Can a multiclass sorcerer make spell slots for their other class using sorcery points?

Question

Can a multiclass sorcerer make spell slots for their other class using sorcery points?

For example: can a multiclassed sorcerer 3 / warlock 1 use Flexible Casting to create a warlock spell slot?

Answer 1

Yes, it can make spell slots used by other classes.

No, it can not make a Warlock spell slot specifically.

Total spell slots from multiclassing are calculated using the method described on page 165 of the PHB (SRD reference):

You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, half your levels (rounded down) in the paladin and ranger classes, and a third of your fighter or rogue levels (rounded down) if you have the Eldritch Knight or the Arcane Trickster feature. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table.

The spell slots are not divided up by class, aside from those provided by the Warlock's Pact Magic feature, so there's no such thing as "Sorcerer spell slots" in this context.

Warlock spell slots are not included in the above section from the SRD because they are not granted by the "Spellcasting" feature, but rather the "Pact Magic" feature, and recharge on a short rest instead of a long rest. The SRD explains separately how Pact Magic is handled for multiclassing:

If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.

They are tracked separately due to the different recharge rate, but otherwise function like any other spell slot.

In addition, this unofficial tweet from Jeremy Crawford that explicitly describes the Sorcerer's Flexible Casting feature as "omnivorous"- able to convert slots from any class into sorcery points. There is nothing in the class feature or this Sage Advice which indicates that the reverse is not also true.

The sorcerer's Flexible Casting feature is omnivorous, able to turn spell slots from any class into sorcery points.

A subsequent tweet in that same chain asks a similar question as OP:

The use case is: convert warlock slots to sorc points, short rest, regain warlock slots. Turning a short rest asset to long

And Crawford responds by explicitly saying that it's possible to turn slots into sorcery points:

Yep, that works. Similarly, a paladin/warlock can use warlock slots for Divine Smite. Warlocks have so few slots on purpose!

But another tweet states that the created slots are themselves not belonging to any specific class:

Multiclassing/Flexible Casting/Pact Magic: Sorcery points can create a slot that's used with Pact Magic. It's not a "warlock slot."

While his responses are unofficial, they do imply that the design intent was for these different features to interact in this way, and this is consistent with how the features are written in the PHB/SRD.

Answer 2

Firstly, multiclassing is a variant rule

In 5th edition Dungeons and Dragons, multiclassing is a variant rule. What this basically means is many of the class features break down as written when understood in a multiclass context. For this reason, the multiclassing rules make some specific exceptions to the general class rules as detailed in the Player's Handbook:

a few features have additional rules when you're multiclassing: Channel Divinity, Extra Attack, Unarmored Defense, and Spellcasting.

Font of Magic, and by extension, Flexible Casting is not considered in these additional rules, so it works the same for a multiclass character as for a single-clased sorcerer. As a result, there may be some strange implications of using the feature as a multiclass character that I will discuss at the end of my answer.

Class-specific spell slots

To start with, Flexible Casting makes no mention of the type of spell slot that you can make:

You can transform unexpended sorcery points into one spell slot as a bonus action on your turn.

Contrast this to features like the ranger's Primeval Awareness or the warlock's Bewitching Whispers which say:

you can use your action and expend one ranger spell slot to focus your awareness on the region around you.

...and...

You can cast compulsion once using a warlock spell slot. You can’t do so again until you finish a long rest.

Clearly, with these features you would need a spell slot that is specific to those classes. In the case of the ranger, though, the multiclassing rules provide a specific revision to the Spellcasting class feature:

You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, half your levels (rounded down) in the paladin and ranger classes, and a third of your fighter or rogue levels (rounded down) if you have the Eldritch Knight or the Arcane Trickster feature. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table.

This means that any spell slot from the Spellcasting class feature should qualify for Primeval Awareness, but still not as a warlock spell slot since your warlock levels don't contribute to that pool (as warlocks have the Pact Magic feature instead of the Spellcasting feature).

Similarly, when Pact Magic allows you to refresh a spell slots on a short rest:

You regain all expended spell slots when you finish a short or long rest.

