diff --git a/src/Client.cc b/src/Client.cc
index fd62a8ac..53f4f394 100644
--- a/src/Client.cc
+++ b/src/Client.cc
@@ -108,9 +108,11 @@ void Client::Config::set_flags_for_version(Version version, int64_t sub_version)
       this->set_flag(Flag::ENCRYPTED_SEND_FUNCTION_CALL);
       this->set_flag(Flag::SEND_FUNCTION_CALL_NO_CACHE_PATCH);
       break;
+    case 0x3A: // GC Ep1&2 US v1.2 (Plus) GMK edition
+      this->set_flag(Flag::IS_CLIENT_CUSTOMIZATION);
+      [[fallthrough]];
     case 0x36: // GC Ep1&2 US v1.2 (Plus)
     case 0x39: // GC Ep1&2 JP v1.5 (Plus)
-    case 0x3A: // GC Ep1&2 US v1.2 (Plus) GMK edition
       this->set_flag(Flag::NO_D6_AFTER_LOBBY);
       this->set_flag(Flag::NO_SEND_FUNCTION_CALL);
       break;
diff --git a/src/Client.hh b/src/Client.hh
index c5fd28a5..5e4310dd 100644
--- a/src/Client.hh
+++ b/src/Client.hh
@@ -69,6 +69,7 @@ public:
     SHOULD_SEND_ARTIFICIAL_FLAG_STATE          = 0x0002000000000000, // Server-side only
     SHOULD_SEND_ENABLE_SAVE                    = 0x0004000000000000,
     SWITCH_ASSIST_ENABLED                      = 0x0000000100000000,
+    IS_CLIENT_CUSTOMIZATION                    = 0x0100000000000000,
 
     // Cheat mode and option flags
     INFINITE_HP_ENABLED                        = 0x0000000200000000,
diff --git a/src/Lobby.cc b/src/Lobby.cc
index 13606585..d0f42567 100644
--- a/src/Lobby.cc
+++ b/src/Lobby.cc
@@ -792,7 +792,8 @@ Lobby::JoinError Lobby::join_error_for_client(std::shared_ptr<Client> c, const s
   if (this->count_clients() >= this->max_clients) {
     return JoinError::FULL;
   }
-  if (!this->version_is_allowed(c->version()) && !c->config.check_flag(Client::Flag::DEBUG_ENABLED)) {
+  bool debug_enabled = c->config.check_flag(Client::Flag::DEBUG_ENABLED);
+  if (!this->version_is_allowed(c->version()) && !debug_enabled) {
     return JoinError::VERSION_CONFLICT;
   }
   if (this->is_game()) {
@@ -805,6 +806,10 @@ Lobby::JoinError Lobby::join_error_for_client(std::shared_ptr<Client> c, const s
     if (this->mode == GameMode::SOLO) {
       return JoinError::SOLO;
     }
+    if (!debug_enabled &&
+        (this->check_flag(Flag::IS_CLIENT_CUSTOMIZATION) != c->config.check_flag(Client::Flag::IS_CLIENT_CUSTOMIZATION))) {
+      return JoinError::VERSION_CONFLICT;
+    }
     if (!c->login->account->check_flag(Account::Flag::FREE_JOIN_GAMES)) {
       if (password && !this->password.empty() && (*password != this->password)) {
         return JoinError::INCORRECT_PASSWORD;
diff --git a/src/Lobby.hh b/src/Lobby.hh
index ce3c6d0a..4a2b7515 100644
--- a/src/Lobby.hh
+++ b/src/Lobby.hh
@@ -70,6 +70,7 @@ struct Lobby : public std::enable_shared_from_this<Lobby> {
     QUEST_IN_PROGRESS               = 0x00000200,
     BATTLE_IN_PROGRESS              = 0x00000400,
     JOINABLE_QUEST_IN_PROGRESS      = 0x00000800,
+    IS_CLIENT_CUSTOMIZATION         = 0x00001000,
     IS_SPECTATOR_TEAM               = 0x00002000, // episode must be EP3 also
     SPECTATORS_FORBIDDEN            = 0x00004000,
     START_BATTLE_PLAYER_IMMEDIATELY = 0x00008000,
diff --git a/src/ReceiveCommands.cc b/src/ReceiveCommands.cc
index 6c90d38e..0b299aeb 100644
--- a/src/ReceiveCommands.cc
+++ b/src/ReceiveCommands.cc
@@ -4149,6 +4149,9 @@ shared_ptr<Lobby> create_game_generic(
     default:
       throw logic_error("invalid quest script version");
   }
+  if (c->config.check_flag(Client::Flag::IS_CLIENT_CUSTOMIZATION)) {
+    game->set_flag(Lobby::Flag::IS_CLIENT_CUSTOMIZATION);
+  }
 
   while (game->floor_item_managers.size() < 0x12) {
     game->floor_item_managers.emplace_back(game->lobby_id, game->floor_item_managers.size());
diff --git a/src/SendCommands.cc b/src/SendCommands.cc
index e7f48340..7c50868b 100644
--- a/src/SendCommands.cc
+++ b/src/SendCommands.cc
@@ -1436,6 +1436,7 @@ void send_game_menu_t(
   for (shared_ptr<Lobby> l : s->all_lobbies()) {
     if (l->is_game() &&
         (client_has_debug || l->version_is_allowed(c->version())) &&
+        (client_has_debug || (l->check_flag(Lobby::Flag::IS_CLIENT_CUSTOMIZATION) == c->config.check_flag(Client::Flag::IS_CLIENT_CUSTOMIZATION))) &&
         (l->check_flag(Lobby::Flag::IS_SPECTATOR_TEAM) == is_spectator_team_list) &&
         (!show_tournaments_only || l->tournament_match)) {
       games.emplace(l);