Commands

Declare commands in plugin-info.commands. When the user runs one, the plugin receives event.command.

commands: vec![Command {
    name: "roll".into(),
    description: "Roll a die".into(),
    options: vec![CommandOption {
        name: "sides".into(),
        description: "Number of sides".into(),
        kind: CommandOptionKind::Integer,
        required: false,
        choices: Vec::new(),
    }],
}],

fn on_event(&mut self, event: Event) {
    let Event::Command(invocation) = event else {
        return;
    };
    let sides = invocation
        .arguments
        .iter()
        .find_map(|argument| match argument.value {
            CommandValue::Integer(sides) if argument.name == "sides" => Some(sides),
            _ => None,
        })
        .unwrap_or(6);
    actions::send_local_message(invocation.channel_id, &format!("Rolling a d{sides}"));
}

actions::send_local_message shows a message only the current user can see. Use actions::send_message to send a real message.

Commands should do something a bot cannot. Simple commands like /cat belong in a user-installable Discord app; see the plugin rules.