How do the Cleric and Druid's Spontaneous Casting interact with other classes' spells?

Question

This is a very RAW vs. RAI question.

When the descriptions of both clerics and druids discuss how prepared spells are instead converted into specific kinds of spells, they always talk about "a" or "any" prepared slot.

For the Cleric:

The cleric can “lose” any prepared spell that is not an orison or domain spell in order to cast any cure spell of the same spell level or lower

For the Druid:

She can “lose” a prepared spell in order to cast any summon nature's ally spell of the same level or lower.

If I were playing an 8th-level wizard / 1st-level druid / 1st-level cleric, according to the wording of their class abilities I should be able to convert the higher-level wizard spells into either cure or summon nature's ally spells.

1. Has Paizo ever confirmed that this does or does not work this way?
2. Is this right by RAW?
3. Should this be allowed? This is obvious with a no.

Answer 1

The answer is no

The Pathfinder Design Team officially addresses this question in an FAQ on their forum:

Spontaneous Casting and Multiple Classes: Can I spontaneously cast spells from one of my classes using a different class’s spell slots?

No. This is only possible if you have a class feature that explicitly allows it, such as Combined Spells. This applies even if the two classes share a spell list or if one of the classes allows you to spontaneously convert that class’s spell slots into certain spells on that class’s spell list, such as cleric and druid.

Answer 2

1. So has Paizo ever confirmed that this does or does not work this way?

They have not. The closest I have found is a similar question in the FAQ about a multiclass sorcerer and the sorcerer class feature bloodline arcana:

Sorcerer: Do the bonuses granted from Bloodline Arcana apply to all of the spells cast by the sorcerer, or just those cast from the sorcerer's spell list?

The Bloodline Arcana powers apply to all of the spells cast by characters of that bloodline, not just those cast using the sorcerer's spell slots.

General rule: If a class ability modifies your spellcasting, it applies to your spells from all classes, not just spells from the class that grants the ability. (The exception is if the class ability specifically says it only applies to spells from that class.)

Based on that precedent, it would imply that a cleric/wizard could sacrifice wizard spell slots to spontaneously cast cure or inflict spells.

However, the even-more-general rule is that the text of classes assumes single-classing (infuriatingly), and should not be assumed to automatically apply generally. So... that’s a contradiction, of sorts.

2. Is this right by RAW?

Yes, it is. It says “any,” so it’s any. Then again, even in RAW, it’s generally accepted that, for example, a multiclass wizard’s banned schools doesn’t affect his non-wizard spellcasting. So that’s... the same contradiction.

Basically, the wording of class features is, and has been since Wizards first started writing them for this family of games, really vague and has an annoying habit of assuming single-classed, starting-from-1st-level characters, and it’s not at all clear when they’re just making that simplification or when they really truly do mean things to be global. Even RAW, as an interpretation hyper-focused on determining an interpretation that can consistently and unambiguously be independently arrived at, falters with this issue.

3. Should this be allowed? This is obvious with a no.

I disagree. Multiclassing is extremely, massively painful for a spellcaster, and no cure, inflict, or summon nature’s ally spell is all that impressive anyway. A cleric/wizard or druid/wizard isn’t appreciably doing anything that a cleric or druid couldn’t do better here. It’s a small bonus for a suboptimal build. Really, I don’t think it deserves much more than a shrug.

Answer 3

Quoting Sean K. Reynolds on Spontaneous Casting:

Seriously, this is basic stuff. The language is a little weak in some places, but you know that there's a realistic way of interpreting it...and an unrealistic way of interpreting it. If there are two interpretations, and one seems too good to be true, go with the other one.

You can't really be confused by this. Because if you are confused by that, this cleric ability is going to blow your mind:

Spontaneous Casting: A good cleric (or a neutral cleric of a good deity) can channel stored spell energy into healing spells that she did not prepare ahead of time. The cleric can “lose” any prepared spell that is not an orison or domain spell in order to cast any cure spell of the same spell level or lower (a cure spell is any spell with “cure” in its name). ...

Heck, the above language doesn't even limit it to spells you have prepared! Why not use this ability to convert someone else's spell into a cure spell? If you know the enemy lich likes to use phantasmal killer, on your turn you should just spontaneously convert his spell into cure critical wounds! Enjoy 4d8+10 damage, lich! ...

Yes, his answer is blunt, but on point.

So, interpreting a single phrase out of the context of the ability, or even out of the context of the class, cannot be taken as rules as written simply because we are using english and it's interpretations as if it was part of the rule system, which it is not. The language is subject to flaws and interpretations, but when something is ambiguous or unclear, we have to look for a context, similar rules, general rules, or even FAQ's (and similar FAQ's) on the subject.

Now, on context, a cleric cannot spontaneously convert spells from another class because the other classes cannot cast cleric spells. Cleric spells are only given by the cleric class, and even oracles, that use the same spell list as clerics, have their own oracle spells. If we look up on the Oracle Class for reference of cleric spells, we will only find that they use the same spell list as the cleric.

A druid casts druid spells, thus those spell slots are invalid for spontaneous casting of the cleric class. Similarly, the spontatenous casting of the cleric class cannot use spell slots from the druid class. Classes that have abilities that can be used on multiple (different) class spells or allow to cast spells from another class, are usually explicitly called out as being able to to do (See Mystic Theurge and Arcanist).

The same issue will be seen on wizard/sorcerer abilities, and though they will often say "your spells" or "your spell slots", the rules as intended are that those spells are from the class you are currently reading.

This is a topic where the community extensively discuss this exact subject: Can a wizard 1/ cleric 1 spontaneously cast?