diff --git a/frontend/src/lib/components/layout/SyncStatusIndicator.svelte b/frontend/src/lib/components/layout/SyncStatusIndicator.svelte
new file mode 100644
index 0000000..c32ed07
--- /dev/null
+++ b/frontend/src/lib/components/layout/SyncStatusIndicator.svelte
@@ -0,0 +1,71 @@
+<script lang="ts">
+	/**
+	 * SyncStatusIndicator.svelte - Compact sync status indicator for TopNav
+	 * Shows connection status with backend: synced, syncing, error, or offline
+	 */
+	import { syncState } from '$lib/backend';
+
+	/** Computed status for display */
+	let displayStatus = $derived.by(() => {
+		if (syncState.status === 'offline' || !syncState.isOnline) {
+			return 'offline';
+		}
+		if (syncState.status === 'error') {
+			return 'error';
+		}
+		if (syncState.status === 'syncing') {
+			return 'syncing';
+		}
+		return 'synced';
+	});
+
+	/** Tooltip text based on status */
+	let tooltipText = $derived.by(() => {
+		switch (displayStatus) {
+			case 'offline':
+				return 'Backend offline - data stored locally only';
+			case 'error':
+				return syncState.lastError
+					? `Sync error: ${syncState.lastError}`
+					: 'Sync error - check backend connection';
+			case 'syncing':
+				return 'Syncing...';
+			case 'synced':
+				if (syncState.lastSyncTime) {
+					const ago = getTimeAgo(syncState.lastSyncTime);
+					return `Synced ${ago}`;
+				}
+				return 'Synced';
+		}
+	});
+
+	/** Format relative time */
+	function getTimeAgo(date: Date): string {
+		const seconds = Math.floor((Date.now() - date.getTime()) / 1000);
+		if (seconds < 60) return 'just now';
+		if (seconds < 3600) return `${Math.floor(seconds / 60)}m ago`;
+		if (seconds < 86400) return `${Math.floor(seconds / 3600)}h ago`;
+		return `${Math.floor(seconds / 86400)}d ago`;
+	}
+</script>
+
+<div class="relative flex items-center" title={tooltipText}>
+	<!-- Status dot -->
+	<span
+		class="inline-block h-2 w-2 rounded-full {displayStatus === 'synced'
+			? 'bg-emerald-500'
+			: displayStatus === 'syncing'
+				? 'animate-pulse bg-amber-500'
+				: 'bg-red-500'}"
+		aria-hidden="true"
+	></span>
+
+	<!-- Pending count badge (only when error/offline with pending items) -->
+	{#if (displayStatus === 'error' || displayStatus === 'offline') && syncState.pendingCount > 0}
+		<span
+			class="ml-1 rounded-full bg-red-500/20 px-1.5 py-0.5 text-[10px] font-medium text-red-500"
+		>
+			{syncState.pendingCount}
+		</span>
+	{/if}
+</div>
diff --git a/frontend/src/lib/components/layout/TopNav.svelte b/frontend/src/lib/components/layout/TopNav.svelte
index 0e2d86c..812a18a 100644
--- a/frontend/src/lib/components/layout/TopNav.svelte
+++ b/frontend/src/lib/components/layout/TopNav.svelte
@@ -9,6 +9,7 @@
 	import ExportDialog from '$lib/components/shared/ExportDialog.svelte';
 	import ConfirmDialog from '$lib/components/shared/ConfirmDialog.svelte';
 	import ContextUsageBar from '$lib/components/chat/ContextUsageBar.svelte';
+	import SyncStatusIndicator from './SyncStatusIndicator.svelte';
 
 	interface Props {
 		/** Slot for the model select dropdown */
@@ -167,8 +168,13 @@
 			</div>
 		{/if}
 
