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.

To see your current arcane spell failure, drag a spell subject to ASF to your hotbar and hover over it. If an item (armor or shield) has ASF, its failure chance is displayed on the item; however, this information does not include global ASF reductions that you might have.

Exceptions
 * Casting a spell from a wand is not subject to ASF, but reading a scroll is.
 * Spells that don't have somatic component (for example Blur or Displacement) are not subject to ASF, either when cast or read from scrolls.
 * Spell-like abilities (SLA) generally ignore ASF (with the exception of some Warlock SLAs).

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
The following effects do not stack with each other; only the highest reduction applies.
 * 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% (Minimum Level 12/16/20). 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.

Additional stacking sources:
 * 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.
 * Otto's Irrevocable Power: Spell Failure Reduction - filigree for a sentient weapon (-10%)
 * Frozen Wanderer: Arcane Spell Failure - filigree for a sentient weapon (-10%)

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. Mithral also reduces the weight category of armor (but not the size class of shields) by one step (Heavy to Medium, etc.) which reduces the PRR provided by the armor since PRR from armor is based in part on the weight category.
 * 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 Eldritch Knight third core enhancement Imbue the Blade reduces ASF from armor, shields and Warforged Body feats by 15% at a cost of 1 AP.
 * The Eldritch Knight fourth core enhancement Subtle Force I reduces ASF from armor, shields and Warforged Body feats by an additional 20% (total 35%) at a cost of 1 AP.
 * The Eldritch Knight tier 2 enhancement Shield Training reduces ASF from shields by 5% and grants proficiency with all shields except Tower Shields at a cost of 2 AP.
 * The Eldritch Knight tier 4 enhancement Armored Arcana grants medium armor proficiency and reduces ASF from light armor, medium armor, and Mithral Body to 0% at a cost of 2 AP.
 * The Battle Engineer Infused Armor core enhancement 3 and 5 each reduce ASF by 10%, for a total of 20% with both.

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
 * Touch of the Mournlands - special attack in the Lord of Blades raid (20% per stack)