SetsulTwiggyAlso the GGPO document mentions that games have to be 'deterministic' in order to use it. Is source engine deterministic?With random spread disabled, probably.
The point is anything that involves a random number generator would be a problem since everyone would get different results.
You can treat rng in the same way as player inputs (which you can't predict), and the whole problem disappears.. even better if you treat the rng seeding as player input. Though it might be technically easier to synchronize the rng by having the server pre-determine the random results before they're needed, (ex. "the next time this scout fires the scattergun the bullets will come out in this pattern") and sending every client the info.
A much bigger problem may be that small hitches in the game's performance seem to have small effects on things like player trajectories. normal rng works deterministically and can be 'predicted', but predicting exactly how the engine interacts with every player's pc might be a nightmare