diff --git a/php/ext/google/protobuf/php-upb.c b/php/ext/google/protobuf/php-upb.c
index 12242a87ed..0ec6ae78a4 100644
--- a/php/ext/google/protobuf/php-upb.c
+++ b/php/ext/google/protobuf/php-upb.c
@@ -10749,6 +10749,9 @@ struct upb_MessageDef {
   upb_inttable itof;
   upb_strtable ntof;
 
+  // Looking up fields by json name.
+  upb_strtable jtof;
+
   /* All nested defs.
    * MEM: We could save some space here by putting nested defs in a contiguous
    * region and calculating counts from offsets or vice-versa. */
@@ -10774,9 +10777,6 @@ struct upb_MessageDef {
   bool in_message_set;
   bool is_sorted;
   upb_WellKnown well_known_type;
-#if UINTPTR_MAX == 0xffffffff
-  uint32_t padding;  // Increase size to a multiple of 8.
-#endif
 };
 
 static void assign_msg_wellknowntype(upb_MessageDef* m) {
@@ -10919,16 +10919,16 @@ bool upb_MessageDef_FindByNameWithSize(const upb_MessageDef* m,
 const upb_FieldDef* upb_MessageDef_FindByJsonNameWithSize(
     const upb_MessageDef* m, const char* name, size_t size) {
   upb_value val;
-  const upb_FieldDef* f;
+
+  if (upb_strtable_lookup2(&m->jtof, name, size, &val)) {
+    return upb_value_getconstptr(val);
+  }
 
   if (!upb_strtable_lookup2(&m->ntof, name, size, &val)) {
     return NULL;
   }
 
-  f = _upb_DefType_Unpack(val, UPB_DEFTYPE_FIELD);
-  if (!f) f = _upb_DefType_Unpack(val, UPB_DEFTYPE_FIELD_JSONNAME);
-
-  return f;
+  return _upb_DefType_Unpack(val, UPB_DEFTYPE_FIELD);
 }
 
 int upb_MessageDef_ExtensionRangeCount(const upb_MessageDef* m) {
@@ -11108,17 +11108,25 @@ void _upb_MessageDef_InsertField(upb_DefBuilder* ctx, upb_MessageDef* m,
       _upb_MessageDef_Insert(m, shortname, shortnamelen, field_v, ctx->arena);
   if (!ok) _upb_DefBuilder_OomErr(ctx);
 
-  if (strcmp(shortname, json_name) != 0) {
-    if (upb_strtable_lookup(&m->ntof, json_name, &v)) {
-      _upb_DefBuilder_Errf(ctx, "duplicate json_name (%s)", json_name);
-    }
+  // TODO: Once editions is supported this should turn into a
+  // check on LEGACY_BEST_EFFORT
+  if (strcmp(shortname, json_name) != 0 &&
+      upb_FileDef_Syntax(m->file) == kUpb_Syntax_Proto3 &&
+      upb_strtable_lookup(&m->ntof, json_name, &v)) {
+    _upb_DefBuilder_Errf(
+        ctx, "duplicate json_name for (%s) with original field name (%s)",
+        shortname, json_name);
+  }
 
-    const size_t json_size = strlen(json_name);
-    const upb_value json_v = _upb_DefType_Pack(f, UPB_DEFTYPE_FIELD_JSONNAME);
-    ok = _upb_MessageDef_Insert(m, json_name, json_size, json_v, ctx->arena);
-    if (!ok) _upb_DefBuilder_OomErr(ctx);
+  if (upb_strtable_lookup(&m->jtof, json_name, &v)) {
+    _upb_DefBuilder_Errf(ctx, "duplicate json_name (%s)", json_name);
   }
 
+  const size_t json_size = strlen(json_name);
+  ok = upb_strtable_insert(&m->jtof, json_name, json_size,
+                           upb_value_constptr(f), ctx->arena);
+  if (!ok) _upb_DefBuilder_OomErr(ctx);
+
   if (upb_inttable_lookup(&m->itof, field_number, NULL)) {
     _upb_DefBuilder_Errf(ctx, "duplicate field number (%u)", field_number);
   }
@@ -11371,6 +11379,9 @@ static void create_msgdef(upb_DefBuilder* ctx, const char* prefix,
   ok = upb_strtable_init(&m->ntof, n_oneof + n_field, ctx->arena);
   if (!ok) _upb_DefBuilder_OomErr(ctx);
 
+  ok = upb_strtable_init(&m->jtof, n_field, ctx->arena);
+  if (!ok) _upb_DefBuilder_OomErr(ctx);
+
   UPB_DEF_SET_OPTIONS(m->opts, DescriptorProto, MessageOptions, msg_proto);
 
   m->oneof_count = n_oneof;
diff --git a/php/ext/google/protobuf/php-upb.h b/php/ext/google/protobuf/php-upb.h
index 392287be18..17700cb3ba 100644
--- a/php/ext/google/protobuf/php-upb.h
+++ b/php/ext/google/protobuf/php-upb.h
@@ -10115,7 +10115,6 @@ typedef enum {
   // Only inside message table.
   UPB_DEFTYPE_FIELD = 0,
   UPB_DEFTYPE_ONEOF = 1,
-  UPB_DEFTYPE_FIELD_JSONNAME = 2,
 } upb_deftype_t;
 
