Does the 'surface thoughts' aspect of Detect Thoughts allow a character to know what spells another creature has prepared?

Question

Before a spellcaster can use a spell, he or she must have the spell firmly fixed in mind...
(PHB, CH 10, Spellcasting, Known and Prepared Spells)

A question came up in play last week along these lines: "Does detect thoughts allow you to know what spells a creature has prepared?¹" I was not sure if it could at all, or if it could, which degree of detect thoughts was necessary for that kind of an application. I wanted to keep playing rather than stop and look at the rule books so I made a ruling in situ.

My ruling was "the target gets to try and make a Wisdom save" and we pressed on. (Since the NPC made the saving throw, we didn't end up going any further with that).

But was I wrong? Could the PC have detected what spells the NPC caster had prepared¹ without there being a saving throw involved? I am still not sure.

Can a PC or NPC detect what spells another creature has prepared¹ by using the spell detect thoughts?

You initially learn the surface thoughts of the creature — what is most on its mind in that moment. As an action, you can either shift your attention to another creature’s thoughts or attempt to probe deeper into the same creature’s mind.

If you probe deeper, the target must make a Wisdom saving throw. If it fails, you gain insight into its reasoning (if any), its emotional state, and something that looms large in its mind (such as something it worries over, loves, or hates). If it succeeds, the spell ends.

Either way, the target knows that you are probing into its mind, and unless you shift your attention to another creature’s thoughts, the creature can use its action on its turn to make an Intelligence check contested by your Intelligence check; if it succeeds, the spell ends.

If knowing what spells a creature has prepared¹ is beyond this spell's capacity, an answer spelling out why that is so would be very helpful.

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¹ The word 'prepared' includes 'spells known' for the purposes of this question, per PHB, Ch 10, Spellcasting: Known and Prepared Spells

Before a spellcaster can use a spell, he or she must have the spell firmly fixed in mind, or must have access to the spell in a magic item. Members of a few classes, including bards and sorcerers, have a limited list of spells they know that are always fixed in mind. The same thing is true of many magic-using monsters. Other spellcasters, such as clerics and wizards, undergo a process of preparing spells. This process varies for different classes, as detailed in their descriptions. In every case, the number of spells a caster can have fixed in min at any given time depends on the character’s level.

Answer 1

Only spells the creature is currently thinking about.

Detect Thoughts has two "levels" of thought-reading. You can read a creature's "surface thoughts" without them getting a saving throw, and you can "probe deeper", with the target getting a Wisdom saving throw to resist. For the sake of completeness, I'll address both of these.

Surface thoughts

Reading a creature's surface thoughts is described thusly:

You initially learn the surface thoughts of the creature—what is most on its mind in that moment.

If the target is currently casting a spell, concentrating on a spell, planning to cast it on their next turn, currently deciding which spell they should cast, etc., then reading their surface thoughts will likely inform you of the specific spell(s) they're (thinking about) casting. Otherwise, reading their surface thoughts would not directly give you any information about their known/prepared spells. Of course, it's the DM's call whether the spell is what is most on the creature's mind, or whether something else is.

A known or prepared spell is "firmly fixed in mind". This does not make it a surface thought. The memory of that really embarrassing thing that happened to me in middle school is still firmly fixed in my mind, but (mercifully) I'm not constantly thinking about it all day every day. It's not the thing that's "most on my mind" at any given time.

Probing deeper

Here is the description of probing deeper:

If you probe deeper, the target must make a Wisdom saving throw. If it fails, you gain insight into its reasoning (if any), its emotional state, and something that looms large in its mind (such as something it worries over, loves, or hates).

None of the information described here includes the creature's list of prepared spells. Again, you may get limited information about spells, for example if the creature's reasoning involves things like selecting a spell, or if the creature is a pyromancer who's really looking forward to the next time they get to cast Fireball. But in general even probing deeper isn't going to give you any information about spells the creature isn't currently thinking about in some way.

Answer 2

Totally up to the DM

This comes down to a semantic question: does “firmly fixed in the mind” automatically imply that the spells are a “surface thought,” that they are among the things “most on their mind” at all times?

