I think I have seen this somewhere before. Probably in one of my caterpillar books. They are currently packed up in boxes. The internet takes me to Monarch and Black Swallowtail with my search terms. Found it on the ground while weeding, so I don’t know what plant it was on. I found a patch with the same kind of weeds. Later on, it looked like it was eating Toadflax, but I only found Buckeye as the caterpillars supposed to be eating that. It was a good inch long.