Dragon Caster Level + PrC

[Vetril]

Hi, I'm statting out a dragon advanced to 35 HD and I can't find anywhere how the dragon innate sorcerer spellcasting interacts with prestige classes. Let's say the dragon enters an arcane PrC by virtue of "being able to cast 3rd level arcane spells" through his dragon racial HD. How does the PrC stack with them?
Should I treat the dragon as being a sorcerer of X level (a mature black dragon being the equivalent of a 5th level sorcerer, for example), and thus make it gain +1 spell level for each 2 additional spellcasting levels gained through the PrC? Following this logic, a mature adult black dragon (Dragon 22/Full CL Prc 9) would cast as a level 14 sorcerer; however, this speeds up the spellcasting progression of the dragon: a 31 HD ancient black dragon only has a sorcerer CL of 11.
Or should I add the PrC levels to the base racial hit dice (assuming the "spellcasting class" is Dragon), and look up the equivalent caster level for a dragon of the resulting level? Following the previous example, the mature adult black dragon (Dragon 22/Full CL Prc 9) would count as a 22+9 = 31 HD dragon, and thus have CL 11.

This problem stems from the fact that I am statting out a Fang Dragon (Dragon 21/Dracolexi 10/Legendary Dreadnaught 4), and I am trying to figure out what spell level it has access to.

[FocusWolf413]

If it's devoting HD to a prc, I'd progress its spllcasting as a sorcerer if it's a full spellcasting prc. Their racial abilities don't progress at all, so it's a fair trade.

I think. Maybe.

[Khatoblepas]

You just add on extra spellcasting levels when the PrC tells you to, you don't follow the HD-Caster Level thing from the racial hit die. If the dragon has 5th level sorcerer casting from HD, and you add 10 levels of a full casting PrC onto it, it now has 15th level casting.

A monster's racial casting always stacks with classes that also grant levels in the casting class they cast as.

[Vetril]

Thanks, now I can complete the stat block!

[atemu1234]

Thanks, now I can complete the stat block!

What're you designing?

[ShurikVch]

Do you know about the Dragon Mystic PrC from Dragon #296?

It's fairly easy to get in (Dragon type, 3rd-level arcane spells, and 5000 gp in diamonds);
PrC is only 5 levels long, and give:
full spellcasting progression;
+1d6 spell damage (any spell);
+2 Cha;
+2 DC to arcane spells from the Enchantment school or an arcane spell that inflicts the same type of energy damage as your damaging breath weapon;
and capstone: Metamagic Breath Weapon - if you have access to Empower/Enlarge/Maximize/Quicken metamagic, you can add it's effect to your breath weapon (rules are like for metabreath feats from Draconomicon)

[WhamBamSam]

RAI you're supposed to be able to advance Dragon casting with PrCs as if you were advancing Sorcerer. So an Adult Red Dragon/Abjurant Champ 1 should cast as a Sorcerer 8. That's how the example dragons with PrC levels (of which there are several in the various books) work.

RAW, you require one level in the actual Sorcerer class before you can advance the Dragon's casting with PrCs because the rules for spellcasting monsters are as follows.
A spellcasting creature is not actually a member of a class unless its entry says so, and it does not gain any class abilities. A creature with access to cleric spells must prepare them in the normal manner and receives domain spells if noted, but it does not receive domain granted powers unless it has at least one level in the cleric class.But spellcasting PrCs use the following language.
At each level, you gain new spells per day and an increase in caster level (and spells known, if applicable) as if you had also gained a level in an arcane spellcasting class to which you belonged before adding the prestige class level.Emphasis mine.

On the bright side, innate casting and casting from class levels do explicitly stack per this line of text.
A spellcasting class is an associated class for a creature that already has the ability to cast spells as a character of the class in question, since the monster’s levels in the spellcasting class stack with its innate spellcasting ability.