ModTagItem

public class ModTagItem extends Item {
    public ModTagItem() {
        super(new Item.Properties().group(ItemGroup.TOOLS));
    }

    public ActionResultType itemInteractionForEntity(ItemStack stack, PlayerEntity playerIn, LivingEntity target, Hand hand) {
        if (!(target instanceof PlayerEntity)) {
            if (!playerIn.world.isRemote && target.isAlive()) {

                System.out.println("Mod tag before: " + target.getCapability(ModTagProvider.MOD_TAG).orElseThrow(
                        () -> new IllegalArgumentException("LazyOptional must not be empty!")).get());

                target.getCapability(ModTagProvider.MOD_TAG).ifPresent(iModTag -> iModTag.set(1));

                System.out.println("Mod tag after: " + target.getCapability(ModTagProvider.MOD_TAG).orElseThrow(
                        () -> new IllegalArgumentException("LazyOptional cannot be empty!")).get());

                stack.shrink(1);
            }

            return ActionResultType.func_233537_a_(playerIn.world.isRemote);
        } else {
            return ActionResultType.PASS;
        }
    }

}