The Summoner class gives the ability to cast arcane spells while wearing light armor. The Mithral special material can make a set of armor count as one category lighter for the purposes of "movement and other limitations". However, "This decrease does not apply to proficiency in wearing the armor." Since the text stating that the Summoner can ignore arcane spell failure on arcane spells cast in light armor is in the "Weapon and Armor Proficiency" section, I'm unsure of whether or not the armor lightening applies to the spell failure text. I've googled this as well, and found several disagreeing sources.

If a medium armor is made of mithral, can a Summoner cast spells in it without spell failure?

  • \$\begingroup\$ Does your summoner have proficiency in Medium Armour? \$\endgroup\$
    – Miniman
    Nov 7, 2014 at 3:42
  • \$\begingroup\$ Nope. He'll be getting a -1 ACP in Mithral Breastplate, but I don't think that matters, as far as ASF is concerned. \$\endgroup\$
    – DuckTapeAl
    Nov 7, 2014 at 3:48
  • 1
    \$\begingroup\$ Extension: The same applies to Bards and lower lvl Magi, and similarly to higher lvl Magi, Bloodragers and Skalds when using Mithral Heavy Armor. \$\endgroup\$
    – MrLemon
    Nov 7, 2014 at 9:17

2 Answers 2


For the purpose of ASF (a "limitation") medium mithral armor is considered light

Armor proficiency (Light/Medium/Heavy)

Benefit: When you wear [type] armor, the armor check penalty for that armor applies only to Dexterity- and Strength-based skill checks.

Normal: A character who is wearing armor with which he is not proficient applies its armor check penalty to attack rolls and to all skill checks that involve moving.

Mithral armor

Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. This decrease does not apply to proficiency in wearing the armor. A character wearing mithral full plate must be proficient in wearing heavy armor to avoid adding the armor's check penalty to all his attack rolls and skill checks that involve moving. Spell failure chances for armors and shields made from mithral are decreased by 10%, maximum Dexterity bonuses are increased by 2, and armor check penalties are decreased by 3 (to a minimum of 0).

If you do not have proficiency in Medium armor, then you will take a penalty to attack rolls and all skill checks that involve moving. This would include spell attack rolls. (Since it is mithral, the penalty will be reduced from the usual -4 down to only -1).

But for purposes of movement and other limitations (which includes class features, eg Ranger's combat styles, Summoner's ASF) it counts as Light.


The Mithral Breatplate stats are as follows:

  • Half weight (as opposed to steel breastplate)
  • Treated as light armor (instead of medium)
  • increased max Dex bonus (+2)
  • reduced armor check penalty (3; min 0)
  • reduced arcane spell failure chance (10%)

So to answer your question, there is a 10% ASF for this item. However, if you choose a mitral buckler is the only item I was able to find with a 0% ASF.

  • \$\begingroup\$ -1: I'm aware that the ASF would be 10%, but it would also be light armor. Summoners ignore ASF from light armor. If you're saying that Summoners take this ASF even though the breastplate counts as medium, could you please make that a bit clearer, and explain why? \$\endgroup\$
    – DuckTapeAl
    Nov 7, 2014 at 3:27
  • \$\begingroup\$ The question here isn't what the ASF is, it's whether it applies. \$\endgroup\$
    – Miniman
    Nov 7, 2014 at 3:37

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