diff --git a/upb/message/copy.c b/upb/message/copy.c
index e9c5f6121a..32d71beeae 100644
--- a/upb/message/copy.c
+++ b/upb/message/copy.c
@@ -286,7 +286,7 @@ upb_Message* _upb_Message_Copy(upb_Message* dst, const upb_Message* src,
   while (upb_Message_NextUnknown(src, &unknowns, &iter)) {
     // Make a copy into destination arena.
     if (!UPB_PRIVATE(_upb_Message_AddUnknown)(dst, unknowns.data, unknowns.size,
-                                              arena)) {
+                                              arena, false)) {
       return NULL;
     }
   }
diff --git a/upb/message/copy_test.cc b/upb/message/copy_test.cc
index af0c4a96b1..d6fe9aa3a2 100644
--- a/upb/message/copy_test.cc
+++ b/upb/message/copy_test.cc
@@ -314,7 +314,7 @@ TEST(GeneratedCode, DeepCloneMessageWithUnknowns) {
   std::string unknown_data(data, len);
   // Add unknown data.
   UPB_PRIVATE(_upb_Message_AddUnknown)
-  (UPB_UPCAST(msg), data, len, source_arena);
+  (UPB_UPCAST(msg), data, len, source_arena, false);
   // Create clone.
   upb_Arena* clone_arena = upb_Arena_New();
   protobuf_test_messages_proto2_TestAllTypesProto2* clone =
diff --git a/upb/message/internal/compare_unknown_test.cc b/upb/message/internal/compare_unknown_test.cc
index d655f66ee9..2d8c32cfa4 100644
--- a/upb/message/internal/compare_unknown_test.cc
+++ b/upb/message/internal/compare_unknown_test.cc
@@ -116,9 +116,9 @@ upb_UnknownCompareResult CompareUnknownWithMaxDepth(UnknownFields uf1,
   std::string buf1 = ToBinaryPayload(uf1);
   std::string buf2 = ToBinaryPayload(uf2);
   UPB_PRIVATE(_upb_Message_AddUnknown)(UPB_UPCAST(msg1), buf1.data(),
-                                       buf1.size(), arena1.ptr());
+                                       buf1.size(), arena1.ptr(), false);
   UPB_PRIVATE(_upb_Message_AddUnknown)(UPB_UPCAST(msg2), buf2.data(),
-                                       buf2.size(), arena2.ptr());
+                                       buf2.size(), arena2.ptr(), false);
   return UPB_PRIVATE(_upb_Message_UnknownFieldsAreEqual)(
       UPB_UPCAST(msg1), UPB_UPCAST(msg2), max_depth);
 }
diff --git a/upb/message/internal/message.h b/upb/message/internal/message.h
index 8f1b09bc47..2f7f0fae40 100644
--- a/upb/message/internal/message.h
+++ b/upb/message/internal/message.h
@@ -88,7 +88,7 @@ void _upb_Message_DiscardUnknown_shallow(struct upb_Message* msg);
 // The data is copied into the message instance.
 bool UPB_PRIVATE(_upb_Message_AddUnknown)(struct upb_Message* msg,
                                           const char* data, size_t len,
-                                          upb_Arena* arena);
+                                          upb_Arena* arena, bool alias);
 
 // Adds unknown data (serialized protobuf data) to the given message.
 // The data is copied into the message instance. Data when concatenated together
diff --git a/upb/message/message.c b/upb/message/message.c
index b4ce505f5d..6ac57887dd 100644
--- a/upb/message/message.c
+++ b/upb/message/message.c
@@ -37,7 +37,8 @@ upb_Message* upb_Message_New(const upb_MiniTable* m, upb_Arena* a) {
 }
 
 bool UPB_PRIVATE(_upb_Message_AddUnknown)(upb_Message* msg, const char* data,
-                                          size_t len, upb_Arena* arena) {
+                                          size_t len, upb_Arena* arena,
+                                          bool alias) {
   UPB_ASSERT(!upb_Message_IsFrozen(msg));
   // TODO: b/376969853  - Add debug check that the unknown field is an overall
   // valid proto field
diff --git a/upb/wire/decode.c b/upb/wire/decode.c
index 4818b7ddc0..55e62ebea5 100644
--- a/upb/wire/decode.c
+++ b/upb/wire/decode.c
@@ -402,7 +402,7 @@ bool _upb_Decoder_CheckEnum(upb_Decoder* d, const char* ptr, upb_Message* msg,
   end = upb_Decoder_EncodeVarint32(v, end);
 
   if (!UPB_PRIVATE(_upb_Message_AddUnknown)(unknown_msg, buf, end - buf,
-                                            &d->arena)) {
+                                            &d->arena, false)) {
     _upb_Decoder_ErrorJmp(d, kUpb_DecodeStatus_OutOfMemory);
   }
   return false;
@@ -1265,8 +1265,9 @@ static const char* _upb_Decoder_DecodeUnknownField(upb_Decoder* d,
     // bounds checks are needed before adding the unknown field.
     _upb_Decoder_IsDone(d, &ptr);
     const char* input_ptr = upb_EpsCopyInputStream_GetInputPtr(&d->input, ptr);
-    if (!UPB_PRIVATE(_upb_Message_AddUnknown)(
-            msg, input_start, input_ptr - input_start, &d->arena)) {
+    if (!UPB_PRIVATE(_upb_Message_AddUnknown)(msg, input_start,
+                                              input_ptr - input_start,
+                                              &d->arena, d->input.aliasing)) {
       _upb_Decoder_ErrorJmp(d, kUpb_DecodeStatus_OutOfMemory);
     }
   } else if (wire_type == kUpb_WireType_StartGroup) {