I've stolen your fireball, now what?

Question

So, for the first time ever, I have someone in my group playing a spellthief. (woohoo!)

However, it got me thinking, and the answer doesn't quite spell itself out (wow, the pun).

When a spellthief sneak attacks a spellcaster and drains a spell (assuming valid target, valid spell level, valid spell, etc..), is the target aware that they've been drained? They are obviously aware that they've been stabbed, but do they know that some magic has been stolen from them? If they do, are they aware of what was stolen?

Example:
The Spellthief steals Disintegrate from Sir Rious-burns the sorceror, who now can't cast Disintegrate for a minute. Sir Rious-burns now wants to Disintegrate the Spellthief, since they stabbed him rather hard. Would he try to cast Disintegrate, only to see it fail and lose his action (something akin to sanctuary result)? Or, would he already know that casting Disintegrate was currently not possible?

Answer 1

A handful of effects that readers might assume are undetectable are explicit that victims recognize them. For example, a creature knows that it can't cast defensively when it's threatened by a foe that possesses the feat Mage Slayer (Monster Manual V 85), and a creature knows that its slightest movement provokes an attack of opportunity from a foe that threatens it and that's in the Devoted Spirit martial stance thicket of blades (Tome of Battle 61). However, the vast majority (and, no, not a small majority) of effects just don't say one way or the other whether an affected creature is aware of them or not (q.v. suggestion, this related Pathfinder question that reverberates from 3.5). Such is the case here: The text details at length what happens to the spellthief when she steals a spell, but it seems to care little about the unfortunate victim.

In short, there's no direction from the text on whether or not the victim knows a spell's been stolen or what spell's been stolen. The DM makes that decision.

This DM would rule that a creature knows if its spell is unavailable

This DM prefers to hew closely to the rules, so when an effect is not described as having an obvious effect, he tends to rule that the effect does not have an obvious effect. That means, for example, spells that seem on the page flashy—perfect for awing pathetic noncasters!—sometimes in this DM's capaigns just aren't. (Yes, I make this clear to my players early on.)

On the other hand, this DM is also a big fan of player agency, speeding play, and friendship. Telling a player that his PC has wasted his time because neither knew that the PC's special abilities had fundamentally changed sounds like a recipe for destroying all three.

That is, it's one thing to tell the player that his sorcerer PC loses the use of his disintegrate spell after the weasely little NPC spellthief jammed a shiv into his sorcerer's gut, and I think it's totally okay not to tell the player that she was a spellthief, and I think that it's also okay not to to tell the player that his sorcerer will get the disintegrate spell back in a minute. (This DM sees such concerns as healthy stress.) But not telling the player that his PC's lost the use of the disintegrate spell then letting the player have his PC go through the motions of casting the gone spell only to see it fail? That's a bridge too far.

This DM can imagine a player having some pretty harsh words for him were he to do that. I mean, not only has every standard action grown exponentially in importance during the course of twelve levels of play (the minimum level a sorcerer needs to be to cast the spell disintegrate), but also the concept of knowing instinctively what spells a caster has available is so fundamental to the character—so bound up with the idea of being a caster—that removing even part of a caster's casting ability secretly—without informing the player—seems, to this DM, like a violation.

So while the rules very well may tacitly—with their silence on the subject—support the DM secretly tracking which spells an NPC spellthief has stolen from the PCs and letting the PCs waste actions trying to cast them, this DM doesn't imagine that being fun for anyone: It's more work for the DM, and it'll make your players angry.

And, therefore, by the same token, an NPC that sees his spell stolen by a PC spellthief should likewise be informed of the loss. The PC spellthief may be initially disappointed at the ruling, but perhaps a mile in another's moccasins will help the player see the wisdom in it. And, if it doesn't, a couple of encounters with NPC spellthieves almost certainly will.

(Arguably, the closest thing to a spellthief's steal spells ability's effect is the supernatural ability energy drain that sees a victim that casts spells lose access to one spell of the highest level as if it had been cast. This, in this DM's opinion, is obvious the caster because it's as though the spell's been cast. This reader imagines the caster's HUD flashing Energy Drain! followed by the caster's disintegrate spell disappearing from her mental list of prepared spells. This is, though, different enough from the spellthief's steal spells ability that this DM wouldn't use it as a precedent, but another DM might.)

Answer 2

From what I read: "The target of a steal spell attack loses one 0-level or 1st-level spell from memory if she prepares spells ahead of time, or one 0-level or 1st-level spell slot if she is a spontaneous caster. A spontaneous caster also loses the ability to cast the stolen spell for 1 minute."

This tells me the victim would temporarily forget that they had the spell memorized. It wouldn't occur to them to cast it as it is not in their memory any longer. For the spontaneous caster it would make sense that they would likewise forget that they knew the spell also for that minute.

I think it would be up to the DM to decide whether the caster would be aware that they knew the spell a moment ago and now "hmm... how did that spell go again?" It would make for some interesting role playing in my opinion.