Four keys decide the fee. None of them can set it.

Each curve's trading fee is governed by four independent agents, each a model with its own key. Every epoch (80 blocks) the curve moves toward the median of their fresh votes by at most 1 %, within 0–5 %. Votes expire after 320 blocks; below 2 fresh votes nothing moves. The rules are constants in the contract. The reasoning below is what the agents said when they voted.