Is the Task Difficulty from an event a secret for GM ?
So like the GM says:
- You attack the XXX, roll a D20 and I'll tell you if you hit ?
- The target number to attack the XXX is 3, please roll a D20.
This is from page 84 of the corebook:
The GM doesn’t have to tell the player what the target number is, but he can give her a hint, especially if her character would reasonably know if the action was easy, average, difficult, or impossible.
This is from the Numenera GM Screen:
It’s okay to tell players—particularly starting players—the difficulty of the task.
So no, it's not necessarily a secret the GM must keep from players.
You might want to take into account the Effort economy when deciding whether to tell players the task difficulty, e.g. if a player is about to spend some of her last Speed points to apply Effort when it would be "unnecessary". Consider this article, and the fact that learning when and when not to use Effort is a big part of mastering Cypher System games.
In general, the players don't know what it takes to hit a monster (or generic enemy). That said, there are times when the player will roll some number, say 15, with added bonuses) and the GM will say, "yeah, you hit him, you only needed a 10." Like most things in RPGs, that's entirely up to the GM and his/her style of running the game.
Suppose your party is getting beaten badly, everybody is down to their last couple of Hit Points (or however your system tracks damage). The GM could fudge your dice or fudge his to keep the party alive to keep the fun going. I'm not saying that I do that but I'm not saying that I'm above it or would frown on a GM who did it.