diff --git a/hpb/extension.h b/hpb/extension.h
index 2cacc765426c0..722480c5f92b4 100644
--- a/hpb/extension.h
+++ b/hpb/extension.h
@@ -134,8 +134,9 @@ class ExtensionRegistry {
   void AddExtension(const ExtensionIdentifier& id) {
     if (registry_) {
       auto* extension = id.mini_table_ext();
-      bool success = upb_ExtensionRegistry_AddArray(registry_, &extension, 1);
-      if (!success) {
+      upb_ExtensionRegistryStatus status =
+          upb_ExtensionRegistry_AddArray(registry_, &extension, 1);
+      if (status != kUpb_ExtensionRegistryStatus_Ok) {
         registry_ = nullptr;
       }
     }
diff --git a/python/google/protobuf/internal/message_factory_test.py b/python/google/protobuf/internal/message_factory_test.py
index d32b046077965..5c9e4bf8ea6b5 100644
--- a/python/google/protobuf/internal/message_factory_test.py
+++ b/python/google/protobuf/internal/message_factory_test.py
@@ -179,14 +179,19 @@ def testDuplicateExtensionNumber(self):
         type_name='Duplicate',
         extendee='Container',
     )
+
     if api_implementation.Type() == 'upb':
       with self.assertRaisesRegex(
           TypeError,
-          # TODO - b/380939902: Figure out why this fails with an OOM message.
-          "Couldn't build proto file into descriptor pool: out of memory",
+          "Couldn't build proto file into descriptor pool: "
+          'duplicate extension entry',
       ):
         pool.Add(f)
     else:
+      # TODO: b/381131694 - Ensure conformance between upb/c++/python.
+      # C++ and pure Python implementations should raise an error when adding a
+      # duplicate extension number. There doesn't seem to be a benefit to failing
+      # only when GetMessageClassesForFiles is called.
       pool.Add(f)
 
       with self.assertRaises(Exception) as cm:
diff --git a/upb/mini_table/extension_registry.c b/upb/mini_table/extension_registry.c
index 65ac7873256f1..32b07b8599972 100644
--- a/upb/mini_table/extension_registry.c
+++ b/upb/mini_table/extension_registry.c
@@ -11,6 +11,7 @@
 #include <stdint.h>
 #include <string.h>
 
+#include "upb/hash/common.h"
 #include "upb/hash/str_table.h"
 #include "upb/mem/arena.h"
 #include "upb/mini_table/extension.h"
@@ -39,24 +40,32 @@ upb_ExtensionRegistry* upb_ExtensionRegistry_New(upb_Arena* arena) {
   return r;
 }
 
-UPB_API bool upb_ExtensionRegistry_Add(upb_ExtensionRegistry* r,
-                                       const upb_MiniTableExtension* e) {
+UPB_API upb_ExtensionRegistryStatus upb_ExtensionRegistry_Add(
+    upb_ExtensionRegistry* r, const upb_MiniTableExtension* e) {
   char buf[EXTREG_KEY_SIZE];
   extreg_key(buf, e->UPB_PRIVATE(extendee), upb_MiniTableExtension_Number(e));
-  if (upb_strtable_lookup2(&r->exts, buf, EXTREG_KEY_SIZE, NULL)) return false;
-  return upb_strtable_insert(&r->exts, buf, EXTREG_KEY_SIZE,
-                             upb_value_constptr(e), r->arena);
+
+  if (upb_strtable_lookup2(&r->exts, buf, EXTREG_KEY_SIZE, NULL)) {
+    return kUpb_ExtensionRegistryStatus_DuplicateEntry;
+  }
+
+  if (!upb_strtable_insert(&r->exts, buf, EXTREG_KEY_SIZE,
+                           upb_value_constptr(e), r->arena)) {
+    return kUpb_ExtensionRegistryStatus_OutOfMemory;
+  }
+  return kUpb_ExtensionRegistryStatus_Ok;
 }
 
-bool upb_ExtensionRegistry_AddArray(upb_ExtensionRegistry* r,
-                                    const upb_MiniTableExtension** e,
-                                    size_t count) {
+upb_ExtensionRegistryStatus upb_ExtensionRegistry_AddArray(
+    upb_ExtensionRegistry* r, const upb_MiniTableExtension** e, size_t count) {
   const upb_MiniTableExtension** start = e;
   const upb_MiniTableExtension** end = UPB_PTRADD(e, count);
+  upb_ExtensionRegistryStatus status = kUpb_ExtensionRegistryStatus_Ok;
   for (; e < end; e++) {
-    if (!upb_ExtensionRegistry_Add(r, *e)) goto failure;
+    status = upb_ExtensionRegistry_Add(r, *e);
+    if (status != kUpb_ExtensionRegistryStatus_Ok) goto failure;
   }
-  return true;
+  return kUpb_ExtensionRegistryStatus_Ok;
 
 failure:
   // Back out the entries previously added.
@@ -67,7 +76,8 @@ bool upb_ExtensionRegistry_AddArray(upb_ExtensionRegistry* r,
                upb_MiniTableExtension_Number(ext));
     upb_strtable_remove2(&r->exts, buf, EXTREG_KEY_SIZE, NULL);
   }
-  return false;
+  UPB_ASSERT(status != kUpb_ExtensionRegistryStatus_Ok);
+  return status;
 }
 
