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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 scripts 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

Mules need to be able to control Avatars and often make use of the information stored in Characters (Mules are typically not applied to Units - probably due to the complexity of handling groups of agents). 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. Ranging from Action Programming to access to a full programming language, this can provide the players with Creative Control and a form of Meta Game to design them as well as possible.

The most common use of Mules is to perform some form of Grinding concerning Character Development or the gathering of a Resource. To modulate the efficiency of using the Mules, 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 Mules.

The use of Mules can become a Risk/Reward tradeoff if there is a chance of having critical failures of the actions being performed, typically leading to the loss of Equipment, attacks by Enemies, and maybe Player Elimination. These challenges may be simple to deal with if players can monitor the Mules as Spectators, but when the Mules are set to do actions without supervision this may leave the Mules incapable of responding appropriately to these failure situations.

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, most often concerning improving Characters. As such, they are also examples of Algorithmic Agents and Agents.

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


Can Instantiate

Creative Control, Meta Games, Proxy Players, Risk/Reward

Can Modulate

Avatars, Characters, Character Development, Grinding

Can Be Instantiated By

Action Programming, AI Players, Algorithmic Agents

Can Be Modulated By

Diminishing Returns, Non-Renewable Resources, Renewable Resources, Skills, Spectators

Possible Closure Effects


Potentially Conflicting With

Diegetic Consistency, Excise


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