Quote Originally Posted by Saint-Just View Post
By the rules it isn't. First Fatigue from labor recovers at point/10 minutes or faster, so keeping the caster at exactly 0 FP is impossibly hard, and with 3 points you can try to do something in some situations (Create Door, 10-yard Teleport, Lockmaster on one lock, Levitation for 1 minute, and many more options become available if you manage to recover say 6 points). Then usually there is an option to get some spells at 0 cost (organizing an escape with only those may be too hard, but unlimited Message usually also more than jailers are willing to allow their prisoners), and finally there is an option to get into negative FP or burn HP to power the spell (but jailers cannot normally keep people at negative FP, it will kill them in days).

For mages all you need is Drain Mana or Suspend Magery, but other magic-users are harder. Divine-powered guys are especially hard to block within the "generic fantasy" options.
I didn't check the book because I'm 90% verse m certain that there's a work around for anything in the GURPS library. But if we assume that most NPCs don't optimise than the average spellcaster probably can't cast spells for free and isn't strong enough to overpower the guards first thing in the morning, needing that for your average message a close eye and hard work will deal with them.

PC mages are very rarely average, so if you wanted to hold them you'll need to get more specialised. And yes, it is hard to hold somebody with divine backing, which makes a lot of sense. The chosen of the god of thieves can probably escape almost any prison.