diff --git a/packages/editor/App.tsx b/packages/editor/App.tsx
index d8131244..c749e3f8 100644
--- a/packages/editor/App.tsx
+++ b/packages/editor/App.tsx
@@ -453,6 +453,8 @@ const App: React.FC = () => {
     clearPendingSharedAnnotations,
     generateShortUrl,
     importFromShareUrl,
+    shareLoadError,
+    clearShareLoadError,
   } = useSharing(
     markdown,
     annotations,
@@ -1277,6 +1279,7 @@ const App: React.FC = () => {
                   isPlanDiffActive={isPlanDiffActive}
                   onPlanDiffToggle={() => setIsPlanDiffActive(!isPlanDiffActive)}
                   hasPreviousVersion={!linkedDocHook.isActive && planDiff.hasPreviousVersion}
+                  showDemoBadge={!isApiMode && !isLoadingShared && !isSharedSession}
                   onOpenLinkedDoc={linkedDocHook.open}
                   linkedDocInfo={linkedDocHook.isActive ? { filepath: linkedDocHook.filepath!, onBack: linkedDocHook.back } : null}
                 />
@@ -1386,6 +1389,16 @@ const App: React.FC = () => {
           showCancel
         />
 
+        {/* Shared URL load failure warning */}
+        <ConfirmDialog
+          isOpen={!!shareLoadError && !isApiMode}
+          onClose={clearShareLoadError}
+          title="Shared Plan Could Not Be Loaded"
+          message={shareLoadError}
+          subMessage="You are viewing a demo plan. This is sample content — it is not your data or anyone else's."
+          variant="warning"
+        />
+
         {/* Save-to-notes toast */}
         {noteSaveToast && (
           <div className={`fixed top-16 right-4 z-50 px-3 py-2 rounded-lg text-xs font-medium shadow-lg transition-opacity ${
diff --git a/packages/ui/components/Viewer.tsx b/packages/ui/components/Viewer.tsx
index 4696dca6..6884e606 100644
--- a/packages/ui/components/Viewer.tsx
+++ b/packages/ui/components/Viewer.tsx
@@ -33,6 +33,8 @@ interface ViewerProps {
   isPlanDiffActive?: boolean;
   onPlanDiffToggle?: () => void;
   hasPreviousVersion?: boolean;
+  /** Show amber "Demo" badge (portal mode, no shared content loaded) */
+  showDemoBadge?: boolean;
 }
 
 export interface ViewerHandle {
@@ -93,6 +95,7 @@ export const Viewer = forwardRef<ViewerHandle, ViewerProps>(({
   isPlanDiffActive,
   onPlanDiffToggle,
   hasPreviousVersion,
+  showDemoBadge,
   onOpenLinkedDoc,
   linkedDocInfo,
 }, ref) => {
@@ -648,8 +651,8 @@ export const Viewer = forwardRef<ViewerHandle, ViewerProps>(({
           linkedDocInfo ? 'border-2 border-primary' : 'border border-border/50'
         }`}
       >
-        {/* Repo info + plan diff badge + linked doc badge - top left */}
-        {(repoInfo || hasPreviousVersion || linkedDocInfo) && (
+        {/* Repo info + plan diff badge + demo badge + linked doc badge - top left */}
+        {(repoInfo || hasPreviousVersion || showDemoBadge || linkedDocInfo) && (
           <div className="absolute top-3 left-3 md:top-4 md:left-5 flex flex-col items-start gap-1 text-[9px] text-muted-foreground/50 font-mono">
             {repoInfo && !linkedDocInfo && (
               <div className="flex items-center gap-1.5">
@@ -674,6 +677,11 @@ export const Viewer = forwardRef<ViewerHandle, ViewerProps>(({
                 hasPreviousVersion={hasPreviousVersion ?? false}
               />
             )}
+            {showDemoBadge && !linkedDocInfo && (
+              <span className="px-1.5 py-0.5 rounded text-[9px] font-mono bg-amber-500/15 text-amber-600 dark:text-amber-400">
+                Demo
+              </span>
+            )}
             {linkedDocInfo && (
               <div className="flex items-center gap-1.5">
                 <button
diff --git a/packages/ui/hooks/useSharing.ts b/packages/ui/hooks/useSharing.ts
index 2a3c1030..8792b079 100644
--- a/packages/ui/hooks/useSharing.ts
+++ b/packages/ui/hooks/useSharing.ts
@@ -8,7 +8,7 @@
  * - Tracking whether current session is from a shared link
  */
 
-import { useState, useEffect, useCallback } from 'react';
+import React, { useState, useEffect, useCallback } from 'react';
 import { Annotation, type ImageAttachment } from '../types';
 import {
   type SharePayload,
@@ -68,6 +68,12 @@ interface UseSharingResult {
 
   /** Import annotations from a teammate's share URL */
   importFromShareUrl: (url: string) => Promise<ImportResult>;
+
+  /** Error message when a shared URL failed to load on mount */
+  shareLoadError: string;
+
+  /** Clear the share load error */
+  clearShareLoadError: () => void;
 }
 
