Here the main question is whether we should make it actual hard rule, or just to fix AI not to do it.
Well, for S3_0 we shouldn't be adjusting rules any more, so there the answer is obviously fixing the AI. S2_6 I don't even consider at this point. The question remains for later branches (at least master).
I've got some kind of patch for this, but it's not (yet) handling multiple advance clauses in the same treaty *together* causing the tech loss.
I'm testing v2.6.2 with the following server settings:
I think these setting are slowing down the AI turn, because AIs (playing on the same team) are continually trading all techs, losing them, and trading them all again every turn.
If diplbulbcost > 0, I'd suggest to forbid tech trading when the bulb count of the player is already negative. Or alternatively forbid it when the bulb count is going to cause techloss already (for being lower than techlossforgiveness). With my suggestion, I think these settings could be used to effectively limit the benefits of tech trading in multiplayer games too.