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 somatic component are not subject to ASF, either when cast or read from scrolls.

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.

Item enchantments
Item enchantments do not stack unless otherwise noted. Only the highest reduction will apply.
 * The Arcane Casting Dexterity reduces ASF by 5%/10%/15%. 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.
 * 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.

Enhancements

 * 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.

Warforged and Bladeforged
Arcane Spell Failure is of particular importance to Warforged and Bladeforged 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 or Warlocks.
 * 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 and Warlocks

 * Bards and Warlocks are not subject to ASF while wearing Light armor.
 * Both have access to a tier 4 enhancement which extends this benefit to Medium armor as well.

Effects that increase spell failure
Certain spells and effects can grant you a chance of spell failure; this can affect also divine casters:
 * Earthquake spell - 30%
 * Deafened condition, for example from Storm of Vengeance
 * Being engulfed in a Gelatinous Cube
 * Something in the Lord of Blades raid