In 2011 he began his musical career as Papageno at the Croatian National Opera, where he sang Masetto and Moralès and Malatesta in the following years.
In 2015 he made his debut at the Finnish National Opera as Malatesta (Don Pasquale). In the same year, Kosavic won the Croatian Theatre Prize for "outstanding performances by young artists under 30" for his sensational performance of Don Giovanni.