The Stunts

Stunt Rubrics

Add a New Action to a Skill

The most basic option for a stunt is to allow a skill to do something that it normally can’t do. It adds a new action onto the base skill in certain situations, for those with this stunt. This new action can be one that’s available to another skill (allowing one skill to swap for another under certain circumstances), or one that’s not available to any skill. (Fate Core, p.89)

Add a Bonus to an Action

Another use for a stunt is to give a skill an automatic bonus under a particular, very narrow circumstance, effectively letting a character specialize in something. The circumstance should be narrower than what the normal action allows, and only apply to one particular action or pair of actions.
The usual bonus is +2 to the skill total. However, if you want, you can also express the bonus as two shifts of additional effect after the roll succeeds, if that makes more sense. Remember, higher shifts on a roll allow your action to be more effective in certain ways.
You can also use this to establish any effect worth two shifts as an additional benefit of succeeding at the skill roll. This might be Fair (+2) passive opposition, the equivalent of a 2-point hit, a mild consequence, or an advantage that takes Fair (+2) opposition to remove.

— p.90
Fate Core

Create a Rules Exception

Finally, a stunt can allow a skill to make a single exception, in a narrow circumstance, for any other game rule that doesn’t precisely fit into the category of an action. The Challenges, Contests, and Conflicts chapter is full of different little rules about the circumstances under which a skill can be used and what happens when you use them. Stunts can break those, allowing your character to stretch the boundaries of the possible.
The only limit to this is that a stunt can’t change any of the basic rules for aspects in terms of invoking, compelling, and the fate point economy. Those always remain the same.

— p.91
Fate Core

Other Rubrics

Flexible Stunts

In Fate Core, stunts are tied explicitly to skills. What if you want your stunts to be skill-agnostic, or tied to multiple skills, or tied to something else entirely different, like an aspect or piece of gear or a stress track?

— p.34
Fate System Toolkit
Aspected Stunts

For stunts which are tied to aspects, you might view some of their effects as narrowly defined free invocations. Other aspected stunts might require an invocation, as Ally’s Shield does, but give something extra or particularly unusual when the aspect is invoked. Such effects should be more potent than a "vanilla" invocation. You could even design a stunt that triggers under particular kinds of compels -just be careful you don’t end up neutering the downside with the resulting benefit.

— p.34
Fate System Toolkit

Triggered Effects

When you use this stunt mechanic, you create stunts that trigger under a specific narrative condition, require a skill roll, and have a specific effect as a result. Stunts like this are a great way to encourage players to do the kinds of things you want to see them do in the game, as those stunts directly reward doing those things.

— p.35
Fate System Toolkit

Broad Stunts

If you’re looking for more variety in your stunts than a +2 or its equivalent, consider the idea of a broad stunt that offers a +1 to two or three things. These could be three different actions within the same skill, or could branch across multiple related skills. If you’re going to allow broad stunts like this, watch out for the overlaps in stunt combinations: you don’t want two broad stunts giving the net effect of three +2s for the price of only two stunts.

— p.35
Fate System Toolkit

Combined Stunts

If you want to offer particularly potent stunts, consider bundling the benefit of multiple stunts together to produce a single big effect. For example, you could create a stunt that provides a monstrous 4-shift effect -that’s a combination of two stunts, and as such would cost two refresh. (You may recognize this as the method used for constructing the supernatural powers in The Dresden Files RPG.) This kind of focused benefit can throw a game out of whack quickly, though. Consider limiting access to such "super-stunts," either in quantity-e.g., "everyone only gets one double-stunt"-or in selection and permission-"only these stunts are available to werewolves."

— p.35
Fate System Toolkit