Disclosed herein are means for the detection and characterization of neurotoxins such as botulinum neurotoxin (BoNT) or tetanus neurotoxin. The present disclosure provides methods for determining potency and activity of neurotoxins in vitro and in vivo. Also disclosed are polypeptides comprising N- and C-terminal fragments of a reporter protein that are split by a linker comprising a neurotoxin cleavage site. Cleavage of the linker by a neurotoxin decreases reporter protein activity, thereby indicating activity of the neurotoxin. Compositions and kits comprising the disclosed polypeptides, nucleic acids comprising nucleotide sequences encoding such polypeptides, and cells expressing such polypeptides are also disclosed.