLASS:
+    definition += ', py::metaclass((PyObject*) &PyType_Type)'
   definition += ', py::release_gil_before_calling_cpp_dtor()'
   if mi_bases:
     definition += ', py::multiple_inheritance()'
@@ -117,7 +121,7 @@ def generate_from(
   for member in class_decl.members:
     if member.decltype == ast_pb2.Decl.Type.CONST:
       for s in consts.generate_from(class_name, member.const):
-        yield I + I + s
+        yield I + s
     elif member.decltype == ast_pb2.Decl.Type.FUNC:
       if member.func.name.native in ('__reduce__', '__reduce_ex__'):
         reduce_or_reduce_ex_defined = True
diff --git a/clif/python/BUILD b/clif/python/BUILD
index 7d239ff0..a970fe89 100644
--- a/clif/python/BUILD
+++ b/clif/python/BUILD
@@ -1,6 +1,7 @@
 # CLIF python frontend
 
 load("@clif_python_deps//:requirements.bzl", "requirement")
+load("//devtools/clif/python:clif_build_rule.bzl", "py_clif_cc")
 load("//third_party/bazel_rules/rules_python/python:py_test.bzl", "py_test")
 
 package(
@@ -412,3 +413,25 @@ py_library(
     srcs_version = "PY2AND3",
     visibility = ["//visibility:public"],
 )
+
+cc_library(
+    name = "meta_ext_lib",
+    hdrs = ["meta_ext.h"],
+)
+
+py_clif_cc(
+    name = "_meta_ext",
+    srcs = ["meta_ext.clif"],
+    deps = [
+        ":meta_ext_lib",
+    ],
+)
+
+py_library(
+    name = "abc_utils",
+    srcs = ["abc_utils.py"],
+    visibility = ["//visibility:public"],
+    deps = [
+        ":_meta_ext",
+    ],
+)
diff --git a/clif/python/CMakeLists.txt b/clif/python/CMakeLists.txt
index a9875fc8..caa81df8 100644
--- a/clif/python/CMakeLists.txt
+++ b/clif/python/CMakeLists.txt
@@ -92,6 +92,7 @@ target_link_libraries(proto_util_cc PRIVATE
   ${PYTHON_LIBRARIES}
 )
 
+add_py_library(abc_utils abc_utils.py)
 add_py_library(callback_exception_guard callback_exception_guard.py)
 add_py_library(type_customization type_customization.py)
 add_py_library(postproc postproc.py)
diff --git a/clif/python/abc_utils.py b/clif/python/abc_utils.py
new file mode 100644
index 00000000..bcd6806c
--- /dev/null
+++ b/clif/python/abc_utils.py
@@ -0,0 +1,24 @@
+"""PyCLIF-pybind11 abc compatibility (in particular abc.ABCMeta).
+
+This is for compatibility between the "default pybind11 metaclass"
+(which is a custom metaclass) and https://docs.python.org/3/library/abc.html.
+
+For background see:
+
+* Description of https://github.com/pybind/pybind11/pull/5015
+
+* Corresponding tests in clif/testing/python/classes_test.py
+"""
+
+import abc
+import typing
+
+PyCLIFMeta = type
+
+
+if typing.TYPE_CHECKING:
+  PyCLIFABCMeta = abc.ABCMeta
+else:
+
+  class PyCLIFABCMeta(abc.ABCMeta, PyCLIFMeta):
+    pass
diff --git a/clif/python/meta_ext.clif b/clif/python/meta_ext.clif
new file mode 100644
index 00000000..2ccb5813
--- /dev/null
+++ b/clif/python/meta_ext.clif
@@ -0,0 +1,4 @@
+from "clif/python/meta_ext.h":
+  namespace `clif_meta_ext`:
+    class empty:
+      pass
diff --git a/clif/python/meta_ext.h b/clif/python/meta_ext.h
new file mode 100644
index 00000000..41862c68
--- /dev/null
+++ b/clif/python/meta_ext.h
@@ -0,0 +1,10 @@
+#ifndef CLIF_PYTHON_META_EXT_H_
+#define CLIF_PYTHON_META_EXT_H_
+
+namespace clif_meta_ext {
+
+struct empty {};
+
+}  // namespace clif_meta_ext
+
+#endif  // CLIF_PYTHON_META_EXT_H_
diff --git a/clif/python/primer.md b/clif/python/primer.md
index 4672020e..a0207560 100644
--- a/clif/python/primer.md
+++ b/clif/python/primer.md
@@ -713,8 +713,8 @@ of the return values as positional arguments. Whatever it returns becomes the
 actual return value.
 
