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.

Such a failure expends the spell's spell points and cooldown as if it had been cast.

The ASF penalty from armor and shields is cumulative - they are added together.

Casting a spell from a wand is not subject to ASF, but reading a scroll is.

Spells that don't have a somatic component are not subject to ASF, either when cast or read from scrolls.

Scrolls and Arcane Spell Failure
Any scroll of a spell on the Sorcerer/Wizard spell list will be susceptible to "Arcane" Spell Failure even if the reader should not be subject to it - ie the reader is an Artificer, Bard wearing light armor, divine caster, etc. Thus, for example, Clerics cannot reliably use Protection from Energy scrolls in their heavy armor. This behavior may be modified in the future, as developers have acknowledged it should not work this way.

Similar to divine spells, Artificer-exclusive spells are Arcane-type spells but are not subject to Arcane Spell Failure. Artificers using Sorcerer/Wizard's list scrolls via UMD 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.

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 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.
 * Lesser Arcane Sigil and Arcane Sigil are normally docent-exclusive enhancements, but can be added to Dragontouched Armor via the appropriate runes. They reduce ASF by 5% and 10% respectively. LAS/AS are typed differently and will stack with normal item enhancement effects and with each other. See discussion (March 2013).
 * Twilight 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 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 ASF by 5%, but this material is generally found on rather heavy armors (Breastplate and Banded) and will not negate ASF alone.

Elven Arcane Fluidity

 * The Elven Arcane Fluidity I/II/III/IV enhancements are available to Elves, Drow, and Half-Elves at level 2/6/10/14 at a cost of 1/2/3/4 AP. Each rank replaces the previous - they are not cumulative.
 * Elven Arcane Fluidity I reduces ASF by 5% on Light armor only.
 * Elven Arcane Fluidity II reduces ASF by 10% on Light armor or 5% on Medium armor.
 * Elven Arcane Fluidity III reduces ASF 15% on Light armor, 10% on Medium armor, and 5% on Heavy armor or shields.
 * Elven Arcane Fluidity IV reduces ASF 20% on Light armor, 15% on Medium armor, and 10% on Heavy armor or shields.

Warforged
Arcane Spell Failure is of particular importance to Warforged arcane casters as their natural body plating imposes at least a 5% ASF penalty by default. Fortunately, there are also warforged-exclusive ways to mitigate ASF:
 * The default Composite Plating imposes 5% ASF, while the optional Mithral Body imposes 15% and Adamantine Body imposes 35%.
 * Composite and Mithral body plating are considered light armor and thus don't impact warforged Bards.
 * The Warforged Inscribed Armor I/II/III enhancements are available to warforged at level 2/6/10, reducing ASF from armor by 5%/10%/15%.
 * Lesser Arcane Sigil and Arcane Sigil are docent-exclusive enhancements, reducing ASF by 5% and 10% respectively. LAS/AS are typed differently and will stack with normal item enhancement effects and with each other.

Bards

 * As noted above, bards are not subject to ASF while wearing Light armor.
 * The Warchanter II bard PRE extends this benefit to Medium armor as well.

Artificers

 * Artificers are not normally subject to ASF.
 * However, the Battle Engineer I artificer PRE grants a 10% reduction in ASF from armor. This is generally only useful when using Scrolls subject to ASF as above.