API Copyrights Revisited

Are application program interfaces (APIs) "original expression" that copyright law protects from unauthorized implementations in computer program code? Or are they too functional to be within the scope of protection that copyright law provides to computer programs? Alternatively, should it be fair use for unauthorized persons to reimplement APIs in independently written code?

Between 1992 and 2014, federal appellate courts in the U.S. were generally in agreement that interfaces necessary to achieving compatibility among programs were unprotectable by copyright law. These rulings made it unnecessary for courts to address fair use defenses for reuses of APIs.