fix: select the fast-transfer fee tier explicitly instead of by array order

[bars26]

What

getBufferedFastTransferFee in src/hooks/use-cross-chain-transfer.ts reads the fee from the first element of the response:

const feePayload = (await response.json()) as FastTransferFeeResponse[];
const feeEntry = feePayload[0];

The /v2/burn/USDC/fees/{src}/{dst} endpoint returns one entry per finality tier, e.g.:

[{ "finalityThreshold": 1000, "minimumFee": 1 }, { "finalityThreshold": 2000, "minimumFee": 0 }]

getBufferedFastTransferFee is only called for fast transfers, which burn with minFinalityThreshold = FAST_FINALITY_THRESHOLD (1000). Taking [0] assumes the fast tier is always first. It currently is, but that ordering is not documented. If the standard tier (minimumFee: 0) is returned first, a fast transfer computes maxFee = 0 and the on-chain depositForBurn reverts, because a fast transfer's maxFee must cover the tier's minimum fee.

FastTransferFeeResponse also does not model finalityThreshold, so there is nothing to select on today.

Fix

• Add finalityThreshold to FastTransferFeeResponse.
• Pick the entry for the tier actually being burned (finalityThreshold <= FAST_FINALITY_THRESHOLD), falling back to feePayload[0] so behaviour is unchanged when nothing matches.

 interface FastTransferFeeResponse {
+  finalityThreshold: number;
   minimumFee: number | string;
 }

-    const feeEntry = feePayload[0];
+    // Select the fee for the tier we actually burn at (fast transfers pass
+    // minFinalityThreshold = FAST_FINALITY_THRESHOLD) rather than trusting the
+    // order of the response array.
+    const feeEntry =
+      feePayload.find(
+        (entry) => entry.finalityThreshold <= FAST_FINALITY_THRESHOLD,
+      ) ?? feePayload[0];

One file. No dependency, ABI, or API changes.

🤖 Generated with Claude Code