Please correct the text, because i am a non native speaker.
Wording stuff:
+2 CardsEach other player gains a Copper.
You may trash a card from your hand, for
+1 Card and
+1 Action.
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When you draw a card during another player's turn, you may discard this and any number of other cards from your hand, then draw that many.
I removed the non-action part, is there a reason to limit it to discarding "non-action cards"? You will almost never want to discard action cards anyway; so it would almost always be used the same way anyway without that extra wording. It only really makes a difference in games with Ruins. I also removed the "have to draw" part, because I think that causes confusion about when that applies... if your opponent plays Vault, and you discard 2 cards, you then have to draw a card. But you also could have chosen not to discard 2 cards, so does "have to draw" apply here?
But I think that reaction wording actually has issues... you discard all the cards at the same time, and you don't have to reveal Counter Magic if it's not the one you put on top of your discard. So for accountability, you would need some really awkward wording... "when you draw a card on another player's turn, you may discard this from your hand. If you do, discard any number of cards, then draw that many, then +1 card."
I don't understand the point of the reaction as a whole though, I think you should just leave it off. It is very rare that you draw a card during another player's turn; so this will do nothing in the vast majority of games. There's only a very small handful of cards that make this happen. And even when it does happen, I think you would rather have this card in your hand for its main ability than you would want to discard it for a Cellar effect.
*Edit* I think there are only a total of 7 cards in Dominion that could cause this reaction to be used... Council Room, Legionary, Lost City, Governor, Margrave, Minion, and Soothsayer.
For the main effect... as a general rule, pure Copper junking like this is bad, because there are a huge number of Copper cards, and that number also differs per game, based on both player count, and whether or not players have chosen to combine treasures from 2 base sets (which the rules say you can do, and should do for higher player-counts). It seems like this should just give Curses instead... I'm not even sure if that would be stronger or weaker, due to the fact that Curses will run out.
The card is also super powerful. Of course Potion-costing cards generally are... but this is way over powered. Look at how strong Masquerade is, just due to having +2 cards, trash a card. This is +3 cards, +1 action, trash a card. That's insane. AND it is a junking attack on top of that. Compare this to Familiar... +1 card, +1 action, junk opponents. This is Familiar, with an extra +2 cards and trashing on top!