 Some commonly needed post-processors have been provided in the above imported
-library with CLIF. You can also
-provide your own Python post-processor library.
+library with CLIF.
+You can also provide your own Python post-processor library.
 
 ## Naming Rules in CLIF Files {#naming_rules}
 
diff --git a/clif/python/types.cc b/clif/python/types.cc
index 75a86d97..0e573331 100644
--- a/clif/python/types.cc
+++ b/clif/python/types.cc
@@ -209,7 +209,6 @@ bool Clif_PyObjAs(PyObject* py, long* c) {  //NOLINT: runtime/int
 }
 
 // int64
-#ifdef HAVE_LONG_LONG
 bool Clif_PyObjAs(PyObject* py, long long* c) {  //NOLINT: runtime/int
   CHECK(c != nullptr);
   if (!PyLong_Check(py)) {
@@ -270,7 +269,6 @@ bool Clif_PyObjAs(PyObject* py, absl::uint128* c) {  // NOLINT: runtime/int
   }
   return !PyErr_Occurred();
 }
-#endif  // HAVE_LONG_LONG
 
 #ifdef ABSL_HAVE_INTRINSIC_INT128
 bool Clif_PyObjAs(PyObject* py, __int128* c) {
diff --git a/clif/python/types.h b/clif/python/types.h
index 174f5c7a..4c635c3e 100644
--- a/clif/python/types.h
+++ b/clif/python/types.h
@@ -102,7 +102,6 @@ inline PyObject* Clif_PyObjFrom(unsigned long c,  // NOLINT runtime/int
   return pc.Apply(PyLong_FromSize_t(c));
 }
 // CLIF use `int64` as int64
-#ifdef HAVE_LONG_LONG
 inline PyObject* Clif_PyObjFrom(long long c,  // NOLINT runtime/int
                                 const py::PostConv& pc) {
   return pc.Apply(PyLong_FromLongLong(c));
@@ -128,7 +127,6 @@ inline PyObject* Clif_PyObjFrom(absl::uint128 c, const py::PostConv& pc) {
   auto lo = PyLong_FromUnsignedLongLong(absl::Uint128Low64(c));
   return pc.Apply(PyNumber_Add(hi, lo));
 }
-#endif
 
 #ifdef ABSL_HAVE_INTRINSIC_INT128
 // CLIF use2 `__int128` as int
@@ -195,12 +193,10 @@ bool Clif_PyObjAs(PyObject*, unsigned char*);
 bool Clif_PyObjAs(PyObject*, unsigned short*);  // NOLINT runtime/int
 bool Clif_PyObjAs(PyObject*, unsigned int*);
 bool Clif_PyObjAs(PyObject*, unsigned long*);  // NOLINT runtime/int
-#ifdef HAVE_LONG_LONG
 bool Clif_PyObjAs(PyObject*, unsigned long long*);  // NOLINT runtime/int
 bool Clif_PyObjAs(PyObject*, long long*);           // NOLINT runtime/int
 bool Clif_PyObjAs(PyObject*, absl::int128*);        // NOLINT runtime/int
 bool Clif_PyObjAs(PyObject*, absl::uint128*);       // NOLINT runtime/int
-#endif
 #ifdef ABSL_HAVE_INTRINSIC_INT128
 bool Clif_PyObjAs(PyObject*, __int128*);
 bool Clif_PyObjAs(PyObject*, unsigned __int128*);
diff --git a/clif/testing/python/CMakeLists.txt b/clif/testing/python/CMakeLists.txt
index 753fe372..911b880b 100644
--- a/clif/testing/python/CMakeLists.txt
+++ b/clif/testing/python/CMakeLists.txt
@@ -93,7 +93,9 @@ add_pyclif_library_for_test(call_method call_method.clif)
 
 add_pyclif_library_for_test(circular circular.clif)
 
