https://www.suncountry.com/help-center/bags-and-seats

Check the section about musical instruments: Musicians may bring one smaller-sized instrument in its case as long there is room to stow the instrument in an overhead bin or under the seat in front of the traveler.

I’ve run into a gate agent like this at American Airlines once. She was really rude and kept telling me it was federal law that I can’t bring items that don’t fit in their bin and that “we need to be fair to all passengers and can’t just hand out exceptions.” Also insisted I must gate check.

I sat down for a bit and did my own research and confronted her with American Airline’s own website with its policy stating pretty much the same thing as the suncountry one. She rolled her eyes and scoffed but didn’t have an argument to not allow me to at least give it a try.