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Here is the board itself. The system prompt is pinned at the top, and each time you press send a message, a new turn drops onto the stack below it.
Keep pressing. For a while the board has room and every turn stays put. Then it fills. From that point on, every new message you add pushes the oldest one off the top. It does not shrink, it does not get summarised, it is simply gone: the model will not see it on the next reply.
Watch what slides off. Early on you told it "keep answers short." A few turns later that instruction is over the edge, and the model has no way to know it was ever there.