Getting the Most CL out of Master Spellthief

[Sleven]

Lots to unpack here, so here's my 15 minutes or less on the subject:

I was not aware of your prior work, though I guess one could see this as the PO version of your TO build, as we're actively avoiding an infinite loop situation.

This and a number of other builds are part of a fair amount of TO and PO builds I've sat on for over a decade, only seeing use in actual games. Occasionally I saw a few of the concepts crop up later, but as with most of the community, I'm by no means original.

It is rather unfortunate though, because at the time I had a little more time on my hands and was considering slowly adding a number of other builds of mine from "back in the day" to that thread but was turned off by the moderation's movement of my posts (I like having them all in one place).

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KotW makes its own CL equal to class level plus other arcane CLs, but since it doesn't increase virtual spellcaster level

You have 3 class levels in warlock, 3 in spellthief, and if we are generous and count eldritch theurge as another arcane casting class that's another 3 (if we aren't generous, by the way, it would come from its own CL granting class extension ability -- that does not provide class levels in what you had before.). Magical training does not grant class levels.

I would argue that "spellcaster levels" refer to effective levels of casting progression, so a Wizard 8 and Wizard 3/Master Specialist 5 would have the same spellcaster level of 8 as they both cast as an 8th level Wizard.

etc.

There seems to be this unfounded assumption that spellcaster levels somehow refers to class levels, rather than caster levels. Where does this come from? I don't know. The glossary is not entirely silent on this subject. There are entries for both "caster level" and "spellcaster", neither of which requires a character capable of casting spells/spellcaster to have its caster level equal to their class level in a spellcasting class (see PHB glossary for details). In fact, you will note that when class levels are mentioned, they are worded in a very specific manner, "the spellcaster's class level." Not, "spellcaster level."

Furthermore, I submit the wording of the Master Spellthief feat itself as evidence of its function. First of all, the feat grants three separate effects, thus has three separate sections. They are worded differently, so let's start by separating them and then note the distinctions:

Your spellthief levels stack with levels of other arcane spellcaster classes (that is, levels of any class that grants arcane spellcasting other than the spellthief) for the purpose of determining what level of spell you can steal. For example, a 4th-level spellthief/4th-level wizard could steal spells of up to 4th level, as if he were an 8th-level spellthief.

It deals with levels in classes that grant (or advance) arcane spellcasting and specifically and repeatedly uses the word "classes" or "class" to make sure we understand the distinction. Note the specific use of the words, "levels of other arcane spellcaster classes," and not, "spellcaster levels."

Your spellthief and arcane spellcaster levels also stack when determining your caster level for all arcane spells. The character described above would have a caster level of 8th for both his spellthief spells and his wizard spells.

Here it clearly deviates from the language used in the first portion of the feat, specifying this time "arcane spellcaster levels" and not just levels in an arcane spellcasting class. The PHB glossary's distinct use of the words "spellcaster's class level" under the glossary entry for caster level is important here when assessing the feat's function. The words "class level" need to follow in order for something to be referring to levels in classes--as they do in the first section of the feat--whereas "spellcaster levels" is most likened to "caster levels".

In addition, you do not incur a chance of arcane spell failure for arcane spells cast or stolen from other classes, but only if you are wearing light armor. You incur the normal arcane spell failure chance when wearing medium or heavy armor or when using a shield.

No one argues about what this does, but it's included here for completion to make sure you're all clear on the distinction between each of the feat's abilities.

TLDR: In the feat Master Spellthief use of the words "spellcaster levels" is equivalent to "caster levels", while use of the words "levels of arcane spellcaster classes" is equivalent to "spellcaster's class levels" in a class that grants arcane spellcasting.

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MT grants us a wizard spellbook. With the exception of Conjuration, Evocation and Necromancy, all wizard spells are spellthief spells as well, and therefore eligible for being copied into our spellbook. As spells we have in our spellbook technically count as known [...]

Are you sure you haven't read one of my threads on the subject before? This is even part of the build I originally referenced you to.

There is text in the Rules Compendium which says that spontaneous casting from class A cannot use spells known from class B. This was a big disappointment for Wizard 1/Archivist n with Uncanny Forethought.

Feats like Versatile Spellcaster and Uncanny Forethought don't grant spontaneous spellcasting and are not considered spontaneous spellcasting. They have specific language for their specific functions and specific trumps general. Reading it any other way is trying to convolute the subject. It's best to read the rules in the simplest manner possible.