diff --git a/csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs b/csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs
deleted file mode 100644
index 208ce1fcb6..0000000000
--- a/csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs
+++ /dev/null
@@ -1,17 +0,0 @@
-#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2008 Google Inc.  All rights reserved.
-//
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file or at
-// https://developers.google.com/open-source/licenses/bsd
-#endregion
-
-namespace Google.Protobuf.Reflection;
-
-internal sealed partial class FeatureSetDescriptor
-{
-    // Canonical serialized form of the edition defaults, generated by embed_edition_defaults.
-    private const string DefaultsBase64 =
-        "ChMYhAciACoMCAEQAhgCIAMoATACChMY5wciACoMCAIQARgBIAIoATABChMY6AciDAgBEAEYASACKAEwASoAIOYHKOgH";
-}
diff --git a/hpb/extension.h b/hpb/extension.h
index 0fa0272768..6dea740af9 100644
--- a/hpb/extension.h
+++ b/hpb/extension.h
@@ -127,18 +127,16 @@ upb_ExtensionRegistry* GetUpbExtensions(
 
 class ExtensionRegistry {
  public:
-  ExtensionRegistry(
-      const std::vector<const upb_MiniTableExtension*>& extensions,
-      const upb::Arena& arena)
-      : registry_(upb_ExtensionRegistry_New(arena.ptr())) {
+  explicit ExtensionRegistry(const upb::Arena& arena)
+      : registry_(upb_ExtensionRegistry_New(arena.ptr())) {}
+
+  template <typename ExtensionIdentifier>
+  void AddExtension(const ExtensionIdentifier& id) {
     if (registry_) {
-      for (const auto extension : extensions) {
-        const auto* ext = extension;
-        bool success = upb_ExtensionRegistry_AddArray(registry_, &ext, 1);
-        if (!success) {
-          registry_ = nullptr;
-          break;
-        }
+      auto* extension = id.mini_table_ext();
+      bool success = upb_ExtensionRegistry_AddArray(registry_, &extension, 1);
+      if (!success) {
+        registry_ = nullptr;
       }
     }
   }
diff --git a/hpb/internal/message_lock_test.cc b/hpb/internal/message_lock_test.cc
index cd82386a8b..a05e4a7bc7 100644
--- a/hpb/internal/message_lock_test.cc
+++ b/hpb/internal/message_lock_test.cc
@@ -18,6 +18,7 @@
 #include "absl/hash/hash.h"
 #include "absl/log/absl_check.h"
 #include "google/protobuf/compiler/hpb/tests/test_model.upb.proto.h"
+#include "google/protobuf/hpb/extension.h"
 #include "google/protobuf/hpb/hpb.h"
 #include "upb/mem/arena.hpp"
 #include "upb/mini_table/extension.h"
@@ -105,24 +106,27 @@ void TestConcurrentExtensionAccess(::hpb::ExtensionRegistry registry) {
 }
 
 TEST(CppGeneratedCode, ConcurrentAccessDoesNotRaceBothLazy) {
-  ::upb::Arena arena;
-  TestConcurrentExtensionAccess({{}, arena});
+  upb::Arena arena;
+  hpb::ExtensionRegistry registry(arena);
+  TestConcurrentExtensionAccess(registry);
 }
 
 TEST(CppGeneratedCode, ConcurrentAccessDoesNotRaceOneLazyOneEager) {
-  ::upb::Arena arena;
-  std::vector<const upb_MiniTableExtension*> e1{theme.mini_table_ext()};
-  TestConcurrentExtensionAccess({e1, arena});
-  std::vector<const upb_MiniTableExtension*> e2{
-      ThemeExtension::theme_extension.mini_table_ext()};
-  TestConcurrentExtensionAccess({e2, arena});
+  upb::Arena arena;
+  hpb::ExtensionRegistry r1(arena);
+  r1.AddExtension(theme);
+  TestConcurrentExtensionAccess(r1);
+  hpb::ExtensionRegistry r2(arena);
+  r2.AddExtension(ThemeExtension::theme_extension);
+  TestConcurrentExtensionAccess(r2);
 }
 
 TEST(CppGeneratedCode, ConcurrentAccessDoesNotRaceBothEager) {
   ::upb::Arena arena;
-  std::vector<const upb_MiniTableExtension*> exts{
-      theme.mini_table_ext(), ThemeExtension::theme_extension.mini_table_ext()};
-  TestConcurrentExtensionAccess({exts, arena});
+  hpb::ExtensionRegistry registry(arena);
+  registry.AddExtension(theme);
+  registry.AddExtension(ThemeExtension::theme_extension);
+  TestConcurrentExtensionAccess(registry);
 }
 
 }  // namespace
diff --git a/hpb_generator/tests/test_generated.cc b/hpb_generator/tests/test_generated.cc
index d42a49c147..6aa436646d 100644
--- a/hpb_generator/tests/test_generated.cc
+++ b/hpb_generator/tests/test_generated.cc
@@ -1019,11 +1019,11 @@ TEST(CppGeneratedCode, ParseWithExtensionRegistry) {
   ::upb::Arena arena;
   auto bytes = ::hpb::Serialize(&model, arena);
   EXPECT_EQ(true, bytes.ok());
-  std::vector<const upb_MiniTableExtension*> exts{
-      theme.mini_table_ext(), other_ext.mini_table_ext(),
-      ThemeExtension::theme_extension.mini_table_ext()};
 
-  ::hpb::ExtensionRegistry extensions(exts, arena);
+  hpb::ExtensionRegistry extensions(arena);
+  extensions.AddExtension(theme);
+  extensions.AddExtension(other_ext);
+  extensions.AddExtension(ThemeExtension::theme_extension);
   TestModel parsed_model =
       ::hpb::Parse<TestModel>(bytes.value(), extensions).value();
   EXPECT_EQ("Test123", parsed_model.str1());
@@ -1287,8 +1287,8 @@ TEST(CppGeneratedCode, HasExtensionAndRegistry) {
   std::string data = std::string(::hpb::Serialize(&source, arena).value());
 
   // Test with ExtensionRegistry
-  std::vector<const upb_MiniTableExtension*> exts{theme.mini_table_ext()};
-  hpb::ExtensionRegistry extensions(exts, arena);
+  hpb::ExtensionRegistry extensions(arena);
+  extensions.AddExtension(theme);
   TestModel parsed_model = ::hpb::Parse<TestModel>(data, extensions).value();
   EXPECT_TRUE(::hpb::HasExtension(&parsed_model, theme));
 }