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. Besides, some argument types can have their default suggestion providers calculated directly in client, which is not focused in this article.

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.

public 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

public 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)) {
            return Suggestions.empty()
        }
 
        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 && CommandSource.shouldSuggest(builder.getRemaining(), attributeId.toString())) {
            ...

The check of CommandSource.shouldSuggest is intended to suggest only results that match the inputed part, which is builder.getRemaining(), which is the part between the beginning of the argument and the cursor.

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 && CommandSource.shouldSuggest(builder.getRemaining(), attributeId.toString())) {
               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:

public 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)) {
            return Suggestions.empty()
        }
 
        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 && CommandSource.shouldSuggest(builder.getRemaining(), attributeId.toString())) {
                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.

In the example above, we can simplify it to:

        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
        return CommandSource.suggestMatching(Iterables.transform(attributeContainer.instances.keySet(), Identifier::toString), builder);
 
        // you can also use stream:
        return CommandSource.suggestMatching(attributeContainer.instances.keySet().stream().map(Identifier::toString), builder);

Most cases the suggestions appear from the beginning of the argument, such as the example above. However, sometimes suggestions appear in the middle of the argument. For example, in a block state argument, you input oak_stairs[, and then suggestions should be property names of the block (facing, waterlogged, …). The suggestion appears after the left square bracket.

In this case, you should use builder.createOffset(offset), where offset is the position of the new suggestion builder.

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