diff --git a/upb/reflection/stage0/google/protobuf/descriptor.upb.h b/upb/reflection/stage0/google/protobuf/descriptor.upb.h
index 33287809bf..9bc4bbbb14 100644
--- a/upb/reflection/stage0/google/protobuf/descriptor.upb.h
+++ b/upb/reflection/stage0/google/protobuf/descriptor.upb.h
@@ -3,7 +3,8 @@
  *     google/protobuf/descriptor.proto
  *
  * Do not edit -- your changes will be discarded when the file is
- * regenerated. */
+ * regenerated.
+ * NO CHECKED-IN PROTOBUF GENCODE */
 
 #ifndef GOOGLE_PROTOBUF_DESCRIPTOR_PROTO_UPB_H_
 #define GOOGLE_PROTOBUF_DESCRIPTOR_PROTO_UPB_H_
diff --git a/upb_generator/common.cc b/upb_generator/common.cc
index 3bda1a3cab..f7f408128e 100644
--- a/upb_generator/common.cc
+++ b/upb_generator/common.cc
@@ -51,7 +51,11 @@ void EmitFileWarning(absl::string_view name, Output& output) {
       " *     $0\n"
       " *\n"
       " * Do not edit -- your changes will be discarded when the file is\n"
-      " * regenerated. */\n\n",
+      " * regenerated.\n"
+      " * NO CHECKED-IN "
+      // Intentional line break.
+      "PROTOBUF GENCODE */\n"
+      "\n",
       name);
 }
 
diff --git a/upb_generator/stage0/google/protobuf/compiler/plugin.upb.h b/upb_generator/stage0/google/protobuf/compiler/plugin.upb.h
index 1ccffa7b7e..50c2d24823 100644
--- a/upb_generator/stage0/google/protobuf/compiler/plugin.upb.h
+++ b/upb_generator/stage0/google/protobuf/compiler/plugin.upb.h
@@ -3,7 +3,8 @@
  *     google/protobuf/compiler/plugin.proto
  *
  * Do not edit -- your changes will be discarded when the file is
- * regenerated. */
+ * regenerated.
+ * NO CHECKED-IN PROTOBUF GENCODE */
 
 #ifndef GOOGLE_PROTOBUF_COMPILER_PLUGIN_PROTO_UPB_H_
 #define GOOGLE_PROTOBUF_COMPILER_PLUGIN_PROTO_UPB_H_