What is the touch AC of an attended object?
Larry the Lizard Wizard casts darkness and tries to touch his opponent's armor. darkness has a range of "touch" and targets an "object touched". Larry's medium sized opponent's touch AC is 14. What is his opponent's armor's touch AC? If Larry were instead to try to touch his medium sized opponent's one-handed weapon (which is a small object) would it have a different touch AC than his opponent's armor?
The "touch" range for spells says only that
Touch: You must touch a creature or object to affect it…
The combat rules specify how spells with a range of touch work in combat.
Touch Spells in Combat: Many spells have a range of touch. To use these spells, you cast the spell and then touch the subject. In the same round that you cast the spell, you may also touch (or attempt to touch) as a free action. You may take your move before casting the spell, after touching the target, or between casting the spell and touching the target. You can automatically touch one friend or use the spell on yourself, but to touch an opponent, you must succeed on an attack roll.
Touch Attacks: Touching an opponent with a touch spell is considered to be an armed attack and therefore does not provoke attacks of opportunity. The act of casting a spell, however, does provoke an attack of opportunity. Touch attacks come in two types: melee touch attacks and ranged touch attacks. You can score critical hits with either type of attack as long as the spell deals damage. Your opponent's AC against a touch attack does not include any armor bonus, shield bonus, or natural armor bonus. His size modifier, Dexterity modifier, and deflection bonus (if any) all apply normally.
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The rules for touch attacks add nothing relevant. The rules do specify an armor class to use when smashing an object.
Best Answer
The RAW doesn't specifically say.
As you note in your research, none of the rules related to hitting objects have any information about what to do if the object is attended.
That said, you can use existing rules to figure out a reasonable alternative. The rules for smashing an object that you link say:
If you try to use the spell rusting grasp on someone's armor:
You can similarly touch a weapon with rusting grasp:
It says that weapons are harder to grasp, but it doesn't actually give any rules for how you should determine the AC of the weapon.
With this in mind, one reasonable ruling would be to apply this rule to attacks against attended objects, but let the object use the attending character's Dex bonus instead of its own, as well as any other modifiers that the character has that apply to touch attacks, like deflection bonuses or a monk's Wisdom to AC. If you try to touch an attended weapon, you provoke an attack of opportunity. This means that it's a little bit harder to hit an object that a character is attending than it is to hit the character, since most objects are smaller than their users.
Another reasonable ruling would be to say that it's a Combat Maneuver, like any other. In this case, I'd say that touching an object that's being attended by another creature would work like Sunder. In this case, you'd simply make a CMB check against your opponent's CMD, provoking an AoO if you don't have Improved Sunder. The drawback of this approach is it means that it's basically impossible for casters to touch an object being attended by a creature except at very low levels, since the CMB of a caster goes up very slowly, but the CMD of most other creatures goes up very quickly.