Going from fighter to Magus Kensai, it says:
A kensai is not proficient with armor or shields and suffers normal
arcane spell failure chance when casting magus spells while armored.
But as a fighter, I'm proficient with armor. Will I still suffer from spell failure?
Note:
A magus is also proficient with light armor. He can cast magus spells
while wearing light armor without incurring the normal arcane spell
failure chance.
Best Answer
Yes.
Here are the rules for Arcane Spell Failure:
The rules do state a few exceptions (spells without somatic components in the quoted portion, and bards getting light armor without ASF elsewhere in the linked document), but proficiency is not one of them.
In other words: Even if you are proficient with armor, you still suffer its Arcane Spell Failure chance.
Yes.
Kensai is an archetype of Magus. The Magus does have a class feature that allows them to use light armor without spell failure:
But this is entirely replaced by the Kensai class feature of the same name:
In other words, the class behaves as though the entire "Weapon and Armor Proficiency" block had been removed from the Magus, and replaced with the one from Kensai.
The Kensai will also not get the Medium Armor (Ex) or Heavy Armor (Ex) class features of the Magus class, as these are replaced by the Iajutsu (Ex), and Iajutsu Focus (Ex) class features.