Example of how to override a WP-CLI command

override-wp-cli-command.php

<?php

if ( defined('WP_CLI') && WP_CLI ) {
	
	class Transient_Mod_Command extends Transient_Command {
		public function delete_all() {
			// new definition of delete-all subcommand
		}
	}
	
	WP_CLI::add_command( 'transient', 'Transient_Mod_Command' );
	
}