... this only applies to the spell slots granted by the Pact Magic feature.

Flexible Casting

Flexible Casting is an edge case among the rules since it doesn't specify what type of spell slot it creates:

You can transform unexpended sorcery points into one spell slot as a bonus action on your turn.

This can have three different interpretations which each have their own consequences:

Sorcerer Spell Slot

One interpretation hinges on the fact that the rules are written without multiclassing in mind by default. This would lend to the idea that the feature creates sorcerer spell slots despite not explicitly mentioning that it does.

If Flexible Casting creates a sorcerer spell slot, then it will work with any feature that would work with the basic Spellcasting spell slots, since a multiclass spellcaster treats them all as the same.

This sort of idea would also mean you couldn't use warlock spell slots on features such as Divine Smite which don't specify a specific class, but would be assumed to mean paladin spell slot with this interpretation.

Generic Spell Slot

This interpretation suggests that the spell slot created by Flexible Casting is not tied to any class, since no class is specified. In this case, it would not work with any feature that requires a specific class's spell slot to function, but could still be used with features that rely on any spell slot such as the actual casting of spells and Divine Smite.

After doing all the reading, this is the interpretation I personally subscribe to as it relies on the fewest assumptions from my point of view.

Any class's spell slot

Finally, one could interpret that the lack of specificity means a sorcerer can make a spell slot that functions as a class of his/her choice. This would mean a sorcerer could create a spell slot specifically designated as a ranger spell slot or a warlock spell slot. These slots could then work for any given set of class's features, but not another.

One thing to stress about this interpretation is that it leads to a question of whether a warlock spell slot created in this way could recover on a short rest or not. Pact Magic is not explicit on what type of spell slots can be recovered during a short rest.

If a sorcerer could make spell slots that would go on to be recoverable on a short rest with Pact Magic, that would be extremely unbalanced compared to the spellcasting capacity of other class combinations.

Another confusion to contend with when making warlock spell slots is whether you can make warlock spell slots of lower levels than your warlock level allows, or what the cost of these spell slots are since Pact Magic only has one level of spell slot:

all of your spell slots are the same level

Ambiguity

As is usually the case with ambiguous rules, you will have to ask your GM, and confer with the rest of your table to figure out which of these interpretations will be used at your table. Remember that the goal of D&D is for everyone to enjoy themselves, so if a rule or an interpretation of a rule will detract from the fun the group is having, you are encouraged to work with the table to play the game how you all see fit.

After all, one of the first things that the Player's Handbook explains is:

To play D&D, and to play it well, you don’t need to read all the rules, memorize every detail of the game, or master the fine art of rolling funny looking dice. None of those things have any bearing on what’s best about the game.

...and, a bit later:

Your DM might set the campaign on one of these worlds or on one that he or she created. Because there is so much diversity among the worlds of D&D, you should check with your DM about any house rules that will affect your play of the game. Ultimately, the Dungeon Master is the authority on the campaign and its setting, even if the setting is a published world.

The rules outlined in the books are merely guidelines to aid in finding common ground. Where they fail in outlining how the game should work, it is up to the group, and ultimately the GM, to determine how the rules will work at any given table.

Answer 3

No.

The rules are written in plain English

A Warlock spell slot is exactly what it sounds like, a spell slot from the Warlock class. The same with a Ranger spell slot, or a Sorcerer spell slot. Spell Slots are spell slots gained from a specific class.

The feature does only what it says it does.

The "Flexible Casting" section of the "Font of Magic" feature says:

Creating Spell Slots. You can transform unexpended sorcery points into one spell slot as a bonus action on your turn.

Flexible Casting does not allow you to specify the class of spell slots gained. Logically they are Sorcerer Spell Slots, but mechanically it doesn't actually matter.

This matters because a number of spells explicitly require spell slots of certain classes. However, excluding these spells that have wording requiring spell slots of specific classes, you can use spell slots for any other spells from any class.

From the question:

There is nothing in the class feature or this Sage Advice which indicates that the reverse is not also true.

Beware of this logic. The rules are not an exhaustive list of things that are not true.