Every trade on fix pays one fee. Its rate is written into the coin at creation: one rate while the coin trades on its curve and one in its pool.
It is the same for everyone. The creator buying their own coin pays it, a follower of a call pays it, and anyone else pays it, at the same rate.
The fee has four lines: the protocol's, the creator's, a share that stays in the pool, and a call slice.
The protocol and pool lines round down and the creator receives the remainder, so the four lines always add up to the fee exactly.
Following a call never raises what you pay. The call slice comes out of the protocol's line.
costs nobody sees
Flashbots described extraction on chain as a cost that lands on users who rarely see it, and argued that the first step is to make it visible.
Paradigm's MEV and Me called the worst of it an invisible tax on the person trading, and pointed out that an application can reduce the extraction it creates.
fix can make the fee one number because it controls the whole trade: the curve and the pool are its own. A launchpad on another platform's pool adds its fee on top of that pool's fee, under that pool's keys.
On fix the fee is one number, the same for every route into the program, and every line of it is shown.
what you see
A quote shows the whole trade before you sign: the total you pay or receive, the fee and each of its lines, the price before and after, the price impact, and what the resulting position would sell for right now.
Every quote carries a bound, a minimum out or a maximum in, and the program enforces it.
The quote is the execution: the amount you are shown is the amount that moves.
Every trade, claim and graduation emits a receipt with each line on it, readable from the transaction itself.
Account rent is shown apart from the fee, together with how it comes back. A follower's position returns its rent when it settles.
what you hold
Positions are shown at exit value: what selling the whole position now would pay, after the fee and the price impact of that size.
Vitalik Buterin's tests for a credibly neutral mechanism ask that anyone can verify it and that it stays simple. One fee, four lines and a receipt for every trade is how fix meets both.