-add_pyclif_library_for_test(classes classes.clif)
+add_pyclif_library_for_test(classes classes.clif
+  PY_DEPS clif_python_abc_utils
+)
 
 add_pyclif_library_for_test(class_module_attr class_module_attr.clif)
 
diff --git a/clif/testing/python/classes_test.py b/clif/testing/python/classes_test.py
index 0a598dd2..65c7e171 100644
--- a/clif/testing/python/classes_test.py
+++ b/clif/testing/python/classes_test.py
@@ -14,11 +14,13 @@
 
 """Tests for clif.testing.python.classes."""
 
+import abc
 from unittest import mock
 
 from absl.testing import absltest
 from absl.testing import parameterized
 
+from clif.python import abc_utils
 from clif.testing.python import classes
 
 
@@ -35,6 +37,20 @@ def testKlass(self):
     # AttributeError on CPython; TypeError on PyPy.
     with self.assertRaises((AttributeError, TypeError)):
       k.i2 = 0
+    self.assertEqual(classes.Klass.C, 1)
+    self.assertEqual(k.C, 1)
+    with self.assertRaisesRegex(AttributeError, 'read-only'):
+      k.C = 0
+
+    # UNDESIRABLE but long established behavior.
+    classes.Klass.C = 0  # C++ const but not read-only in Python.
+    self.assertEqual(classes.Klass.C, 0)
+    self.assertEqual(k.C, 0)
+    # Restore original value, to minimize the potential for surprises,
+    # just in case.
+    classes.Klass.C = 1
+    self.assertEqual(classes.Klass.C, 1)
+    self.assertEqual(k.C, 1)
 
   def testMockIsRejected(self):
     k_inst = classes.Klass(3)
@@ -212,6 +228,42 @@ def testNestedUnproperty(self, attr, expected, new_value, new_expected):
     ret = getattr(obj, getter)()
     self.assertEqual(ret, new_expected)
 
+  def testABCMeta(self):
+    # Purely to help the linter.
+    ABCMeta = abc.ABCMeta  # pylint: disable=unused-variable
+
+    if abc_utils.PyCLIFMeta is type:
+
+      class KlassABCMeta(classes.Klass, metaclass=ABCMeta):
+        pass
+
+      self.assertEqual(type(classes.Klass).__name__, 'type')
+      km = KlassABCMeta(5)
+      self.assertEqual(km.i, 5)
+
+    else:
+      self.assertEqual(classes.__pyclif_codegen_mode__, 'pybind11')
+      with self.assertRaisesRegex(TypeError, 'metaclass conflict'):
+
+        class KlassABCMeta(classes.Klass, metaclass=ABCMeta):
+          pass
+
+  def testPyCLIFABCMeta(self):
+    # Purely to help the linter.
+    PyCLIFABCMeta = abc_utils.PyCLIFABCMeta  # pylint: disable=unused-variable,invalid-name
+
+    class KlassPyCLIFABCMeta(classes.Klass, metaclass=PyCLIFABCMeta):
+      pass
+
+    if abc_utils.PyCLIFMeta is type:
+      expected_type_name = 'type'
+    else:
+      self.assertEqual(classes.__pyclif_codegen_mode__, 'pybind11')
+      expected_type_name = 'pybind11_type'
+    self.assertEqual(type(classes.Klass).__name__, expected_type_name)
+    km = KlassPyCLIFABCMeta(5)
+    self.assertEqual(km.i, 5)
+
 
 if __name__ == '__main__':
   absltest.main()