fix(sdk-coin-flrp): pad base fee for Flare atomic exports

modules/sdk-coin-flrp/src/lib/ExportInCTxBuilder.ts

@@ -13,6 +13,13 @@ import {
 import utils from './utils';
 import { Tx, FlareTransactionType, ExportEVMOptions, DecodedUtxoObj } from './iface';
 
+/**
+ * Buffer applied to a caller-supplied base fee to absorb C-chain base-fee volatility
+ * between transaction signing and broadcast. Expressed in basis points (1000 = 10%).
+ * Mirrors the padding applied to Flare atomic imports (CECHO-1821).
+ */
+const BASE_FEE_PADDING_BPS = 1000n;
+
 export class ExportInCTxBuilder extends AtomicInCTransactionBuilder {
   private _nonce: bigint;
 
@@ -147,7 +154,13 @@ export class ExportInCTxBuilder extends AtomicInCTransactionBuilder {
       throw new BuildTransactionError('context is required');
     }
 
-    const fee = BigInt(this.transaction._fee.fee);
+    // The supplied fee is a C-chain base fee (per gas), not a literal total fee: flarejs's
+    // newExportTxFromBaseFee derives the actual export cost from the real tx size/inputs.
+    // Pad it to absorb base-fee volatility between signing and broadcast, mirroring the
+    // padding applied to Flare atomic imports (CECHO-1821), and fixing insufficient-funds
+    // broadcast failures caused by an unpadded base fee (CECHO-2004).
+    const suppliedBaseFee = BigInt(this.transaction._fee.fee);
+    const fee = (suppliedBaseFee * (10000n + BASE_FEE_PADDING_BPS)) / 10000n;
     const fromAddressBytes = this.transaction._fromAddresses[0];
     const sortedToAddresses = [...this.transaction._to].sort((a, b) => {
       const aHex = Buffer.from(a).toString('hex');

modules/sdk-coin-flrp/test/unit/lib/exportInCTxBuilder.ts

@@ -179,6 +179,37 @@ describe('ExportInCTxBuilder', function () {
         });
     });
 
+    it('should pad the supplied base fee by 10% to absorb C-chain base-fee volatility (CECHO-2004)', async () => {
+      const buildWithFee = async (fee: string) => {
+        const tx = await factory
+          .getExportInCBuilder()
+          .fromPubKey(testData.cHexAddress)
+          .nonce(testData.nonce)
+          .amount(testData.amount)
+          .threshold(testData.threshold)
+          .locktime(testData.locktime)
+          .to(testData.pAddresses)
+          .fee(fee)
+          .context(CONTEXT as FlrpContext)
+          .build();
+        const json = tx.toJson();
+        return BigInt(json.inputs[0].value) - BigInt(json.outputs[0].value);
+      };
+
+      const suppliedBaseFee = BigInt(testData.fee);
+      const actualFee = await buildWithFee(testData.fee);
+      // actualFee = paddedBaseFee * gasUnits, where paddedBaseFee = suppliedBaseFee * 1.10.
+      // gasUnits is recovered by dividing by the padded base fee and must be a whole number.
+      const paddedBaseFee = (suppliedBaseFee * 11000n) / 10000n;
+      const gasUnits = actualFee / paddedBaseFee;
+      const expectedFee = paddedBaseFee * gasUnits;
+      actualFee.should.equal(expectedFee);
+
+      // Sanity check the padding is actually non-trivial (not silently a no-op).
+      const unpaddedEquivalentFee = suppliedBaseFee * gasUnits;
+      (actualFee > unpaddedEquivalentFee).should.be.true();
+    });
+
     it('should compute correct tx id for on-chain verified signed export', async () => {
       const signedExportHex =
         '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';