The most recent version of the Unearthed Arcana Mystic (V3) can be found here.
The way I read it RAW Counterspell has no effect on Psionic Disciplines but this seems extremely over powered and I find it difficult to believe that this is RAI.
Can Counterspell be used to prevent a Mystic from using a Discipline?
you cannot counterspell a discipline, simply because casting/activating a discipline is imperceptible.
It doesn't matter whether a discipline can be regarded as spell or not.
Counterspell requires you to be able to see the caster casting a spell:
which you take when you see a creature within 60 feet of you casting a spell
which you cannot do against a Mystic:
Disciplines don't require the components that many spells require. Using a discipline requires no spoken words, gestures, or materials.
Counterspell states that:
You attempt to interrupt a creature in the process of casting a spell
This costs one reaction and further clarify's exactly when a reaction can be used to cast Counterspell:
which you take when you see a creature within 60 feet of you casting a spell
This raises a couple of questions:
The Mystic V3 UA-Mystic pdf offers some help here but is far from conclusive and arguably contradictory. In regards to the first question, it states the following at various different points in the document:
Psionics is a special form of magic use, distinct from spellcasting
This would seem to suggest that using a Mystic Discipline is not the same as casting a spell.
Psionic Disciplines are magical and function similarly to spells
This statement suggests that Mystic disciplines and spells function similarly and that using a Discipline may well count as 'casting a spell'
Psionics and spells are separate effects, and therefore their benefits and drawbacks overlap. A psionic effect that reproduces a spell is an exception to this rule.
This statement suggests that using a psionic discipline may count as 'casting a spell' if it is a psionic effect that reproduces a spell.
I find this statement particularly unhelpful as nowhere in the document does it clarify what Mystic Disciplines duplicate spell effects or how similar the two have to be for this to apply.
Some disciplines are very similar to existing spells in 5e, others are not and some, while similar in some aspects, differ considerably in others. For example take the Immortal discipline 'Bestial Form' - Flight:
You transform your body, gaining traits of different beasts. Wings sprout from your back. You gain a flying speed equal to your walking speed.
when compared to the transmutation spell fly:
You touch a willing creature. The target gains a flying speed of 60 feet for the duration. When the spell ends, the target falls if it is still aloft, unless it can stop the fall.
Both abilities allow the target to fly so you can argue that this duplicates the effect of a fly spell. However, the Mystic Discipline does not require concentration, last an hour instead of 10 mins, and in most cases will give you a fly speed of 30 ft rather then 60 ft, making it far from clear cut.
There are numerous other examples like Occluded Mind from the Telepathic Contact Discipline and zone of truth, which have similar effects but certainly not an exact duplicate.
My approach would be to use quite broad brush as to when a psionic discipline 'duplicates' a spell effect to catch as many disciplines as possible in this definition. That said we still have the issue raised in question 2 above to address which makes all of the above something of a mute argument.
As stated above you can use a reaction to cast Counterspell when you see a creature within 60 feet of you casting a spell.
Even assuming the Mystic Discipline counts as casting a spell it would still not be possible for a normal caster to counterspell it because Mystic disciplines don’t require the components that many spells require.
Using discipline requires no spoken words, gestures, or materials. The power of psionics comes from the mind.'
RAW this means a mystic using a discipline shows no outward sign that they are doing so, so there is nothing for the person casting Counterspell to see that can trigger the reaction needed to cast Counterspell.
So RAW Counterspell cannot be used on Psionic Disciplines.
A possible solution to this problem is to include the following revisions/homebrew to the UA Mystic (I did not come up with these see below).
All use of Discipline has to be expressed though some sort of somatic component, these maybe straight forward and simple, such as touching a finger to your fore-head, thrusting out your hand for a blast, or a suggestive wave of your fingers.
Discipline use can be done covertly, but are NOT completely covert. You may roll a Deception check to perform somatic components covertly, with a DC equal to 10+the Psi spent, to either minimize the needed somatic component, or to covertly disguise your somatic component as casual gestures of expression.
Alternatively, you may spend 2 additional Psi on a discipline effect to completely negate the need for this gestural component. Your Psi limit applies to this additional expenditure. Sort of like Subtle Spell.
(I saw these suggestions on a mystic reddit post but cannot give credit as I can't now find the post).
