I think that is too crazy. Most cantrip workshops have a downside like being delayed (Cobbler), being conditional (Sculptor, Falconer) or being in a split pile (Hill Fort).
Unlike Hill Fort, this is immediately available and the self-gaining makes this far too good. I mean, which game in which this is in will not end in three-piling?
A game in which there aren't any Kingdom cards at $4 or less worth piling out. If it's all weak trashers and terminal Silver, those aren't the kind of cards that you want to fill your deck with.
I'd also point out that, even without SC's autogain and Hill Fort's gain-to-hand, HF would be a better card, because it's cantrip happens after the gaining, meaning that if you trigger the shuffle your new card doesn't miss it.
An empty deck/discard with a Sewing Circle and at least one other cantrip in hand can empty the pile instantly which I'm never a fan of. That said, I still think it's a neat idea. The card it reminds me most of is actually Groom, which can also cantrip gain, can gain and play copies of itself (it loops pretty easily with +Actions, Groom+Horse can gain Groom+Horse, etc.), usually ends games in 3 piles, and it only costs $4. The biggest issue is that it's way easier to multiply these that to multiply Grooms since you don't need the +Actions. I wonder if adding a "(if you haven't gained one this turn)" after the "gain a Sewing Circle" might be enough to mitigate the autopiling issues.
I'm not sure that's correct. Because SC gains after it draws, there has to be one waiting in the discard pile when you play another. Otherwise, you could pay a SC (drawing nothing because your deck/discard is empty) to gain a SC, then play the other cantrip to draw the 2nd SC, but when you played that, it wouldn't draw anything, and the other SC would be stuck in your discard. To autopile, you'd either need 2 SC in your hand or 1 in hand and there other one to be the only card in your discard/deck. (Rats works exactly the same way, but it also trashes cards). There's a fairly big difference between needing 2 of them and needing just 1, but there is still the possibility of autopiling. (You can also simulate having 2 SCs with a throne variant, and you can accomplish the chaining by playing a card that lets you topdeck on-gain [Tracker, WotSeal]).
I hadn't thought of the autopiling issue. I may have to come up with something to deal with that.