Selective 5-HT2AR agonism without 5-HT2BR activation is critical for safe psychedelic-inspired therapeutics. Here, we systematically probe the ligand-binding pockets of 5-HT2AR and 5-HT2BR and identify steric and conformational constraints at the side-extended pocket (SEP) and extended binding pocket (EBP) that dictate ligand orientation and receptor signaling. Guided by these insights, we...