diff --git a/packages/ui/src/apps/MobileApp.tsx b/packages/ui/src/apps/MobileApp.tsx
index 234df4b7..0d1dbab9 100644
--- a/packages/ui/src/apps/MobileApp.tsx
+++ b/packages/ui/src/apps/MobileApp.tsx
@@ -27,7 +27,7 @@ import { readTabletLayout, useOrientation, useTabletLayout } from '@/lib/device'
 import { useHardwareKeyboard } from '@/lib/hardwareKeyboard';
 import { useI18n } from '@/lib/i18n';
 import { runtimeFetch } from '@/lib/runtime-fetch';
-import { getRuntimeApiBaseUrl, getRuntimeKey, subscribeRuntimeEndpointChanged, switchRuntimeEndpoint } from '@/lib/runtime-switch';
+import { getRuntimeApiBaseUrl, getRuntimeKey, subscribeRuntimeEndpointChanged, switchRuntimeEndpoint, MOBILE_DISCONNECTED_RUNTIME_KEY } from '@/lib/runtime-switch';
 import { refreshGlobalSessions, resolveGlobalSessionDirectory } from '@/stores/useGlobalSessionsStore';
 import { clearLastActiveSession, readLastActiveSession } from '@/sync/last-session-cache';
 import { cn } from '@/lib/utils';
@@ -686,7 +686,7 @@ export function MobileApp({ apis }: MobileAppProps) {
     };
     const disconnect = (reason: string) => {
       logMobileConnectEvent('resume:disconnect', { reason });
-      switchRuntimeEndpoint({ apiBaseUrl: '', clientToken: null, runtimeKey: 'mobile-disconnected' });
+      switchRuntimeEndpoint({ apiBaseUrl: '', clientToken: null, runtimeKey: MOBILE_DISCONNECTED_RUNTIME_KEY });
       setConnectionEpoch((value) => value + 1);
     };
 
@@ -896,7 +896,7 @@ export function MobileApp({ apis }: MobileAppProps) {
     const dropToConnectScreen = (notice: MobileConnectionNotice | null) => {
       logMobileConnectEvent('cold-launch:drop', { kind: notice?.kind ?? 'unknown' });
       if (notice) setAutoConnectNotice(notice);
-      switchRuntimeEndpoint({ apiBaseUrl: '', clientToken: null, runtimeKey: 'mobile-disconnected' });
+      switchRuntimeEndpoint({ apiBaseUrl: '', clientToken: null, runtimeKey: MOBILE_DISCONNECTED_RUNTIME_KEY });
       setConnectionEpoch((value) => value + 1);
     };
     void reprobeActiveConnection().then(async (outcome) => {
@@ -1196,7 +1196,7 @@ export function MobileApp({ apis }: MobileAppProps) {
                   type="button"
                   variant="outline"
                   onClick={() => {
-                    switchRuntimeEndpoint({ apiBaseUrl: '', clientToken: null, runtimeKey: 'mobile-disconnected' });
+                    switchRuntimeEndpoint({ apiBaseUrl: '', clientToken: null, runtimeKey: MOBILE_DISCONNECTED_RUNTIME_KEY });
                     setConnectionEpoch((value) => value + 1);
                   }}
                 >
@@ -1279,7 +1279,7 @@ export function MobileApp({ apis }: MobileAppProps) {
               <OpenCodeUpdateToast />
               <MobileAppUpdateToast />
               <MobileShell onActiveConnectionDeleted={() => {
-                switchRuntimeEndpoint({ apiBaseUrl: '', clientToken: null, runtimeKey: 'mobile-disconnected' });
+                switchRuntimeEndpoint({ apiBaseUrl: '', clientToken: null, runtimeKey: MOBILE_DISCONNECTED_RUNTIME_KEY });
                 setConnectionEpoch((value) => value + 1);
               }} />
               <AppLinkConfirmDialog />
diff --git a/packages/ui/src/contexts/theme-storage.test.ts b/packages/ui/src/contexts/theme-storage.test.ts
index 92acc2ba..ecba5ad0 100644
--- a/packages/ui/src/contexts/theme-storage.test.ts
+++ b/packages/ui/src/contexts/theme-storage.test.ts
@@ -5,12 +5,12 @@ import { DEFAULT_DARK_THEME_ID, DEFAULT_LIGHT_THEME_ID } from '@/lib/theme/theme
 import {
   adoptThemePreferencesForRuntime,
   getThemePreferencesStorageKey,
-  isTransientRuntimeKey,
   readThemePreferencesForRuntime,
   resolveThemePreferencesForRuntime,
   resolveThemePreferencesFromStorageEvent,
   writeThemePreferencesForRuntime,
 } from './theme-storage';
+import { isTransientRuntimeKey } from '@/lib/runtime-switch';
 
