2008-04-01 : If the controller of the original spell chose to pay an alternative cost (such as prowl) or an additional cost, that choice is also copied.

2008-04-01 : If the spell is modal (that is, it says “choose one —” or “choose two —”), the choice of mode(s) can’t be changed.

2008-04-01 : Sigil Tracer’s ability can copy an instant or sorcery spell that isn’t targeted.

2008-04-01 : Since the activated ability doesn’t have a tap symbol in its cost, you can tap creatures (including Sigil Tracer itself) that haven’t been under your control since your most recent turn began to pay the cost.