Originally Posted by
bid
If you are wizard 1 / cleric 16, your MC combination gives you the spell slot of a level 17 caster which includes a 9th slot.
Wish is a 9th spell.
A wizard's spellcasting feature allows you to add spells to your spellbook, "of a level for which you have spell slots"
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If you allow character level on warlock invocation, you necessarily allow (gate, true resurrection, true polymorph, wish) on character level. Which includes a bard 1 / cleric 1 / druid 13 / sorcerer 1 / wizard 1 character or any other combo with a 9th slot.