Brigadier allows specification of custom suggestions for arguments. In Minecraft, these suggestions are sent to the client as a user is typing out the command.

A SuggestionProvider is used to make a list of suggestions that will be sent to the client. A suggestion provider is a functional interface that takes a CommandContext and a SuggestionBuilder and returns some Suggestions. The SuggestionProvider returns a CompletableFuture as the suggestions may not be available immediately.

Suggestions can be contextual since a suggestion provider gives you access to the current command context.

For example let's say you want to suggest all attributes an entity could have.

class AttributeSuggestionProvider implements SuggestionProvider<ServerCommandSource> {
    @Override
    public CompletableFuture<Suggestions> getSuggestions(CommandContext<ServerCommandSource> context, SuggestionsBuilder builder) throws CommandSyntaxException {
        Identifier entityTypeId = context.getArgument("type", Identifier.class);
        ...

Since we have a command context, we can check the context for the value of any arguments before the argument this suggestion provider targets.

Next is a bunch of boilerplate code

class AttributeSuggestionProvider implements SuggestionProvider<ServerCommandSource> {
    @Override
    public CompletableFuture<Suggestions> getSuggestions(CommandContext<ServerCommandSource> context, SuggestionsBuilder builder) {
        Identifier entityTypeId = context.getArgument("type", Identifier.class);
        EntityType<?> entityType = Registries.ENTITY_TYPE.getOrEmpty(entityTypeId).orElse(null);
        // For versions before 1.19.3, use ''Registry.ENTITY_TYPE'' instead.
 
        if (!DefaultAttributeContainer.hasDefinitionFor(entityType)) {
            // TODO: Fail
        }
 
        DefaultAttributeContainer attributeContainer = DefaultAttributeRegistry.get(entityType);
        // You will need mixin to get the 'instances map'. Lets assume we can just access it for the sake of the tutorial
        for (EntityAttribute attribute : attributeContainer.instances().keySet()) {
            Identifier attributeId = Registries.ATTRIBUTE.getId(attribute);
            if (attributeId != null) {
            ...

In order to have a suggestion appear on the client, you must add the suggestion to the builder. This can be done through one of the suggest methods. Some of these handle numbers, and a few support showing a tooltip.

for (EntityAttribute attribute : attributeContainer.instances().keySet()) {
    Identifier attributeId = Registries.ATTRIBUTE.getId(attribute);
    if (attributeId != null) {
       builder.suggest(attributeId.toString());
    }
}

You can also combine the result of another suggestion builder using the add(SuggestionBuilder) method.

Finally, you need to build the suggestions to be returned. This is done with the buildFuture method.

return builder.buildFuture();

And the final product:

class AttributeSuggestionProvider implements SuggestionProvider<ServerCommandSource> {
    @Override
    public CompletableFuture<Suggestions> getSuggestions(CommandContext<ServerCommandSource> context, SuggestionsBuilder builder) {
        Identifier entityTypeId = context.getArgument("type", Identifier.class);
        EntityType<?> entityType = Registries.ENTITY_TYPE.getOrEmpty(entityTypeId).orElse(null);
 
        if (!DefaultAttributeContainer.hasDefinitionFor(entityType)) {
            // TODO: Fail
        }
 
        DefaultAttributeContainer attributeContainer = DefaultAttributeRegistry.get(entityType);
        // You will need mixin to get the 'instances map'. Lets assume we can just access it for the sake of the tutorial
        for (EntityAttribute attribute : attributeContainer.instances().keySet()) {
            Identifier attributeId = Registries.ATTRIBUTE.getId(attribute);
            if (attributeId != null) {
                builder.suggest(attributeId.toString());
            }
        }
 
        return builder.buildFuture();
    }
}

Now that you have a suggestion provider, it is time to use the provider. Do note suggestion providers cannot be applied to literals.

When registering an argument, you can set the suggestion provider using the suggests(SuggestionProvider) method. This is applied to the RequiredArgumentBuilder as shown below:

argument(argumentName, word())
    .suggests(CompletionProviders.suggestedStrings())
        .then(/*Rest of the command*/));

Minecraft includes a few builtin suggestion providers, which includes:

CommandSource contains a few utility methods to help remove boilerplate when making the suggestions. Many of the utility methods involve returning completed suggestions from a Stream or Set of Identifiers, positions or matching strings.

Click here to view the other command tutorials.