Online play with rollback netcode requires a 1.02 ISO to function correctly.
If you're looking for the actual file, search engines won't help much – instead, visit the Slippi or Project Slippi documentation, or look up "Melee 1.02 ISO redump hash" to verify a copy you already have access to. super smash bros melee 102 iso top
Unlike versions 1.00 and 1.01, which contain various game-breaking freezing bugs, 1.02 corrected many of these issues, making it the most stable for long-form bracket play. Online play with rollback netcode requires a 1