We can’t really know. We have no real-life frame of reference for a prepared spell—the descriptions and lore make it clear that preparing a spell is a mental experience beyond anything non-spellcasters can know. That goes all the way back to Jack Vance’ novels that inspired Gygax’s original implementation of the spell system (though D&D has moved further and further from that inspiration over time). Consider: a prepared spell is apparently a “thing,” that exists and can be interacted with. You can consume it. Gods can just give you one, ready to go (this is how clerics work). Certain characters and creatures can even steal it (or at least, have been able to, in past editions of D&D; not sure any have yet been printed for 5e). Our actual, real-life thoughts do not work that way.

Does that make a prepared spell a surface thought? I don’t know. The game certainly doesn’t say. We have a couple of reasonable answers arguing it does not, but the argument is based around real-world English language, and real-world experience with thoughts and memories, and that premise is questionable, at best, in this case. Perhaps the best argument against it, really, is that as a special, unreal aspect of the game that the readers cannot be expected to just have an intuitive understanding for from their own experience, we expect the rules to explicitly tell us whenever prepared spells are going to do anything special. You could argue that the lack of mention here implies that they aren’t covered, as a form of “spells do only what they say they do.” But even that is still reading between the lines.

If it’s up to the DM, what should the DM consider?

The only real answer is that it’s going to be up to the DM, and it’s going to be a setting detail that will affect your campaign. So perhaps instead of a semantic argument, the more useful approach to this question is to consider the ramifications of each choice. To my thinking, “no” is the “safe” answer—as DM, things that are hidden from players are things you don’t have to necessarily define. You could imagine someone going around with detect thoughts all the time, picking up these “surface thought” spell lists, and now you have to define what every spellcaster has prepared each day. That sounds pretty miserable DMing experience to me. Plus there’s always the possibility that someday, you want to keep the fact that someone has a certain thing prepared a secret—if you have already established that detect thoughts can do this, that could become a problem.

On the other hand, if you answer “yes,” that potentially creates some interesting scenarios that could be a part of your world. This is more work, but if you wanted to make it a major facet of the campaign, it could be really cool. It does make detect thoughts a much more useful spell, to the point that many mages are going to want to keep it available most of the time. (This could imply it’s too strong for a 2nd-level spell, though.) But there are protections that characters could use, and that could make for some really interesting interactions. It could make surprises that much more surprising, if the players think they have strong evidence of someone’s plans and it turns out those are illusion.

Ultimately, though, I don’t think anyone here can tell you “the” answer. It’s really up to the DM and what they want to do with the campaign.

Answer 3

Only if they happen to be thinking about their spell list at the time.

The spell tells you what "surface thoughts" means:

what is most on its mind in that moment

So if the DM determines that at the time you detect the target's surface thoughts, they happen to be recounting their prepared spell list in their mind, then you would learn those prepared spells they thought about. You might be able to get them to think about their prepared spells via the "white bear problem" by shouting, "don't think about your prepared spells!", but that again depends on the DM.

As for probing deeper, the target's prepared spells seems out of scope for what the spell detects there:

its reasoning (if any), its emotional state, and something that looms large in its mind (such as something it worries over, loves, or hates)".

Answer 4

No

The spell say you can read surface thoughts.

You initially learn the surface thoughts of the creature—what is most on its mind in that moment.

Or things that they are currently thinking.

If you probe deeper, the target must make a Wisdom saving throw. If it fails, you gain insight into its reasoning (if any), its emotional state, and something that looms large in its mind (such as something it worries over, loves, or hates).

Alternatively, you can interogate the caster, and probably read what come to their mind, but you have to ask it the good way.

Answer 5

Consider the name of the spell as well. That it detects thoughts, is assuming it is actively thinking about something. That informs us a lot about the Surface Thoughts.

You initially learn the surface thoughts of the creature—what is most on its mind in that moment.

Probing Deeper might then be the act of motivating their mind toward your curiosity 'the probe', where the Wisdom Saving Throw is the recognition that their mind being motioned toward 'this morning's spell preparation' is an outside influence or an alien thought.

If you probe deeper, the target must make a Wisdom saving throw.

Once your seed is planted within its mind you can start to steer thoughts, much like Trojan Malware can't be distinguished from a computational point of view.

If it fails, you gain insight into its reasoning (if any), its emotional state, and something that looms large in its mind (such as something it worries over, loves, or hates).

Someone in world, specially those capable of spell casting, are likely to know and understand the value of maintaining a blank mind in situations. This is why later in life many mages develop Mind Blank. Something similar to many meditations on mindfulness and essentially recognizing a thought for what it is, then releasing it. Slowly gaining more and more control, Wis Save, of their conscious mind.