diff --git a/static/js/presets.js b/static/js/presets.js
index cbb902f..4922000 100644
--- a/static/js/presets.js
+++ b/static/js/presets.js
@@ -8,6 +8,24 @@ let API_BASE = '';
 let selectedPreset = null;
 let presets = {};
 
+export function loadStoredArray(key) {
+  try {
+    const value = JSON.parse(localStorage.getItem(key) || '[]');
+    return Array.isArray(value) ? value : [];
+  } catch (e) {
+    return [];
+  }
+}
+
+export function loadStoredObject(key) {
+  try {
+    const value = JSON.parse(localStorage.getItem(key) || '{}');
+    return value && typeof value === 'object' && !Array.isArray(value) ? value : {};
+  } catch (e) {
+    return {};
+  }
+}
+
 // Built-in prompt templates (moved from cot_prompts.py)
 export const PROMPT_TEMPLATES = [
   {
@@ -228,7 +246,7 @@ function initNameDropdown() {
         }
         // Hide built-in preset
         if (isBuiltin) {
-          const hidden = JSON.parse(localStorage.getItem('odysseus-hidden-presets') || '[]');
+          const hidden = loadStoredArray('odysseus-hidden-presets');
           if (!hidden.includes(charName)) hidden.push(charName);
           localStorage.setItem('odysseus-hidden-presets', JSON.stringify(hidden));
         }
@@ -311,7 +329,7 @@ function _populateCharSelect() {
     select.appendChild(group);
   }
 
-  const hiddenPresets = JSON.parse(localStorage.getItem('odysseus-hidden-presets') || '[]');
+  const hiddenPresets = loadStoredArray('odysseus-hidden-presets');
   const builtins = PROMPT_TEMPLATES.filter(t => !savedNames.has(t.name) && !hiddenPresets.includes(t.name));
   if (builtins.length) {
     const group = document.createElement('optgroup');
@@ -405,7 +423,7 @@ function initPersistentChat() {
       await fetch(`${API_BASE}/api/session/${sessionId}/important`, { method: 'POST', body: favFd });
 
       // Save session → character mapping so it restores on switch
-      const charSessions = JSON.parse(localStorage.getItem('odysseus-char-sessions') || '{}');
+      const charSessions = loadStoredObject('odysseus-char-sessions');
       charSessions[sessionId] = charName;
       localStorage.setItem('odysseus-char-sessions', JSON.stringify(charSessions));
 
@@ -1011,7 +1029,7 @@ function _syncCharIndicator() {
 let _prevSessionId = null;
 
 export function onSessionSwitch(sessionId) {
-  const charSessions = JSON.parse(localStorage.getItem('odysseus-char-sessions') || '{}');
+  const charSessions = loadStoredObject('odysseus-char-sessions');
 
   // Leaving a persistent chat — deactivate for this switch only
   if (window._persistentChatSession) {
@@ -1059,7 +1077,7 @@ export function isPersistentChat() {
  * Remove a session from persistent chat mappings (call when session is deleted).
  */
 export function removePersistentChat(sessionId) {
-  const charSessions = JSON.parse(localStorage.getItem('odysseus-char-sessions') || '{}');
+  const charSessions = loadStoredObject('odysseus-char-sessions');
   if (charSessions[sessionId]) {
     delete charSessions[sessionId];
     localStorage.setItem('odysseus-char-sessions', JSON.stringify(charSessions));
diff --git a/tests/test_preset_local_storage_js.py b/tests/test_preset_local_storage_js.py
new file mode 100644
index 0000000..2da3f54
--- /dev/null
+++ b/tests/test_preset_local_storage_js.py
@@ -0,0 +1,53 @@
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+_REPO = Path(__file__).resolve().parent.parent
+_MODULE = _REPO / "static" / "js" / "presets.js"
+_HAS_NODE = shutil.which("node") is not None
+
+
+def _load_values():
+    js = f"""
+    globalThis.localStorage = {{
+      getItem(key) {{
+        return {{
+          broken: '{{',
+          list: '[]',
+          object: '{{"session":"Socrates"}}',
+        }}[key] ?? null;
+      }},
+    }};
+    const presets = await import('{_MODULE.as_posix()}');
+    console.log(JSON.stringify({{
+      brokenArray: presets.loadStoredArray('broken'),
+      wrongArray: presets.loadStoredArray('object'),
+      brokenObject: presets.loadStoredObject('broken'),
+      wrongObject: presets.loadStoredObject('list'),
+      object: presets.loadStoredObject('object'),
+    }}));
+    """
+    proc = subprocess.run(
+        ["node", "--input-type=module"],
+        input=js,
+        capture_output=True,
+        text=True,
+        cwd=str(_REPO),
+        timeout=30,
+    )
+    assert proc.returncode == 0, proc.stderr
+    return json.loads(proc.stdout.strip())
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_preset_storage_helpers_fall_back_for_bad_values():
+    assert _load_values() == {
+        "brokenArray": [],
+        "wrongArray": [],
+        "brokenObject": {},
+        "wrongObject": {},
+        "object": {"session": "Socrates"},
+    }