How Does Master Spellthief Grant Full Caster Level?

[White_Drake]

As the title. Everybody says it does, but after reading the feat I still can't figure out how.

[Douglas]

Benefit section, second paragraph, first sentence:

Your spellthief and arcane spellcaster levels also stack when determining your caster level for all arcane spells.

That's the part that does it.

[White_Drake]

Is that because although stock spellthief levels only grant 1/2 caster, every level in spellthief is a level in an arcane spellcaster, and would therefore allow stacking with itself for full caster level?

[Talionis]

Is that because although stock spellthief levels only grant 1/2 caster, every level in spellthief is a level in an arcane spellcaster, and would therefore allow stacking with itself for full caster level?

Almost.

The feat changes the arcane caster level from 1/2 per Spellthief level to a full (1) arcane caster level per Spellthief level.

Without the feat a Spellthief 20 has an arcane caster level of ten (10). With the feat same Spellthief 20 has an arcane caster level of 20.

So even a single class Spellthief gets several bonuses from Master Spellthief feat.

But there are plenty of classes that are arcane casters that do not have full class level as a caster level.