From what I am reading of the Spellthief, he only ever gets 3 spell slots. These spell slot are always 1st level until the spell thief reaches 7th level as a rogue. His spell slots are now 2nd level, just like a warlock's. At 13th level they become 3rd level and at 19th they peak out at 4th level slots.
Quote:
~ Max Spell Levels Array
focus.field[cArrKnLev].arrayvalue[2] += 1
focus.field[cArrKnLev].arrayvalue[6] += 2
focus.field[cArrKnLev].arrayvalue[12] += 3
focus.field[cArrKnLev].arrayvalue[18] += 4
|
The above sets what level a spellthief's slots are and when they hit those levels.
Also, just like a warlock, you only ever have 2-3 spell slots based on you rogue level. You do not get spell slots like a sorcerer according to what I am seeing.
From what I see the spellthief is an Arcane trickster who uses sorcerer spells and a modified warlock's spell slot progression.