Multi-Hotbar Core is a core mod that increases the player's hotbar size to 36 items.
Features:
Changes the integer value 9 to 36 via bytecode manipulation in specific classes/methods with the end goal of increasing the hotbar size from the perspective of the game code.
Without this modification Minecraft will not accept a client's request to set the currently held item to a value other than 0-8 (the first 9 slots on the hotbar). This allows for any item in the standard inventory to be set as the current item and thus allows for mods that increase the hotbar size visually on the client.