The general rule for Contractors is could they hand the work to a 3rd party, though the actual test gets complex. https://www.irs.gov/taxtopics/tc762.html

Basically if you want a sink replaced you can define what the sink will look like and function etc but not which tools they use to fix it. When someone is providing core business functions for a year+ receiving constant directions for short term tasks has direct customer interactions thus representing your business etc they just don't have much wiggle room.