-		<!-- Right section: Theme toggle + Chat actions -->
+		<!-- Right section: Sync status + Theme toggle + Chat actions -->
 		<div class="flex items-center gap-1">
+			<!-- Sync status indicator (always visible) -->
+			<div class="mr-1 px-2">
+				<SyncStatusIndicator />
+			</div>
+
 			<!-- Theme toggle (always visible) -->
 			<button
 				type="button"
diff --git a/frontend/src/lib/components/shared/SyncWarningBanner.svelte b/frontend/src/lib/components/shared/SyncWarningBanner.svelte
new file mode 100644
index 0000000..fce6061
--- /dev/null
+++ b/frontend/src/lib/components/shared/SyncWarningBanner.svelte
@@ -0,0 +1,154 @@
+<script lang="ts">
+	/**
+	 * SyncWarningBanner.svelte - Warning banner for sync failures
+	 * Shows when backend is disconnected for >30 seconds continuously
+	 */
+	import { syncState } from '$lib/backend';
+	import { onMount } from 'svelte';
+
+	/** Threshold before showing banner (30 seconds) */
+	const FAILURE_THRESHOLD_MS = 30_000;
+
+	/** Track when failure started */
+	let failureStartTime = $state<number | null>(null);
+
+	/** Whether banner has been dismissed for this failure period */
+	let isDismissed = $state(false);
+
+	/** Whether enough time has passed to show banner */
+	let thresholdReached = $state(false);
+
+	/** Interval for checking threshold */
+	let checkInterval: ReturnType<typeof setInterval> | null = null;
+
+	/** Check if we're in a failure state */
+	let isInFailureState = $derived(
+		syncState.status === 'error' || syncState.status === 'offline' || !syncState.isOnline
+	);
+
+	/** Should show the banner */
+	let shouldShow = $derived(isInFailureState && thresholdReached && !isDismissed);
+
+	/** Watch for failure state changes */
+	$effect(() => {
+		if (isInFailureState) {
+			// Start tracking failure time if not already
+			if (failureStartTime === null) {
+				failureStartTime = Date.now();
+				isDismissed = false;
+				thresholdReached = false;
+
+				// Start interval to check threshold
+				if (checkInterval) clearInterval(checkInterval);
+				checkInterval = setInterval(() => {
+					if (failureStartTime && Date.now() - failureStartTime >= FAILURE_THRESHOLD_MS) {
+						thresholdReached = true;
+						if (checkInterval) {
+							clearInterval(checkInterval);
+							checkInterval = null;
+						}
+					}
+				}, 1000);
+			}
+		} else {
+			// Reset on recovery
+			failureStartTime = null;
+			isDismissed = false;
+			thresholdReached = false;
+			if (checkInterval) {
+				clearInterval(checkInterval);
+				checkInterval = null;
+			}
+		}
+	});
+
+	onMount(() => {
+		return () => {
+			if (checkInterval) {
+				clearInterval(checkInterval);
+			}
+		};
+	});
+
+	/** Dismiss the banner */
+	function handleDismiss() {
+		isDismissed = true;
+	}
+</script>
+
+{#if shouldShow}
+	<div
+		class="fixed left-0 right-0 top-12 z-50 flex items-center justify-center px-4 animate-in"
+		role="alert"
+	>
+		<div
+			class="flex items-center gap-3 rounded-lg border border-red-500/30 bg-red-500/10 px-4 py-2 text-red-400 shadow-lg backdrop-blur-sm"
+		>
+			<!-- Warning icon -->
+			<svg
+				xmlns="http://www.w3.org/2000/svg"
+				class="h-5 w-5 flex-shrink-0"
+				fill="none"
+				viewBox="0 0 24 24"
+				stroke="currentColor"
+				stroke-width="1.5"
+			>
+				<path
+					stroke-linecap="round"
+					stroke-linejoin="round"
+					d="M12 9v3.75m-9.303 3.376c-.866 1.5.217 3.374 1.948 3.374h14.71c1.73 0 2.813-1.874 1.948-3.374L13.949 3.378c-.866-1.5-3.032-1.5-3.898 0L2.697 16.126ZM12 15.75h.007v.008H12v-.008Z"
+				/>
+			</svg>
+
+			<!-- Message -->
+			<span class="text-sm font-medium">
+				Backend not connected. Your data is only stored in this browser.
+			</span>
+
+			<!-- Pending count if any -->
+			{#if syncState.pendingCount > 0}
+				<span
+					class="rounded-full bg-red-500/20 px-2 py-0.5 text-xs font-medium"
+				>
+					{syncState.pendingCount} pending
+				</span>
+			{/if}
+
+			<!-- Dismiss button -->
+			<button
+				type="button"
+				onclick={handleDismiss}
+				class="ml-1 flex-shrink-0 rounded p-0.5 opacity-70 transition-opacity hover:opacity-100"
+				aria-label="Dismiss sync warning"
+			>
+				<svg
+					xmlns="http://www.w3.org/2000/svg"
+					class="h-4 w-4"
+					fill="none"
+					viewBox="0 0 24 24"
+					stroke="currentColor"
+					stroke-width="2"
+				>
+					<path stroke-linecap="round" stroke-linejoin="round" d="M6 18 18 6M6 6l12 12" />
+				</svg>
+			</button>
+		</div>
+	</div>
+{/if}
+
+<style>
+	@keyframes slide-in-from-top {
+		from {
+			transform: translateY(-100%);
+			opacity: 0;
+		}
+		to {
+			transform: translateY(0);
+			opacity: 1;
+		}
+	}
+
+	.animate-in {
+		animation: slide-in-from-top 0.3s ease-out;
+	}
+</style>
diff --git a/frontend/src/routes/+layout.svelte b/frontend/src/routes/+layout.svelte
index 9b89824..256c1e6 100644
--- a/frontend/src/routes/+layout.svelte
+++ b/frontend/src/routes/+layout.svelte
@@ -17,6 +17,7 @@
 	import ModelSelect from '$lib/components/layout/ModelSelect.svelte';
 	import { ToastContainer, ShortcutsModal } from '$lib/components/shared';
 	import UpdateBanner from '$lib/components/shared/UpdateBanner.svelte';
+	import SyncWarningBanner from '$lib/components/shared/SyncWarningBanner.svelte';
 
 	import type { LayoutData } from './$types';
 	import type { Snippet } from 'svelte';
@@ -184,5 +185,8 @@
 <!-- Update notification banner -->
 <UpdateBanner />
 
+<!-- Sync warning banner (shows when backend disconnected) -->
+<SyncWarningBanner />
+
 <!-- Keyboard shortcuts help -->
 <ShortcutsModal isOpen={showShortcutsModal} onClose={() => (showShortcutsModal = false)} />