Fees

​

The formula

fee = k × P × (1 − P) × outcomeTokens

• P is the execution price, 0 < P < 1.
• k is a period-specific coefficient.
• outcomeTokens is the number of outcome tokens traded.
• The curve peaks at P = 0.5 (max uncertainty) and decays to 0 at the extremes.
• Both sides of every fill pay this fee. CTFExchange._chargeFee fires on each leg of _matchOrders / _fillFacingExchange — the same feeRateBps value the user signed into the EIP-712 Order is deducted from the asset they’re receiving (collateral on SELL, outcome tokens on BUY). Earlier docs claimed “maker fee is always 0”; that matched a transitional MVP path where signed orders carried feeRateBps = 0. As of the on-chain-fees rollout, every signed Order carries the live feeTakerBps and the contract takes the fee on both legs.

​

Why quadratic (not linear)?

​

Three fee surfaces — which one to use

​

Charging direction

• Seller (whichever side is sending outcome tokens for collateral): fee deducted from collateral proceeds. fee_collateral = k × P × (1−P) × outcomeTokens.
• Buyer (whichever side is sending collateral for outcome tokens): fee deducted in outcome tokens. fee_tokens = k × (1−P) × outcomeTokens.

​

How fees appear in the response

{
  "id": "...",
  "price": "0.52",
  "quantity": "100",
  "fee": "0.2496",
  "side": "buy"
}

​

On-chain equivalence

​

Fee cap

​

Examples

// BUY 100 YES at P=0.52 with k=0.04 (feeRateBps = 400)
const price = 0.52;
const quantity = 100;
const k = 0.04;
const feeTokens = k * (1 - price) * quantity;           // 1.92 YES tokens
const feeUsd    = k * price * (1 - price) * quantity;   // 1.00 USDC equiv

// SELL 100 YES at P=0.80 with k=0.014 (feeRateBps = 140)
const feeUsd = 0.014 * 0.80 * 0.20 * 100;               // 0.224 USDC

​

Rebates