Custom arguments

Custom arguments are a quality-of-life feature that the CommandAPI offers which allows you to perform pre-processing on an argument in the argument instance rather than in your executes() method for a command. They are designed to be used for multiple commands - you can define the argument once and can use it wherever you want when declaring commands.

The CustomArgument<T, B> has the following constructor:

public CustomArgument(Argument<B> base, CustomArgumentInfoParser<T, B> parser);

This constructor takes in two parameters:

  • A "base argument", which is the argument that it'll use as the underlying parser. For example, if this is a StringArgument, it'll use the StringArgument's parsing rules ( alphanumeric characters (A-Z, a-z and 0-9), and the underscore character) and if this is a LocationArgument, it'll take three numerical values.

  • A "parser", which lets you process the argument based on its input. This is described in more detail below.

Type params

The custom argument requires two type parameters, <T> and <B>:

  • <T> refers to the type that this argument will return when parsing the arguments for a command. For instance, if you have a CustomArgument<Player, ...>, then when parsing the arguments for the command, you would cast it to a Player object.

  • <B> refers to the type that the base argument will return. This can be found in the Argument Casting section. For example, if the base argument is a StringArgument, you'd have CustomArgument<..., String>.

The CustomArgumentInfoParser class

To create a parser for a CustomArgument, you need to provide a CustomArgumentInfoParser function to the constructor. The CustomArgumentInfoParser class is a functional interface which accepts CustomArgumentInfo and returns T, an object of your choosing:

public interface CustomArgumentInfoParser<T, B> {

    public T apply(CustomArgumentInfo<B> info) throws CustomArgumentException;


The CustomArgumentInfo record is very similar to the SuggestionInfo record for declaring argument suggestions. This record contains the following methods:

public record CustomArgumentInfo<B> {
    CommandSender sender();
    CommandArguments previousArgs(); 
    String input();
    B currentInput();

These fields are as follows:

  • CommandSender sender();

    sender() represents the command sender that is typing the command. This is normally a Player, but can also be a console command sender if using a Paper server.

  • CommandArguments previousArgs();

    previousArgs() represents the previously declared arguments, which are parsed and interpreted as if they were being used to execute the command.

  • String input();

    input() represents the current input for the custom argument that the user has typed. For example, if a user is typing /mycommand hello and the first argument is a CustomArgument, the input() would return "hello".

  • B currentInput();

    currentInput() represents the current input, as parsed by the base argument. For example, if your base argument was an IntegerArgument, the return type of currentInput() would be an int.

Example - World argument

Say we want to create an argument to represents the list of available worlds on the server. We want to have an argument which always returns a Bukkit World object as the result. Here, we create a method worldArgument() that returns our custom argument that returns a World. First, we retrieve our String[] of world names to be used for our suggestions. We then write our custom argument that creates a World object from the input (in this case, we simply convert the input to a World using Bukkit.getWorld(String)). We perform error handling before returning our result:

// Function that returns our custom argument
public Argument<World> customWorldArgument(String nodeName) {

    // Construct our CustomArgument that takes in a String input and returns a World object
    return new CustomArgument<World, String>(new StringArgument(nodeName), info -> {
        // Parse the world from our input
        World world = Bukkit.getWorld(info.input());

        if (world == null) {
            throw CustomArgumentException.fromMessageBuilder(new MessageBuilder("Unknown world: ").appendArgInput());
        } else {
            return world;
    }).replaceSuggestions(ArgumentSuggestions.strings(info ->
        // List of world names on the server
// Function that returns our custom argument
fun worldArgument(nodeName: String): Argument<World> {

    // Construct our CustomArgument that takes in a String input and returns a World object
    return CustomArgument<World, String>(StringArgument(nodeName)) { info ->
        // Parse the world from our input
        val world = Bukkit.getWorld(info.input())

        if (world == null) {
            throw CustomArgumentException.fromMessageBuilder(MessageBuilder("Unknown world: ").appendArgInput())
        } else {
    }.replaceSuggestions(ArgumentSuggestions.strings { _ ->
        // List of world names on the server
        Bukkit.getWorlds().map{ }.toTypedArray()

In our error handling step, we check if the world is equal to null (since the Bukkit.getWorld(String) is @Nullable). To handle this case, we throw a CustomArgumentException with an error from a MessageBuilder. The CustomArgumentException has various static factory methods tailored to your desired printing method, so a message builder isn't required each time:

CustomArgumentException fromBaseComponents(BaseComponent[] errorMessage);
CustomArgumentException fromString(String errorMessage);
CustomArgumentException fromAdventureComponent(Component errorMessage);
CustomArgumentException fromMessageBuilder(MessageBuilder errorMessage);

We can use our custom argument like any other argument. Say we wanted to write a command to teleport to a specific world. We will create a command of the following syntax:

/tpworld <world>

Since we have defined the method worldArgument() which automatically generates our argument, we can use it as follows:

new CommandAPICommand("tpworld")
    .executesPlayer((player, args) -> {
        player.teleport(((World) args.get(0)).getSpawnLocation());
    .executesPlayer(PlayerCommandExecutor { player, args ->
        player.teleport((args[0] as World).spawnLocation)
commandAPICommand("tpworld") {
    worldArgument("world") // This method is actually also built into the Kotlin DSL
    playerExecutor { player, args ->
        player.teleport((args[0] as World).spawnLocation)

By using a CustomArgument (as opposed to a simple StringArgument and replacing its suggestions), we are able to provide a much more powerful form of error handling (automatically handled inside the argument), and we can reuse this argument for other commands.

Message Builders

The MessageBuilder class is a class to easily create messages to describe errors when a sender sends a command which does not meet the expected syntax for an argument. It acts in a similar way to a StringBuilder, where you can append content to the end of a String.

The following methods are as follows:

appendArgInput()Appends the argument that failed that the sender submitted to the end of the builder. E.g. /foo bar will append bar
appendFullInput()Appends the full command that a sender submitted to the end of the builder. E.g. /foo bar will append foo bar
appendHere()Appends the text <--[HERE] to the end of the builder
append(Object)Appends an object to the end of the builder

Example - Message builder for invalid objective argument

To create a MessageBuilder, simply call its constructor and use whatever methods as you see fit. Unlike a StringBuilder, you don't have to "build" it when you're done - the CommandAPI does that automatically:

new MessageBuilder("Unknown world: /").appendFullInput().appendHere();