fix(relayer): correct recommended processing fees layers (#21252)

Co-authored-by: Matus Kysel <MatusKysel@users.noreply.github.com>
Co-authored-by: David <david@taiko.xyz>

Author: 3 people

packages/relayer/pkg/http/get_recommended_processing_fees.go

@@ -115,15 +115,15 @@ func (srv *Server) GetRecommendedProcessingFees(c echo.Context) error {
 	for _, f := range feeTypes {
 		fees = append(fees, fee{
 			Type:        f.String(),
-			Amount:      srv.getCost(uint64(f), destGasTipCap, destBaseFee, Layer1).String(),
-			DestChainID: srcChainID.Uint64(),
+			Amount:      srv.getCost(uint64(f), destGasTipCap, destBaseFee, Layer2).String(),
+			DestChainID: destChainID.Uint64(),
 			GasLimit:    strconv.Itoa(int(f)),
 		})
 
 		fees = append(fees, fee{
 			Type:        f.String(),
-			Amount:      srv.getCost(uint64(f), srcGasTipCap, srcBaseFee, Layer2).String(),
-			DestChainID: destChainID.Uint64(),
+			Amount:      srv.getCost(uint64(f), srcGasTipCap, srcBaseFee, Layer1).String(),
+			DestChainID: srcChainID.Uint64(),
 			GasLimit:    strconv.Itoa(int(f)),
 		})
 	}

packages/relayer/pkg/http/get_recommended_processing_fees_test.go

@@ -0,0 +1,36 @@
+package http
+
+import (
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetCost_LayerBehaviour(t *testing.T) {
+	srv := &Server{processingFeeMultiplier: 2.5}
+
+	gasLimit := uint64(10)
+	gasTipCap := big.NewInt(1)
+	baseFee := big.NewInt(2)
+
+	// Calculate base cost used by both branches: gasLimit * (gasTipCap + baseFee*2).
+	baseCost := new(big.Int).Mul(
+		new(big.Int).SetUint64(gasLimit),
+		new(big.Int).Add(gasTipCap, new(big.Int).Mul(baseFee, big.NewInt(2))),
+	)
+
+	gotLayer2 := srv.getCost(gasLimit, gasTipCap, baseFee, Layer2)
+	gotLayer1 := srv.getCost(gasLimit, gasTipCap, baseFee, Layer1)
+
+	// For Layer2 we expect raw base cost without processingFeeMultiplier.
+	assert.Equal(t, baseCost, gotLayer2)
+
+	// For Layer1 we expect base cost multiplied by processingFeeMultiplier.
+	costRat := new(big.Rat).SetInt(baseCost)
+	multiplierRat := new(big.Rat).SetFloat64(srv.processingFeeMultiplier)
+	costRat.Mul(costRat, multiplierRat)
+	expectedLayer1 := new(big.Int).Div(costRat.Num(), costRat.Denom())
+
+	assert.Equal(t, expectedLayer1, gotLayer1)
+}