 let createdWindow = false;
 let createdLocalStorage = false;
diff --git a/packages/ui/src/contexts/theme-storage.ts b/packages/ui/src/contexts/theme-storage.ts
index 8fc94ef6..863b9efc 100644
--- a/packages/ui/src/contexts/theme-storage.ts
+++ b/packages/ui/src/contexts/theme-storage.ts
@@ -1,5 +1,6 @@
 import type { ThemeMode } from '@/types/theme';
 import { DEFAULT_DARK_THEME_ID, DEFAULT_LIGHT_THEME_ID, getThemeById } from '@/lib/theme/themes';
+import { isTransientRuntimeKey } from '@/lib/runtime-switch';
 
 type StoredThemePreferences = {
   themeMode: ThemeMode;
@@ -21,56 +22,56 @@ const THEME_PREFERENCES_KEY_PREFIX = 'openchamber.theme.v2:';
 export const getThemePreferencesStorageKey = (runtimeKey: string): string =>
   `${THEME_PREFERENCES_KEY_PREFIX}${encodeURIComponent(runtimeKey)}`;
 
-// Runtime keys that mean "no instance connected" — the uninitialized default
-// and the mobile disconnect state. They carry no instance theme, so scoped
-// storage must not read or write them: a write would pin whatever theme was
-// current at that moment (e.g. cold-boot defaults) to a key every future
-// launch resolves before connecting, and a read would surface that stale
-// entry on the mobile connect splash. The global splash hints are the right
-// fallback for those phases.
-const TRANSIENT_RUNTIME_KEYS = new Set(['', 'url:default', 'mobile-disconnected']);
+const THEME_MODES: readonly ThemeMode[] = ['light', 'dark', 'system'];
 
-export const isTransientRuntimeKey = (runtimeKey: string): boolean =>
-  TRANSIENT_RUNTIME_KEYS.has(runtimeKey);
+const isThemeMode = (value: string): value is ThemeMode =>
+  THEME_MODES.some((mode) => mode === value);
 
-export const readThemePreferencesForRuntime = (runtimeKey: string): StoredThemePreferences | null => {
-  if (typeof window === 'undefined' || isTransientRuntimeKey(runtimeKey)) {
-    return null;
-  }
-  let raw: string | null = null;
+// Boundary parser for the scoped entry. A malformed or partial payload is a
+// failure (`null`), never a valid default: the caller then falls back to the
+// legacy seed or keeps its current preferences.
+const parseStoredThemePreferences = (raw: string): StoredThemePreferences | null => {
   try {
-    raw = localStorage.getItem(getThemePreferencesStorageKey(runtimeKey));
-  } catch {
-    return null;
-  }
-  if (!raw) {
-    return null;
-  }
-  try {
-    const parsed = JSON.parse(raw) as unknown;
-    if (!parsed || typeof parsed !== 'object') {
+    // SAFETY: this key is written only by `writeThemePreferencesForRuntime`
+    // with exactly this shape. Every field is still re-checked below, and a
+    // field of the wrong type throws on `.trim()` into the catch.
+    const candidate = JSON.parse(raw) as Partial<StoredThemePreferences> | null;
+    if (candidate === null) {
       return null;
     }
-    const candidate = parsed as Record<string, unknown>;
-    if (candidate.themeMode !== 'light' && candidate.themeMode !== 'dark' && candidate.themeMode !== 'system') {
+    const themeMode = candidate.themeMode ?? '';
+    if (!isThemeMode(themeMode)) {
       return null;
     }
-    if (typeof candidate.lightThemeId !== 'string' || typeof candidate.darkThemeId !== 'string') {
-      return null;
-    }
-    const lightThemeId = candidate.lightThemeId.trim();
-    const darkThemeId = candidate.darkThemeId.trim();
+    const lightThemeId = (candidate.lightThemeId ?? '').trim();
+    const darkThemeId = (candidate.darkThemeId ?? '').trim();
     if (!lightThemeId || !darkThemeId) {
       return null;
     }
-    return { themeMode: candidate.themeMode, lightThemeId, darkThemeId };
+    return { themeMode, lightThemeId, darkThemeId };
   } catch {
     return null;
   }
 };
 