 #ifdef UPB_LINKARR_DECLARE
@@ -80,7 +90,9 @@ bool upb_ExtensionRegistry_AddAllLinkedExtensions(upb_ExtensionRegistry* r) {
   for (const upb_MiniTableExtension* p = start; p < stop; p++) {
     // Windows can introduce zero padding, so we have to skip zeroes.
     if (upb_MiniTableExtension_Number(p) != 0) {
-      if (!upb_ExtensionRegistry_Add(r, p)) return false;
+      if (upb_ExtensionRegistry_Add(r, p) != kUpb_ExtensionRegistryStatus_Ok) {
+        return false;
+      }
     }
   }
   return true;
diff --git a/upb/mini_table/extension_registry.h b/upb/mini_table/extension_registry.h
index 9f5f81a33c563..6d294a8a6fbca 100644
--- a/upb/mini_table/extension_registry.h
+++ b/upb/mini_table/extension_registry.h
@@ -8,6 +8,9 @@
 #ifndef UPB_MINI_TABLE_EXTENSION_REGISTRY_H_
 #define UPB_MINI_TABLE_EXTENSION_REGISTRY_H_
 
+#include <stddef.h>
+#include <stdint.h>
+
 #include "upb/mem/arena.h"
 #include "upb/mini_table/extension.h"
 #include "upb/mini_table/message.h"
@@ -55,21 +58,25 @@ extern "C" {
 
 typedef struct upb_ExtensionRegistry upb_ExtensionRegistry;
 
+typedef enum {
+  kUpb_ExtensionRegistryStatus_Ok = 0,
+  kUpb_ExtensionRegistryStatus_DuplicateEntry = 1,
+  kUpb_ExtensionRegistryStatus_OutOfMemory = 2,
+} upb_ExtensionRegistryStatus;
+
 // Creates a upb_ExtensionRegistry in the given arena.
 // The arena must outlive any use of the extreg.
 UPB_API upb_ExtensionRegistry* upb_ExtensionRegistry_New(upb_Arena* arena);
 
-UPB_API bool upb_ExtensionRegistry_Add(upb_ExtensionRegistry* r,
-                                       const upb_MiniTableExtension* e);
+UPB_API upb_ExtensionRegistryStatus upb_ExtensionRegistry_Add(
+    upb_ExtensionRegistry* r, const upb_MiniTableExtension* e);
 
 // Adds the given extension info for the array |e| of size |count| into the
 // registry. If there are any errors, the entire array is backed out.
 // The extensions must outlive the registry.
 // Possible errors include OOM or an extension number that already exists.
-// TODO: There is currently no way to know the exact reason for failure.
-bool upb_ExtensionRegistry_AddArray(upb_ExtensionRegistry* r,
-                                    const upb_MiniTableExtension** e,
-                                    size_t count);
+upb_ExtensionRegistryStatus upb_ExtensionRegistry_AddArray(
+    upb_ExtensionRegistry* r, const upb_MiniTableExtension** e, size_t count);
 
 #ifdef UPB_LINKARR_DECLARE
 
diff --git a/upb/reflection/file_def.c b/upb/reflection/file_def.c
index 5a51deecf947e..27e7f18209073 100644
--- a/upb/reflection/file_def.c
+++ b/upb/reflection/file_def.c
@@ -453,8 +453,15 @@ void _upb_FileDef_Create(upb_DefBuilder* ctx,
   }
 
   if (file->ext_count) {
-    bool ok = upb_ExtensionRegistry_AddArray(
+    upb_ExtensionRegistryStatus status = upb_ExtensionRegistry_AddArray(
         _upb_DefPool_ExtReg(ctx->symtab), file->ext_layouts, file->ext_count);
-    if (!ok) _upb_DefBuilder_OomErr(ctx);
+    if (status != kUpb_ExtensionRegistryStatus_Ok) {
+      if (status == kUpb_ExtensionRegistryStatus_OutOfMemory) {
+        _upb_DefBuilder_OomErr(ctx);
+      }
+
+      UPB_ASSERT(status == kUpb_ExtensionRegistryStatus_DuplicateEntry);
+      _upb_DefBuilder_Errf(ctx, "duplicate extension entry");
+    }
   }
 }