[SolAndLuna]

I don't disagree that the dagger skills also need something, definitely, but at the same time, adjustments like that may be a bit much?

The Flying Dagger one in particular bugs me, since it kind of leads back to the same issues skills like Grenade Launcher have - a 1 momentum skill that consistently adds a fair deal of extra damage with no real counter available. If I understood the way it's written correctly, it would add more Lingering Damage each time it's used, when... well, if set up properly, daggers can reach about 20 Power, so it'd add up to a lot of damage very quickly. If you meant that it would simply be a Lingering Damage status that can't be powered up, then that sounds reasonable enough, but I'm personally still a bit wary of adding statuses like that to something that only costs 1 momentum.

Beyond that, for the Mystic Dagger suggestion... well, being able to hit every enemy, regardless of distance, with a basic attack that also knocks down if the opponent is blinded (something that the Spellthief can get Altera, Divine Shower or even Pocket Sand for pretty easily) sounds like a tad much to me, personally, especially when you're saying they'd appear behind their opponent and daggers like the Dancing Shiv, Vorpal Fang or even stuff like Tsukikage exist. Maybe having it only warp once, to a predictable location (for example, the enemy closest/furthest away from you) sounds fair? Definitely not "I get to attack everyone every round", though, especially when it's not even a main class skill.

I do kind of agree on Goose Bite, though - more sources of Silence are definitely welcome at the moment, and it makes the skill somewhat more usable.

As for this...

Personally, I do think the class needs a lot more attention to the part of it that includes daggers more than it needs buffs to spell use, since that feels tacked on. Not going to suggest my own ideas on how that could be achieved. Just that a lot of the steal skills go unused outside of being used to get an initial roster of pre-determined spells, and a lot of the dagger spells seem underwhelming.

Well, I agree that Spellthief's other skills need some love, too, and admittedly, I only mentioned the copy spells because... well, the class is called "Spellthief", magic is quite literally their main specialty just from the name alone. That being said, at least from my understanding, classes generally shouldn't only be capable of using one specific kind of thing well, and especially not the Rogue classes. Void Assassin can equally be a mage or a dagger build, for example, and Engineer has the ability to summon bots, fly around with Jetpack, toss bombs/grenades, give others the ability to do some of the same things and so on.
As it is now, Spellthief is (technically) capable of being a support class, a dagger build or a mage class, and ideally, I'd rather it was still capable of all of those to some degree. The problem as it is now is that it is far, far more capable of being a support class than any of the other things it is capable of - buffs and debuffs don't need you to use them more than once every few rounds anyway, after all. For a Rogue class that literally has "spell" in the name, that just sounds wrong, personally.

For a dagger-wielding Spellthief, it's pretty clear that the dagger skills, aside from Final Flare/Hot Potato, need some sort of improvement, regardless of whether I agree with some of the written suggestions. As for having Spellthief be a mage class... Sure, it has copy spells to use, and even with a 2 round cooldown, it would generally mean you have at least one attack to use, but the problem is that it won't always be the attack you really want to use. Between resistances/immunities/absorbs making specific spells a bad option, crippled ability to use utility spells (Blink/Swap Position, among others) making certain invocations difficult, spells that lack the range/utility/damage that their original classes can offer and a general inability to repeat things like status inflicting spells the way other mages can, their magic definitely needs a boost of some kind as well.

TLDR - dagger spells definitely need to be adjusted, but I'd still appreciate copy spells being looked at as well.