 
@@ -76,8 +82,8 @@ export function useSharing(
   annotations: Annotation[],
   globalAttachments: ImageAttachment[],
   setMarkdown: (m: string) => void,
-  setAnnotations: (a: Annotation[]) => void,
-  setGlobalAttachments: (g: ImageAttachment[]) => void,
+  setAnnotations: React.Dispatch<React.SetStateAction<Annotation[]>>,
+  setGlobalAttachments: React.Dispatch<React.SetStateAction<ImageAttachment[]>>,
   onSharedLoad?: () => void,
   shareBaseUrl?: string,
   pasteApiUrl?: string
@@ -91,12 +97,15 @@ export function useSharing(
   const [shortUrlError, setShortUrlError] = useState('');
   const [pendingSharedAnnotations, setPendingSharedAnnotations] = useState<Annotation[] | null>(null);
   const [sharedGlobalAttachments, setSharedGlobalAttachments] = useState<ImageAttachment[] | null>(null);
+  const [shareLoadError, setShareLoadError] = useState('');
 
   const clearPendingSharedAnnotations = useCallback(() => {
     setPendingSharedAnnotations(null);
     setSharedGlobalAttachments(null);
   }, []);
 
+  const clearShareLoadError = useCallback(() => setShareLoadError(''), []);
+
   // Load shared state from URL hash (or paste-service short URL)
   const loadFromHash = useCallback(async () => {
     try {
@@ -135,9 +144,11 @@ export function useSharing(
         }
         // Paste fetch failed — short URL path can't fall back to hash parsing
         // (the hash contains #key=, not plan data).
+        setShareLoadError('Failed to load shared plan — the link may be expired or incomplete.');
         return false;
       }
 
+      const hash = window.location.hash.slice(1);
       const payload = await parseShareHash();
 
       if (payload) {
@@ -173,9 +184,15 @@ export function useSharing(
 
         return true;
       }
+
+      // Hash was present but failed to decompress (likely truncated by browser)
+      if (hash) {
+        setShareLoadError('Failed to load shared plan — the URL may have been truncated by your browser.');
+      }
       return false;
     } catch (e) {
       console.error('Failed to load from share hash:', e);
+      setShareLoadError('Failed to load shared plan — an unexpected error occurred.');
       return false;
     }
   }, [setMarkdown, setAnnotations, setGlobalAttachments, onSharedLoad, pasteApiUrl]);
@@ -297,8 +314,9 @@ export function useSharing(
         return { success: true, count: 0, planTitle, error: 'No annotations found in share link' };
       }
 
-      // Deduplicate: skip annotations that already exist (by originalText + type + text)
-      const newAnnotations = importedAnnotations.filter(imp =>
+      // Estimate count from current closure (may be slightly stale, but
+      // the actual merge below uses the latest state via functional updater)
+      const estimatedNew = importedAnnotations.filter(imp =>
         !annotations.some(existing =>
           existing.originalText === imp.originalText &&
           existing.type === imp.type &&
@@ -306,26 +324,36 @@ export function useSharing(
         )
       );
 
-      if (newAnnotations.length > 0) {
-        // Merge: append new annotations to existing ones
-        setAnnotations([...annotations, ...newAnnotations]);
-
-        // Set as pending so they get applied to DOM highlights
-        setPendingSharedAnnotations(newAnnotations);
+      if (estimatedNew.length > 0) {
+        // Merge using functional updater to avoid stale closure
+        setAnnotations(prev => {
+          const newAnnotations = importedAnnotations.filter(imp =>
+            !prev.some(existing =>
+              existing.originalText === imp.originalText &&
+              existing.type === imp.type &&
+              existing.text === imp.text
+            )
+          );
+          if (newAnnotations.length === 0) return prev;
+          const merged = [...prev, ...newAnnotations];
+          // Set ALL annotations as pending so DOM highlights include originals
+          setPendingSharedAnnotations(merged);
+          return merged;
+        });
 
         // Handle global attachments (deduplicate by path)
         if (payload.g?.length) {
           const parsed = parseShareableImages(payload.g) ?? [];
-          const existingPaths = new Set(globalAttachments.map(g => g.path));
-          const newAttachments = parsed.filter(p => !existingPaths.has(p.path));
-          if (newAttachments.length > 0) {
-            setGlobalAttachments([...globalAttachments, ...newAttachments]);
-          }
+          setGlobalAttachments(prev => {
+            const existingPaths = new Set(prev.map(g => g.path));
+            const newAttachments = parsed.filter(p => !existingPaths.has(p.path));
+            return newAttachments.length > 0 ? [...prev, ...newAttachments] : prev;
+          });
           setSharedGlobalAttachments(parsed);
         }
       }
 
-      return { success: true, count: newAnnotations.length, planTitle };
+      return { success: true, count: estimatedNew.length, planTitle };
     } catch (e) {
       const errorMessage = e instanceof Error ? e.message : 'Failed to decompress share URL';
       return { success: false, count: 0, planTitle: '', error: errorMessage };
@@ -346,5 +374,7 @@ export function useSharing(
     refreshShareUrl,
     generateShortUrl,
     importFromShareUrl,
+    shareLoadError,
+    clearShareLoadError,
   };
 }