A long snake, G. jansenii may attain a total length of 199 cm (6.53 ft), which includes a tail length of 45 cm (1.48 ft). Adults are olive or yellowish brown on the anterior and middle portions of the body, with some scales black-edged. They are entirely black on the posterior portion of the body and tail (Boulenger, 1894).
Bleeker P (1858). "Gonyosoma Jansenii Blkr, eene nieuwe slang van Manado". Natuurkundig Tijdschrift voor Nederlandsch Indië16: 242. (Gonyosoma jansenii, new species, p. 242). (in Dutch).
Boulenger GA (1894). Catalogue of the Snakes in the British Museum (Natural History). Volume II., Containing the Conclusion of the Colubridæ Aglyphæ. London: Trustees of the British Museum (Natural History). (Taylor and Francis, printers). xi + 382 pp. + Plates I–XX. (Coluber janseni, pp. 57–58 + Plate I, figure 2, dorsal and lateral views of head).
Dowling HG (1958). "A Taxonomic Study of the Ratsnakes VI. Validation of the Genera GonyosomaWagler and ElapheFitzinger". Copeia1958 (1): 29–40.
Koch A (2012). Discovery, Diversity, and Distribution of the Amphibians and Reptiles of Sulawesi and its offshore islands. Frankfurt am Main, Germany: Edition Chimaira. 374 pp. ISBN978-3-89973-432-4.