Arcane spell failure
 Arcane Spell Failure
Arcane Spell Failure (ASF) is the chance that an arcane spell — that is, one from the Sorcerer/Wizard spell list or Bard spell list — will fail to be cast due to interference from the caster's armor or shield.
The ASF penalty from armor and shields is cumulative - they are added together.
 Scrolls and Arcane Spell Failure
Similar to divine spells, Artificer-exclusive spells are not subject to Arcane Spell Failure, but Artificers using Sorcerer/Wizard list scrolls are subject to Arcane Spell Failure. So, for example, anyone using a Lightning Motes scroll will suffer no Arcane Spell Failure, but an Artificer is susceptible to Arcane Spell Failure when using a scroll of Reconstruct (available to Artificer but from the Sorcerer/Wizard spell list).
Similarly, Bards wearing light armor using scrolls on the Sorcerer/Wizard's list will be susceptible to Arcane Spell Failure even for spells also on the Bard spell list. A Bard wearing light armor has no chance of Arcane Spell Failure when casting Greater Heroism as a spell, but is susceptible to failure when casting it from a scroll.
 Reducing Arcane Spell Failure
 Item Enhancements
Item enhancements do not stack unless otherwise noted. Only the highest reduction will apply.
- The Lesser Arcane Casting Dexterity/Arcane Casting Dexterity prefix reduces ASF by 5%/10%. Arcane Casting Dexterity reduces total ASF, and as such can be on any item rather than needing to be on the armor or shield that's causing ASF.
- A Sapphire of Spell Agility +5/+10/+15 in a Blue Augment Slot reduces ASF by 5%/10%/15%. This reduces total ASF, and as such can be on any item rather than needing to be on the armor or shield that's causing ASF.
- The Lesser Arcane Sigil and Arcane Sigil prefixes are normally docent-exclusive enhancements, but can be added to Dragontouched Armor via the appropriate runes. They reduce ASF by 5% and 10% respectively. Prior to U17 LAS/AS were untyped and stacked with normal item enhancement effects and with each other. After U17, LAS/AS were apparently changed to global enhancement bonuses and no longer stack with other item enhancements or with each other. 
- The Twilight armor prefix reduces ASF by 10%, and Greater Twilight reduces ASF by 20%. Twilight affects the armor or shield it is on only, however. It is applied before item effects granting global reduction of arcane spell failure and stacks with them.
 Armor/Shield Materials
- Mithral is exceptionally light, reducing the Arcane Spell Failure chance of an armor or shield made with it by 10%. For very light types of armor or shields, this can completely negate ASF by itself.
- Darkleaf reduces the ASF of the armor or shield made with it by 5%, but this material is generally found on rather heavy armors (Breastplate and Banded) and will not negate ASF alone.
- Alchemical Crystal reduces the ASF of an Alchemical Shield by 10% when applied to it through reforging.
- The Arcane Fluidity enhancement is available to Elves, Half-Elves, Drow, and Morninglords (iconic) as a Tier 1 racial enhancement. Rank 1/2/3 reduce ASF from armor (but not shields) by a total of 5%/10%/15% at a cost of 1 AP per rank.
- The Wizard/Sorcerer Eldritch Knight Spellsword core enhancement 2 and 4 each reduce ASF from armor and shields by 5%, for a total of 10% with both.
- The Eldritch Knight tier 2 enhancement Light Armor Proficiency reduces ASF from armor by 5% and grants Light armor proficiency at a cost of 2 AP.
- The Eldritch Knight tier 4 enhancement Medium Armor Proficiency reduces ASF from armor by 5% and grants medium armor proficiency at a cost of 2 AP.
- The Eldritch Knight tier 5 enhancement Still Spell reduces ASF from armor and shields by a total of 5%/10%/15% at a cost of 1 AP per rank.
- The default Composite Plating imposes 5% ASF, while the optional Mithral Body imposes 15% and Adamantine Body imposes 35%.
Fortunately, there are also warforged-exclusive ways to mitigate ASF:
- Composite and Mithral body plating are considered light armor and thus don't impact warforged Bards.
- The Inscribed Armor enhancement is available to Warforged and Bladeforged as a tier 1 racial enhancement, reducing ASF from body plating by 5%/10%/15% for a cost of 1 AP per rank.
- Lesser Arcane Sigil and Arcane Sigil are docent-exclusive enhancements, reducing ASF by 5% and 10% respectively. Prior to U17 LAS/AS were untyped and stacked with normal item enhancement effects and with each other. After U17, LAS/AS were apparently changed to global enhancement bonuses and no longer stack with other item enhancements or with each other. 
- Bards are not subject to ASF while wearing Light armor.
- The Warchanter tier 4 enhancement Armorer extends this benefit to Medium armor as well.
 Default ASF Penalties
|Table: Arcane Spell Failure — Armor|
|Armor Name||Failure %||Armor Type|
|Composite Plating (Warforged only)||5%||Light|
|Padded Armor (pre-MotU)||5%||Light|
|Leather Armor (pre- and post-MotU)||10%||Light|
|Mithral Body (Warforged only)||15%||Light|
|Studded Leather Armor (pre-MotU)||15%||Light|
|Chain armors (post-MotU)||15%||Light|
|Chain Shirt (pre-MotU)||20%||Light|
|Hide Armor (pre-MotU)||20%||Medium|
|Half Plate (post-MotU)||30%||Heavy|
|Adamantine Body (Warforged only)||35%||Heavy|
|Banded Mail (pre-MotU)||35%||Heavy|
|Full Plate (pre-MotU)||35%||Heavy|
|Half Plate (pre-MotU)||40%||Heavy|
|Splint Mail (pre-MotU)||40%||Heavy|
|Table: Arcane Spell Failure — Shields|
|Shield Name||Failure %|