@ELGainsborough, I think it would be in our best interest to carry on for the time being, with or without @Yamazaki's response and just assuming their character. You still have my pledge.
Weapon and Armor Proficiency: Spellthieves are proficient with all simple weapons and with light armor but not with shields. Because the somatic components required for spellthief spells are simple, a spellthief can cast spellthief spells while wearing light armor without incurring the normal arcane spell failure chance. However, a spellthief wearing medium or heavy armor or using a shield incurs a chance of arcane spell failure (see page 123 of the Player's Handbook) if the spell in question has a somatic component (most do). A multiclass spellthief still incurs the normal arcane spell failure chance for arcane spells received from other classes, including those stolen from arcane casters (see the steal spell ability, below).