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Code algorithms that can take over the role of players from repetitious tasks.

Some games, especially online games, require players to perform smaller or larger sequences of actions repetitiously. One way to let players avoid this is through letting them relinquish control to a program, a Mule, for parts of the gameplay. While this may let them escape boring sequences of gameplay, if they are allowed to also create these Mules they are provided with a possibility of being inventive and creative.


The first possibilities to creates Mules appeared in Multiuser Dungeons. While DragonMud provided a limited way to add more content, Kingdoms made a programming language similar to C available to trusted players. The commercial massively multiplayer game Ultima Online provided similar but more restricted functionality through macros[1]. This was followed in World of Warcraft, which provides players with a set of functions that can be used for writing scripts. These scripts can vary from being practical macros that reduce the amount of excise but can also be more complex scrips capable of interacting with the environment.[2].

Although not created by the players themselves, the bots that can take over their role in the Left 4 Dead series can be seen as a form of Mules. This since they can let players take pauses during less important sections of them game.

Using the pattern

The main difference of designing Mules and AI Players is that Mules only need to be able to fill in for players for limited amount of time and limited sets of activities. Since this makes it easier to design them, it is also easier to help players create them. This can provide the players with Creative Control and a form of Meta Game to design them as well as possible.

Several features of components other than the character need to be considered when making a Mule possible. First, players must be able to define scripts or sequences of actions for game elements (although the game clients may be hacked to perform such functions). Second, monotonous actions must be possible, i. e. an action must be possible to repeat many times with a chance of gaining a positive outcome each time. Third, the actions of a Mule usually require the presence of some resource that is refined into another resource, in practice making the Mule a Converter.

The use of a Mule can become a Risk/Reward tradeoff if there is a chance of having critical failures of the actions being performed, typically leading to the destruction of Tools or attacks by Enemies, or if the game permits, Player Elimination from attacks by the other players or the game elements under the game systems control. These challenges may be simple to deal with if the player is monitoring the character, but when the character is set to do actions without supervision this may leave the character incapable of responding appropriately to these failure situations.

To modulate the efficiency of using a Mule, the resource generated may have Diminishing Returns, require skills, or require the presence of Non-Renewable Resources or Renewable Resources with a lower replenishment rate than the rate of consumption set for the Mule.

Diegetic Aspects

Depending on what activities the Mules should be able to handle, this can put various difficulties in achieving Diegetic Consistency, where Dialogues can be an example of a difficult area. When players can create the Mules, this of course puts the responsibility of maintaining Diegetic Consistency into their hands.

Interface Aspects

If players should be able to create or modify Mules they need Optional Interfaces.


Mules are a limited form of AI Players that work as Proxy Players to make players escape Excise.

When players can create the Mules themselves, the pattern also provides Creative Control and a form of Meta Game.

Agents Avatars Algorithmic Agents


Can Instantiate

AI Players Creative Control Meta Games Proxy Players

Can Modulate

Algorithmic Agents

Can Be Instantiated By

Can Be Modulated By

Possible Closure Effects

Potentially Conflicting With



A revised version of the pattern Mule that was part of the original collection in the book Patterns in Game Design[3].


  1. Entry about creating macros on the UOGuide site.
  2. Entry describing the World of Warcraft API on the WoWWiki.
  3. Björk, S. & Holopainen, J. (2004) Patterns in Game Design. Charles River Media. ISBN1-58450-354-8.


Karl-Petter Åkesson