This is a classic "rules lawyering/powergaming" tactic I like to call "intentional ignorance." The rules are quite clear, yet you reach for the clearly unintended AND unwritten interpretation that grants outrageously overpowered options by ignoring a) the specific v. general guideline, and b) the EXPLICIT multiclassing rules.
The rules are pretty clear, I agree, but you're the one who seems to be ignoring them. In order to support the argument that a Wizard19/Cleric1 can only prepare 1st level cleric spells, you need to argue that the phrase in the multiclassing rules that says that you prepare spells as a single-classed spellcaster also says you determine what spell slots you possess as a single-classed spellcaster. The problem is that the multiclassing rules don't say that -- there's an entire paragraph on how you determine what caster level you are for determining your character's spell slots, and it makes no distinction between your classes -- the rules even explicitly state that you can cast spells from any of your Spellcasting classes with the slots you gain from the Multiclassing Spell Slot chart.
In other words, if you are a multiclass caster, you determine your character's spell slots from the Multiclass Spell Slot chart; those are the spell slots your character has. You don't have a subset or different number of those slots only to determine what spells you can prepare -- those are your spell slots.
Obviously, no one can tell you ["you" = anyone suggesting the 9th level cleric spells are ok] how to play
Actually, someone can, since I play a cleric/wizard in Adventurer's League, and to this point, that's how the interaction has been ruled in Organized Play.
There is no conflict here [in the book] except the one being created, specifically, by those who want to get outrageously broken power.
I'd suggest you try a Wizard19/Cleric1 compared with other multiclasses to see how 'broken' it is before just spouting off:
- Compared to a Cleric20, a Wizard19/Cleric 1 has no domains or domain abilities, which also means no free prepared spells from those domains. A mass heal from a Life Domain cleric will be far more effective than one from a Wizard/Cleric; similarly other cleric-specific spells will be more effective coming from an actual cleric.
- A Cleric19/Wizard1 can prepare 9th level wizard spells, but has the in-game requirement of actually finding 9th level spells on scrolls or in other spellbooks to copy into his own. (The wizard can copy any spell he can prepare, and he can prepare any spell for which he has spell slots -- so if the cleric can do it, so can the wizard.) This makes the Wizard19/Cleric1 nearly identical to the Cleric19/Wizard1, instead of making one vastly more powerful than the other.
- If your interpretation of the rule is correct, then a Wizard10/Cleric10 is one of the worst multi-class options you can take, since a Wizard is already restricted based on what spells he gets for free in his spellbook. (Unlike the rule on preparing spells, the rule for gaining free spells specifically lists the Wizard chart for determining what spell level of spell can be gained for free.) Under my interpretation, Wizard10/Cleric10 is of basically equivalent spellcasting ability to any other cleric/wizard multiclass, and the differences lie in which class abilities the character has access to.
An interpretation where one multi-class combo is vastly more powerful than the others is more 'broken', to my mind, than one where all multi-class combos are largely equivalent.
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Pauper