Add pre-confirmation trade fee estimate

src/components/common/NetworkFeeHint.tsx

@@ -3,20 +3,29 @@ import { Zap } from 'lucide-react';
 
 interface NetworkFeeHintProps {
 	fee?: string;
+	label?: string;
 	className?: string;
 	variant?: 'chip' | 'text';
 }
 
 const NetworkFeeHint = ({
 	fee = '~0.0001 ETH',
+	label = 'Network fee',
 	className,
 	variant = 'chip',
 }: NetworkFeeHintProps) => {
 	if (variant === 'text') {
 		return (
-			<div className={cn('flex items-center gap-1.5 text-xs text-white/40', className)}>
+			<div
+				className={cn(
+					'flex items-center gap-1.5 text-xs text-white/40',
+					className
+				)}
+			>
 				<Zap className="size-3 text-amber-500/50" />
-				<span>Network fee: {fee}</span>
+				<span>
+					{label}: {fee}
+				</span>
 			</div>
 		);
 	}

src/components/common/TradeDialog.tsx

@@ -15,7 +15,11 @@ import { formatDisplayKeyPrice } from '@/utils/keyPriceDisplay.utils';
 import PercentageBadge from '@/components/common/PercentageBadge';
 import NetworkFeeHint from '@/components/common/NetworkFeeHint';
 import { TRADE_FEE_ESTIMATE } from '@/constants/fees';
-import { formatTransactionFeeDisplay } from '@/utils/transactionFee.utils';
+import {
+	fetchTradeNetworkFeeEstimate,
+	formatTransactionFeeDisplay,
+	type NetworkFeeDataProvider,
+} from '@/utils/transactionFee.utils';
 import { normalizeCreatorDisplayName } from '@/utils/creatorDisplayName.utils';
 
 export type TradeSide = 'buy' | 'sell';
@@ -30,8 +34,13 @@ export interface TradeDialogProps {
 	onOpenChange: (open: boolean) => void;
 	onConfirm: (amount: number) => Promise<void> | void;
 	isSubmitting?: boolean;
+	networkFeeEstimateProvider?: NetworkFeeDataProvider;
 }
 
+type NetworkFeeEstimateState =
+	| { status: 'idle' | 'loading' | 'error'; fee: null }
+	| { status: 'success'; fee: number };
+
 const TradeDialog: React.FC<TradeDialogProps> = ({
 	open,
 	side,
@@ -41,8 +50,11 @@ const TradeDialog: React.FC<TradeDialogProps> = ({
 	onOpenChange,
 	onConfirm,
 	isSubmitting = false,
+	networkFeeEstimateProvider,
 }) => {
 	const [amountText, setAmountText] = useState('1');
+	const [networkFeeEstimate, setNetworkFeeEstimate] =
+		useState<NetworkFeeEstimateState>({ status: 'idle', fee: null });
 	const amountInputRef = useRef<HTMLInputElement | null>(null);
 
 	useEffect(() => {
@@ -65,9 +77,60 @@ const TradeDialog: React.FC<TradeDialogProps> = ({
 	const title = side === 'buy' ? 'Buy keys' : 'Sell keys';
 	const confirmLabel = side === 'buy' ? 'Confirm buy' : 'Confirm sell';
 	const estimatedNetworkFee = formatTransactionFeeDisplay(
-		TRADE_FEE_ESTIMATE.DEFAULT_NETWORK_FEE,
+		networkFeeEstimate.status === 'success'
+			? networkFeeEstimate.fee
+			: TRADE_FEE_ESTIMATE.DEFAULT_NETWORK_FEE,
 		{ unit: TRADE_FEE_ESTIMATE.UNIT }
 	);
+	const networkFeeCopy =
+		networkFeeEstimate.status === 'loading'
+			? 'Estimating...'
+			: networkFeeEstimate.status === 'error'
+				? 'Cannot estimate network fee'
+				: estimatedNetworkFee;
+
+	useEffect(() => {
+		if (!open) {
+			setNetworkFeeEstimate({ status: 'idle', fee: null });
+			return;
+		}
+
+		if (!amountValid || !networkFeeEstimateProvider) {
+			setNetworkFeeEstimate({ status: 'error', fee: null });
+			return;
+		}
+
+		let cancelled = false;
+		setNetworkFeeEstimate({ status: 'loading', fee: null });
+
+		fetchTradeNetworkFeeEstimate(networkFeeEstimateProvider, {
+			side,
+			amount: parsedAmount,
+		})
+			.then(fee => {
+				if (cancelled) return;
+				setNetworkFeeEstimate(
+					fee == null
+						? { status: 'error', fee: null }
+						: { status: 'success', fee }
+				);
+			})
+			.catch(() => {
+				if (!cancelled) {
+					setNetworkFeeEstimate({ status: 'error', fee: null });
+				}
+			});
+
+		return () => {
+			cancelled = true;
+		};
+	}, [
+		amountValid,
+		networkFeeEstimateProvider,
+		open,
+		parsedAmount,
+		side,
+	]);
 
 	return (
 		<Dialog
@@ -147,13 +210,12 @@ const TradeDialog: React.FC<TradeDialogProps> = ({
 								/>
 							)}
 					</div>
-					{side === 'buy' && (
-						<NetworkFeeHint
-							variant="text"
-							fee={estimatedNetworkFee}
-							className="text-white/45"
-						/>
-					)}
+					<NetworkFeeHint
+						variant="text"
+						label="Approx. network fee"
+						fee={networkFeeCopy}
+						className="text-white/45"
+					/>
 					{side === 'sell' && parsedAmount > availableHoldings && (
 						<div className="text-xs text-red-300">
 							You can’t sell more than your current holdings.

