diff --git a/frontend/src/lib/components/PdfViewer.svelte b/frontend/src/lib/components/PdfViewer.svelte
index 2574e8d0..359c9700 100644
--- a/frontend/src/lib/components/PdfViewer.svelte
+++ b/frontend/src/lib/components/PdfViewer.svelte
@@ -1,9 +1,10 @@
 <script lang="ts">
-import { onMount } from 'svelte';
+import { onMount, setContext } from 'svelte';
 import type { PDFDocumentProxy, PDFPageProxy, RenderTask } from 'pdfjs-dist';
 import AnnotationLayer from './AnnotationLayer.svelte';
 import type { Annotation } from '$lib/types';
 import { m } from '$lib/paraglide/messages.js';
+import { parseBackendError, getErrorMessage } from '$lib/errors';
 
 type DrawRect = { x: number; y: number; width: number; height: number; pageNumber: number };
 
@@ -55,6 +56,7 @@ let pdfjsReady = $state(false);
 
 let annotations = $state<Annotation[]>([]);
 let showAnnotations = $state(true);
+let annotationUpdateError = $state<string | null>(null);
 
 const TRANSCRIPTION_COLOR = '#00C7B1';
 
@@ -186,6 +188,29 @@ async function loadAnnotations(docId: string) {
 	}
 }
 
+async function updateAnnotation(
+	annotationId: string,
+	coords: { x: number; y: number; width: number; height: number }
+) {
+	if (!documentId) return;
+	const res = await fetch(`/api/documents/${documentId}/annotations/${annotationId}`, {
+		method: 'PATCH',
+		headers: { 'Content-Type': 'application/json' },
+		body: JSON.stringify(coords)
+	});
+	if (!res.ok) {
+		const err = await parseBackendError(res);
+		const msg = getErrorMessage(err?.code ?? 'ANNOTATION_UPDATE_FAILED');
+		annotationUpdateError = msg;
+		setTimeout(() => (annotationUpdateError = null), 4000);
+		throw new Error(msg);
+	}
+	const updated = await res.json();
+	annotations = annotations.map((a) => (a.id === annotationId ? updated : a));
+}
+
+setContext('annotationUpdate', updateAnnotation);
+
 async function handleDraw(rect: { x: number; y: number; width: number; height: number }) {
 	if (!documentId || !transcribeMode) return;
 	await onTranscriptionDraw?.({ ...rect, pageNumber: currentPage });
@@ -306,6 +331,26 @@ function zoomOut() {
 				<span class="font-sans text-xs text-amber-300">{m.annotation_outdated_notice()}</span>
 			</div>
 		{/if}
+		{#if annotationUpdateError}
+			<div
+				class="flex shrink-0 items-center gap-2 border-b border-red-500/30 bg-red-500/10 px-4 py-2"
+				aria-live="assertive"
+				role="alert"
+			>
+				<svg
+					class="h-4 w-4 shrink-0 text-red-400"
+					viewBox="0 0 24 24"
+					fill="none"
+					stroke="currentColor"
+					stroke-width="2"
+				>
+					<circle cx="12" cy="12" r="10" />
+					<line x1="15" y1="9" x2="9" y2="15" />
+					<line x1="9" y1="9" x2="15" y2="15" />
+				</svg>
+				<span class="font-sans text-xs text-red-300">{annotationUpdateError}</span>
+			</div>
+		{/if}
 		<!-- Controls -->
 		<div
 			class="flex shrink-0 items-center justify-between gap-2 border-b border-pdf-ctrl px-4 py-2"