The instinct is to blame the model. But the same model, used interactively across an equally long session, usually doesn't fail this way nearly as often. The difference isn't capability, it's that someone was in the loop, implicitly signaling "that part's done, move on." An unattended agent has no equivalent. Nothing marks a finished subtask as closed, so it stays active in context at the same rate the task grows, until a meaningful share of what the agent's weighing at any moment is work that's already done.