src/components/common/__tests__/TradeDialog.focusOrder.test.tsx

@@ -101,4 +101,33 @@ describe('TradeDialog focus order', () => {
 
 		expect(ordered).toEqual(['1', '2', '3']);
 	});
+
+	it('shows an approximate network fee estimate before confirmation', async () => {
+		renderDialog({
+			networkFeeEstimateProvider: {
+				getFeeData: vi.fn().mockResolvedValue({
+					gasPrice: 1_000_000_000n,
+				}),
+			},
+		});
+
+		expect(screen.getByTestId('trade-dialog-confirm')).toBeInTheDocument();
+		expect(
+			await screen.findByText('Approx. network fee: ~0.00018 ETH')
+		).toBeInTheDocument();
+	});
+
+	it('shows a cannot estimate message when the fee estimate fails', async () => {
+		renderDialog({
+			networkFeeEstimateProvider: {
+				getFeeData: vi.fn().mockRejectedValue(new Error('RPC unavailable')),
+			},
+		});
+
+		expect(
+			await screen.findByText(
+				'Approx. network fee: Cannot estimate network fee'
+			)
+		).toBeInTheDocument();
+	});
 });

src/constants/fees.ts

@@ -15,4 +15,6 @@ export const KEY_PRICE_BOUNDS = {
 export const TRADE_FEE_ESTIMATE = {
 	DEFAULT_NETWORK_FEE: 0.0001,
 	UNIT: 'ETH',
+	BUY_GAS_LIMIT: 180_000n,
+	SELL_GAS_LIMIT: 150_000n,
 } as const;

src/pages/LandingPage.tsx

@@ -29,6 +29,7 @@ import EmptyTransactionTimelineState from '@/components/common/EmptyTransactionT
 import TradeDialog, { type TradeSide } from '@/components/common/TradeDialog';
 import NetworkMismatchBanner from '@/components/common/NetworkMismatchBanner';
 import StellarConnectionQualityBadge from '@/components/common/StellarConnectionQualityBadge';
+import { useEthersProvider } from '@/hooks/useEthersProvider';
 import { useNetworkMismatch } from '@/hooks/useNetworkMismatch';
 import showToast from '@/utils/toast.util';
 import { getSignatureErrorMessage } from '@/utils/errorHandling.utils';
@@ -249,6 +250,7 @@ function LandingPage() {
 	const [tradeSide, setTradeSide] = useState<TradeSide>('buy');
 	const [tradeDialogOpen, setTradeDialogOpen] = useState(false);
 	const [tradeSubmitting, setTradeSubmitting] = useState(false);
+	const tradeFeeEstimateProvider = useEthersProvider();
 	const prefersReducedMotion = usePrefersReducedMotion();
 	const [sortOption, setSortOption] = useState<SortOption>(() => {
 		if (typeof window === 'undefined') return 'featured';
@@ -1027,6 +1029,7 @@ function LandingPage() {
 				availableHoldings={featuredHoldings}
 				keyPriceStroops={resolveCreatorKeyPriceStroops(featuredCreator)}
 				isSubmitting={tradeSubmitting}
+				networkFeeEstimateProvider={tradeFeeEstimateProvider}
 				onOpenChange={setTradeDialogOpen}
 				onConfirm={handleConfirmTrade}
 			/>

src/utils/transactionFee.utils.ts

@@ -1,3 +1,5 @@
+import { formatEther } from 'ethers';
+import { TRADE_FEE_ESTIMATE } from '@/constants/fees';
 import { formatNumber } from '@/utils/numberFormat.utils';
 
 export interface FormatTransactionFeeOptions {
@@ -6,6 +8,20 @@ export interface FormatTransactionFeeOptions {
 	prefix?: string;
 }
 
+export interface NetworkFeeDataProvider {
+	getFeeData: () => Promise<{
+		gasPrice?: bigint | null;
+		maxFeePerGas?: bigint | null;
+	}>;
+}
+
+export type TradeFeeEstimateSide = 'buy' | 'sell';
+
+export interface TradeNetworkFeeEstimateRequest {
+	side: TradeFeeEstimateSide;
+	amount: number;
+}
+
 /**
  * Formats a transaction fee for confirmation UIs.
  *
@@ -29,3 +45,28 @@ export function formatTransactionFeeDisplay(
 		minimumFractionDigits: 0,
 	})} ${unit}`;
 }
+
+export function getTradeFeeGasLimit(side: TradeFeeEstimateSide): bigint {
+	return side === 'buy'
+		? TRADE_FEE_ESTIMATE.BUY_GAS_LIMIT
+		: TRADE_FEE_ESTIMATE.SELL_GAS_LIMIT;
+}
+
+export async function fetchTradeNetworkFeeEstimate(
+	provider: NetworkFeeDataProvider,
+	request: TradeNetworkFeeEstimateRequest
+): Promise<number | null> {
+	if (!Number.isFinite(request.amount) || request.amount <= 0) {
+		return null;
+	}
+
+	const feeData = await provider.getFeeData();
+	const gasPrice = feeData.maxFeePerGas ?? feeData.gasPrice;
+
+	if (gasPrice == null) {
+		return null;
+	}
+
+	const estimatedFeeWei = gasPrice * getTradeFeeGasLimit(request.side);
+	return Number(formatEther(estimatedFeeWei));
+}