work around 6xD9 client bug

src/CommandFormats.hh

@@ -6356,7 +6356,10 @@ struct G_AddSRankWeaponSpecial_BB_6xD8 {
 } __packed_ws__(G_AddSRankWeaponSpecial_BB_6xD8, 0x24);
 
 // 6xD9: Momoka item exchange (BB; handled by server)
-// The client sends this when it executes an F95B quest opcode.
+// The client sends this when it executes an F95B quest opcode. The client has
+// an unfortunate bug where it doesn't set the size field when generating this
+// command, so the size ends up as an uninitialized value and the client sends
+// more (or less!) data than necessary.
 
 struct G_MomokaItemExchange_BB_6xD9 {
   G_ClientIDHeader header;

src/ReceiveSubcommands.cc

@@ -5194,7 +5194,8 @@ static asio::awaitable<void> on_momoka_item_exchange_bb(shared_ptr<Client> c, Su
     throw runtime_error("6xD9 command sent during free play");
   }
 
-  const auto& cmd = msg.check_size_t<G_MomokaItemExchange_BB_6xD9>();
+  // See notes in CommandFormats.hh about why we allow larger commands here
+  const auto& cmd = msg.check_size_t<G_MomokaItemExchange_BB_6xD9>(0xFFFF);
   auto s = c->require_server_state();
   auto p = c->character_file();
   try {