IlluminatiJoe

Got it to work, for anyone that has this same question, here's the code: - In the EntityMenu this.blockEntity.getCapability(ForgeCapabilities.ITEM_HANDLER).ifPresent(iItemHandler -> { this.addSlot(new SlotItemHandler(iItemHandler, 0, 12, 15) { @Override public int getMaxStackSize() { return 1; } @Override public int getMaxStackSize(@NotNull ItemStack stack) { return 1; } @Override public void set(@NotNull ItemStack stack) { ItemStack copiedItemStack = stack.copyWithCount(1); super.set(copiedItemStack); return; } }); this.addSlot(new SlotItemHandler(iItemHandler, 1, 86, 15)); this.addSlot(new SlotItemHandler(iItemHandler, 2, 86, 60)); }); Adding an anonymous class to the slot you want to have this, and in the class you override the three methods. Now if you want other number of items, replace the 1's in the methods with the number you want. PS. The return line at the end of set() is not needed.

I tried that, and the problem is I can still insert a full stack by hand.

I am trying to make a machine that has an upgrade slot that has an item limit of 1. I've tried overriding a lot of functions and even making a custom SlotItemHandler class, but nothing worked. How would one go about making this?