Fate as solo engine has been already discussed on several blogs, like Solo Roleplayer, No one to play with or Solo RPG voyages.
Following them, I just wanted to add my own 5 cents.
When I first started with Fate, I planed to use driver like 9Qs to run my game, but it turned out there is no need for that.
As Roryb from No one to play with correctly noticed - Aspects is really where it’s at.
To achieve something in Fate, player needs Fate points, and the pool is running empty fast. In order to fill it back, player needs to compel to one of the aspects and include complication, and that is usually enough to drive the story forward.
Of course, spicing things up is always good.
Solo roleplayer offers this through Fate oracle, a way to answer yes/no questions with a twist. The problem with this is the fact that oracle is already incorporated into Fate game, and there is no need to remember another one.
Binary questions in Fate are answered through Create advantage action using in-game mechanic. Simply, instead of asking a question "Is the door locked?" and assigning probability, player can try to create "Unlocked" aspect on the same door, by assigning difficulty for the task. If he fails, door is locked, and then the player can use Overcome action to try to unlock them. On the other hand, this approach offers no twist in AND/BUT range.
Solo RPG voyages introduces twists through random events triggered when all marked sides of the dice show only + or only - signs. The idea is further extended on the blog.
Following this idea, I tried to incorporate it into Fate to add AND/BUT twist to the built in oracle.
Here it is:
Every time the player rolls dice against fixed difficulty (usually to make Create advantage or Overcome action) look at the symbols on the dice.
If two or more blank sides are visible, result is success of failure as per rules with added AND.
If at least one combination of + and - is visible, result is success of failure as per rules with added BUT.
In case of the dice combination + - _ _ BUT and AND result can either cancel each other, or you may select special case of success/failure with both AND and BUT.
In case of the re-roll use the second set of dice.
The issue that AND or BUT relate to can be selected by the player or invoked using some sort of Random idea generator.
Chances table if AND and BUT cancel each other:
AND ------21/81 or 26%
BUT-------37/81 or 46%
No result-23/81 or 28%
Examples of this system in action can be seen here.