A rudimentary implementation of client side commands.

gistfile1.txt

@Mod.EventBusSubscriber(modid = MODID, value = Dist.CLIENT)
public class ClientChatListener {

	private static CommandDispatcher<CommandSource> commands = new CommandDispatcher();

	static{
		commands.register(Commands.literal("foo").executes(context -> {
			context.getSource().sendFeedback(new StringTextComponent("bar"), false);
			return 1;
		}));
	}

	@SubscribeEvent
	public static void playerChat(ClientChatEvent event){
		if(event.getMessage().startsWith("/")){
			try{
				ParseResults<CommandSource> parse = commands.parse(event.getMessage().substring(1), Minecraft.getInstance().player.getCommandSource());
				if(parse.getContext().getNodes().size() > 0){
					event.setCanceled(true);
					commands.execute(parse);
				}
			} catch (CommandSyntaxException e) {}
		}
	}
}