This question already has an answer here:
The polymorph spell description is kind of elusive about this:
The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast Spells, or take any other action that requires hands or Speech. (5th Edition SRD, p. 169)
We were in an arena doing some gladiator fights and one of the foes, controlled by the DM, was a sorcerer.
My DM took it this way: spells don't require hands or speech while casting a Subtle Spell, so he could transform his character into a Giant Ape with polymorph, buffed with Mage Armor, and kept counter-spelling me since I was the only caster in the fight.
I think this is the wrong way to interpret the whole thing, but I am running out of proof. Here's the question: Who is wrong and why?
I searched a lot about the topic, but couldn't find anything. Was it included in an errata I could not find?