 #ifdef __cplusplus
diff --git a/ruby/ext/google/protobuf_c/ruby-upb.c b/ruby/ext/google/protobuf_c/ruby-upb.c
index b47c57f37e..13a44754ae 100644
--- a/ruby/ext/google/protobuf_c/ruby-upb.c
+++ b/ruby/ext/google/protobuf_c/ruby-upb.c
@@ -10263,6 +10263,9 @@ struct upb_MessageDef {
   upb_inttable itof;
   upb_strtable ntof;
 
+  // Looking up fields by json name.
+  upb_strtable jtof;
+
   /* All nested defs.
    * MEM: We could save some space here by putting nested defs in a contiguous
    * region and calculating counts from offsets or vice-versa. */
@@ -10288,9 +10291,6 @@ struct upb_MessageDef {
   bool in_message_set;
   bool is_sorted;
   upb_WellKnown well_known_type;
-#if UINTPTR_MAX == 0xffffffff
-  uint32_t padding;  // Increase size to a multiple of 8.
-#endif
 };
 
 static void assign_msg_wellknowntype(upb_MessageDef* m) {
@@ -10433,16 +10433,16 @@ bool upb_MessageDef_FindByNameWithSize(const upb_MessageDef* m,
 const upb_FieldDef* upb_MessageDef_FindByJsonNameWithSize(
     const upb_MessageDef* m, const char* name, size_t size) {
   upb_value val;
-  const upb_FieldDef* f;
+
+  if (upb_strtable_lookup2(&m->jtof, name, size, &val)) {
+    return upb_value_getconstptr(val);
+  }
 
   if (!upb_strtable_lookup2(&m->ntof, name, size, &val)) {
     return NULL;
   }
 
-  f = _upb_DefType_Unpack(val, UPB_DEFTYPE_FIELD);
-  if (!f) f = _upb_DefType_Unpack(val, UPB_DEFTYPE_FIELD_JSONNAME);
-
-  return f;
+  return _upb_DefType_Unpack(val, UPB_DEFTYPE_FIELD);
 }
 
 int upb_MessageDef_ExtensionRangeCount(const upb_MessageDef* m) {
@@ -10622,17 +10622,25 @@ void _upb_MessageDef_InsertField(upb_DefBuilder* ctx, upb_MessageDef* m,
       _upb_MessageDef_Insert(m, shortname, shortnamelen, field_v, ctx->arena);
   if (!ok) _upb_DefBuilder_OomErr(ctx);
 
-  if (strcmp(shortname, json_name) != 0) {
-    if (upb_strtable_lookup(&m->ntof, json_name, &v)) {
-      _upb_DefBuilder_Errf(ctx, "duplicate json_name (%s)", json_name);
-    }
+  // TODO: Once editions is supported this should turn into a
+  // check on LEGACY_BEST_EFFORT
+  if (strcmp(shortname, json_name) != 0 &&
+      upb_FileDef_Syntax(m->file) == kUpb_Syntax_Proto3 &&
+      upb_strtable_lookup(&m->ntof, json_name, &v)) {
+    _upb_DefBuilder_Errf(
+        ctx, "duplicate json_name for (%s) with original field name (%s)",
+        shortname, json_name);
+  }
 
-    const size_t json_size = strlen(json_name);
-    const upb_value json_v = _upb_DefType_Pack(f, UPB_DEFTYPE_FIELD_JSONNAME);
-    ok = _upb_MessageDef_Insert(m, json_name, json_size, json_v, ctx->arena);
-    if (!ok) _upb_DefBuilder_OomErr(ctx);
+  if (upb_strtable_lookup(&m->jtof, json_name, &v)) {
+    _upb_DefBuilder_Errf(ctx, "duplicate json_name (%s)", json_name);
   }
 
+  const size_t json_size = strlen(json_name);
+  ok = upb_strtable_insert(&m->jtof, json_name, json_size,
+                           upb_value_constptr(f), ctx->arena);
+  if (!ok) _upb_DefBuilder_OomErr(ctx);
+
   if (upb_inttable_lookup(&m->itof, field_number, NULL)) {
     _upb_DefBuilder_Errf(ctx, "duplicate field number (%u)", field_number);
   }
@@ -10885,6 +10893,9 @@ static void create_msgdef(upb_DefBuilder* ctx, const char* prefix,
   ok = upb_strtable_init(&m->ntof, n_oneof + n_field, ctx->arena);
   if (!ok) _upb_DefBuilder_OomErr(ctx);
 
+  ok = upb_strtable_init(&m->jtof, n_field, ctx->arena);
+  if (!ok) _upb_DefBuilder_OomErr(ctx);
+
   UPB_DEF_SET_OPTIONS(m->opts, DescriptorProto, MessageOptions, msg_proto);
 
   m->oneof_count = n_oneof;
diff --git a/ruby/ext/google/protobuf_c/ruby-upb.h b/ruby/ext/google/protobuf_c/ruby-upb.h
index 0c4dbb81f7..6d98c1c6b9 100755
--- a/ruby/ext/google/protobuf_c/ruby-upb.h
+++ b/ruby/ext/google/protobuf_c/ruby-upb.h
@@ -10619,7 +10619,6 @@ typedef enum {
   // Only inside message table.
   UPB_DEFTYPE_FIELD = 0,
   UPB_DEFTYPE_ONEOF = 1,
-  UPB_DEFTYPE_FIELD_JSONNAME = 2,
 } upb_deftype_t;
 
 #ifdef __cplusplus