Does the Wizard's Spell Mastery spells need to be prepared as a Wizard or can they be "known"?

Question

At 18th level, a Wizard gains the feature Spell Mastery. It reads:

At 18th level, you have achieved such mastery over certain spells that you can cast them at will. Choose a 1st-level wizard spell and a 2nd-level wizard spell that are in your spellbook. You can cast those spells at their lowest level without expending a spell slot when you have them prepared. If you want to cast either spell at a higher level, you must expend a spell slot as normal.

Early in Merlin's career, they learned the shield spell. It's in their spellbook and they have it prepared almost every day. At some point in their life, they realized that the magic was in them all along and gained a level of Sorcerer. As such, they learned shield as a Sorcerer to free up a Wizard Prepared Spell slot for something else.

When Merlin reaches level 19 (18 Wizard/1. Sorcerer) they get Spell Mastery.

Can they still use the spell prepared as a Sorcerer for the Wizard feature?

The checkboxes I see are:

• [X] A 1st-level Wizard spell
• [X] A spell that is in your spellbook
• [?] You have [it] prepared

To my reading, it doesn't need to be prepared AS A WIZARD, just prepared. The question is, Sorcerers don't "prepare" spells, they "know" spells. But the PHB lumps those terms together. Is there a mechanical/rules difference and can Merlin keep the spell as a Sorcerer and use it with the Spell Mastery feature?

Answer 1

A spell known/prepared through another class is not a wizard spell

The multiclassing rules regarding spellcasting contain the following excerpt (emphasis added):

You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. If you are a ranger 4/wizard 3, for example, you know three 1st-level ranger spells based on your levels in the ranger class. As 3rd-level wizard, you know three wizard cantrips, and your spellbook contains ten wizard spells, two of which (the two you gained when you reached 3rd level as a wizard) can be 2nd-level spells. If your Intelligence is 16, you can prepare six wizard spells from your spellbook.

Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell. Similarly, a spellcasting focus, such as a holy symbol, can be used only for the spells from the class associated with that focus.

When multiclassing as a wizard/sorcerer, the spells known through your sorcerer levels are sorcerer spells, the spells prepared from your spellbook are wizard spells. Even though you have some version of shield known to you, it is not the same as the shield spell written in your spellbook. To use shield for the Spell Mastery feature, you would need to have the spell from your spellbook prepared.

The Sage Advice Compendium does not have this exact situation, but one similar is referenced here:

A wizard multiclasses into Wild Magic sorcerer. Do spells cast from their spellbook trigger Wild Magic Surge if they are on the sorcerer spell list, or do they have to gain them from sorcerer to trigger?

From the multiclassing rules: “Each spell you know and prepare is associated with one of your classes”. This rule means only the spells gained from levels in sorcerer trigger Wild Magic Surge.

Answer 2

It must be a wizard spell you have prepared; you can’t prepare sorcerer spells

The text you reference from the Players Handbook is (emphasis mine):

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.

While both sorcerers and wizards can have spells “fixed in mind”, it’s clear from the passage above that “knowing” and “preparing” spells are not the same, and that of the two classes, only wizards prepare spells.

Furthermore, you have to choose a wizard spell that’s in your spell book for this feature. The multiclass rules allow you to use one set of spell slots for both classes, but they don’t allow you to use any class feature with any other class’s spell. From the Players Handbook section on multiclassing, under the Spellcasting feature (emphasis mine):

Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell.

This is further clarified in the Sage Advice Compendium, which uses the rule above to make it clear that wizard spells you cast as a multiclass wizard/sorcerer aren’t subject to sorcerer features like Wild Magic, which specifies it only works with sorcerer spells.

Some features, like the sorcerer’s Metamagic, say they work when you cast “a spell” and so can be used with a spell from any of your classes. But Spell Mastery specifies that you can only choose wizard spells, with the additional requirement that they be prepared to be cast using this feature.

So while you also know shield as a sorcerer spell, you will still need to have shield prepared as one of your wizard spells in order to cast it using Spell Mastery. This would be true even if you had multiclassed as another spell preparing class, but in this case it’s doubly clear, as you cannot prepare a sorcerer spell.