If you are invisible and you cast a spell to counterspell does it make you visible? What if you counterspell an on-going effect?
Based on the three noted sections of the description of the Invisibility spell, I'd have to say the answer is no for both cases. Targeting an effect generated by or an area containing a foe is not an attack.
The spell ends if the subject attacks any creature. For purposes of this spell, an attack includes any spell targeting a foe or whose area or effect includes a foe. Exactly who is a foe depends on the invisible character's perceptions. Actions directed at unattended objects do not break the spell. Causing harm indirectly is not an attack. Thus, an invisible being can open doors, talk, eat, climb stairs, summon monsters and have them attack, cut the ropes holding a rope bridge while enemies are on the bridge, remotely trigger traps, open a portcullis to release attack dogs, and so forth. If the subject attacks directly, however, it immediately becomes visible along with all its gear. Spells such as bless that specifically affect allies but not foes are not attacks for this purpose, even when they include foes in their area.
So if where to dispel an effect that is already existing WOULD make you visible because it is an attack on an enemy? (e.g. Dispelling Mirror Image on an ememy caster that had already cast is before your turn)
So if where to dispel an effect that is already existing WOULD make you visible because it is an attack on an enemy? (e.g. Dispelling Mirror Image on an ememy caster that had already cast is before your turn)
Pretty much... but you can dispel something that is an area effect NOT cast on a target without breaking invis... so you could dispel a Darkness spell for example, and stay invisible.
Dispelling a buff on a target is definitely a hostile act.
Based on the three noted sections of the description of the Invisibility spell, I'd have to say the answer is no for both cases. Targeting an effect generated by or an area containing a foe is not an attack.
The spell ends if the subject attacks any creature. For purposes of this spell, an attack includes any spell targeting a foe or whose area or effect includes a foe.
Italics highlighted.
That covers Fireball area spells, and should also include Targetted Dispells since their effect includes the foe (negating effect currently including the foe).