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Thanks a bunch! I just finished Heretic Episode 1 with a couple of friends and it worked perfectly (I was expecting out-of-sync messages but there were none). By the way, you've released a version of zdoom that doesn't spawn extra weapons in co-op. I assume you changed this bit in p_mobj.cpp: // [RH ...

I'd love to try this out, but the links aren't working. I looked for a solution for proper heretic co-op (i.e. without the extra stuff in levels) years ago but couldn't find one. This seems to be it. And it's source-port-agnostic!

PS. the other posters seem slightly confused