Encapsulation of a cationic carbodiimide condensing agent within a self-assembled hexameric capsule made of resorcin[4]arene units provides a nano-environment that efficiently steers the substrate selectivity in the amide synthesis reaction between carboxylic acids and primary amines. While in solution pairs of acids react similarly with a given amine, in the presence of the capsule the formation of the shorter amide is greatly favored.