Fees and quotes

A Quote is the engine's answer to "if I send X to Y, what happens?". It's pure data until the user clicks confirm, no transaction is submitted. You can build a quote, throw it away, build another one, do whatever you want with it before the user commits.

What a Quote contains

type Quote = {
  route: Route; // same-chain "send" or cross-chain "bridge"
  sourceChain: Chain;
  destinationChain: Chain;
  amountIn: string; // what the user types
  amountOut: string; // what the recipient receives
  fees: FeeBreakdown; // every line item
  expiresAt: number; // ms-epoch, quotes get stale
  recipient: ResolvedRecipient;
  token: Token;
};

amountIn - amountOut === sumFees(fees). Whisk never sneaks in a fee that isn't enumerated in the breakdown. If the math doesn't add up, that's a bug — file it.

Where fees come from

Two sources contribute:

• App Kit fees. Gas, the bridge protocol fee, the forwarder fee when applicable. These come from Circle's quote API; Whisk passes them through.
• Your platform fee. Optional. If you've configured a feePolicy on the config, Whisk adds your line item and (if you've set a recipient) routes the cut to your treasury.

The review step renders each line so the user can see what they're paying before they sign.

Adding a platform fee

Pass feePolicy to createWhiskConfig:

const config = createWhiskConfig({
  wallets: [evm({ projectId })],
  chains: ["Base"],
  feePolicy: {
    flat: "0.05", // 5¢ flat
    percent: 0.001, // plus 0.1%
    recipient: "0xYourTreasury…", // fee wallet
    label: "Platform fee", // shown in the review UI
  },
});

Every field is optional. Mix flat + percent, leave recipient off to display the fee without collecting (useful when you're staging a rollout), or skip feePolicy entirely if you don't charge.

Building a quote without rendering

Want a fee preview on a checkout summary before the widget mounts? Call the engine directly. The engine doesn't render, so this works on the server or in a button click handler:

const quote = await engine.quote({
  recipient,
  amount: "100",
  sourceChain: "Base",
  adapter,
});

console.log(quote.amountOut); // "99.92"
console.log(quote.fees);
// → [
//     { kind: "gas",      label: "Gas",          amount: "0.04"  },
//     { kind: "platform", label: "Platform fee", amount: "0.04"  },
//   ]

Stale quotes

expiresAt is in milliseconds since epoch. Quotes go stale fast because gas prices and bridge availability shift. If a user lingers on a confirmation screen for a few minutes, re-quote before they sign — engine.quote(...) is idempotent, and the new quote will have a fresh expiresAt.

The bundled <WhiskSend> handles this automatically. The check matters when you're driving the engine yourself.

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