+const readLocalStorageItem = (key: string): string | null => {
+  try {
+    return localStorage.getItem(key);
+  } catch {
+    return null;
+  }
+};
+
+export const readThemePreferencesForRuntime = (runtimeKey: string): StoredThemePreferences | null => {
+  if (isTransientRuntimeKey(runtimeKey)) {
+    return null;
+  }
+  const raw = readLocalStorageItem(getThemePreferencesStorageKey(runtimeKey));
+  return raw ? parseStoredThemePreferences(raw) : null;
+};
+
 export const writeThemePreferencesForRuntime = (runtimeKey: string, preferences: StoredThemePreferences): void => {
-  if (typeof window === 'undefined' || isTransientRuntimeKey(runtimeKey)) {
+  if (isTransientRuntimeKey(runtimeKey)) {
     return;
   }
   try {
@@ -118,16 +119,12 @@ const readLegacyThemePreferences = (): StoredThemePreferences => {
   let lightThemeId: string = DEFAULT_LIGHT_THEME_ID;
   let darkThemeId: string = DEFAULT_DARK_THEME_ID;
 
-  if (typeof window === 'undefined') {
-    return { themeMode, lightThemeId, darkThemeId };
-  }
+  const legacyMode = readLocalStorageItem('themeMode');
+  const legacyUseSystem = readLocalStorageItem('useSystemTheme');
+  const legacyThemeId = readLocalStorageItem('selectedThemeId');
+  const legacyVariant = readLocalStorageItem('selectedThemeVariant');
 
-  const legacyMode = localStorage.getItem('themeMode');
-  const legacyUseSystem = localStorage.getItem('useSystemTheme');
-  const legacyThemeId = localStorage.getItem('selectedThemeId');
-  const legacyVariant = localStorage.getItem('selectedThemeVariant');
-
-  if (legacyMode === 'light' || legacyMode === 'dark' || legacyMode === 'system') {
+  if (legacyMode !== null && isThemeMode(legacyMode)) {
     themeMode = legacyMode;
   } else if (legacyUseSystem !== null) {
     const useSystem = legacyUseSystem === 'true';
@@ -148,13 +145,13 @@ const readLegacyThemePreferences = (): StoredThemePreferences => {
     themeMode = legacyVariant;
   }
 
-  const legacyLightId = localStorage.getItem('lightThemeId');
-  const legacyDarkId = localStorage.getItem('darkThemeId');
-  if (typeof legacyLightId === 'string' && legacyLightId.trim().length > 0) {
-    lightThemeId = legacyLightId.trim();
+  const legacyLightId = readLocalStorageItem('lightThemeId')?.trim();
+  const legacyDarkId = readLocalStorageItem('darkThemeId')?.trim();
+  if (legacyLightId) {
+    lightThemeId = legacyLightId;
   }
-  if (typeof legacyDarkId === 'string' && legacyDarkId.trim().length > 0) {
-    darkThemeId = legacyDarkId.trim();
+  if (legacyDarkId) {
+    darkThemeId = legacyDarkId;
   }
 
   return { themeMode, lightThemeId, darkThemeId };
diff --git a/packages/ui/src/lib/runtime-switch.ts b/packages/ui/src/lib/runtime-switch.ts
index 8b11ef90..a6a6eecc 100644
--- a/packages/ui/src/lib/runtime-switch.ts
+++ b/packages/ui/src/lib/runtime-switch.ts
@@ -51,6 +51,17 @@ const normalizeRuntimeUrlKey = (value: string): string => {
   }
 };
 
+// Runtime keys that mean "no instance connected": the uninitialized default
+// (`normalizeRuntimeUrlKey` of an empty/unparseable base URL) and the mobile
+// disconnect state (`MobileApp` switches to it when the connection drops).
+// Per-instance client state (e.g. the scoped theme entry) must not be read
+// from or written under them.
+export const MOBILE_DISCONNECTED_RUNTIME_KEY = 'mobile-disconnected';
+const UNINITIALIZED_RUNTIME_KEY = 'url:default';
+
+export const isTransientRuntimeKey = (runtimeKey: string): boolean =>
+  runtimeKey === '' || runtimeKey === UNINITIALIZED_RUNTIME_KEY || runtimeKey === MOBILE_DISCONNECTED_RUNTIME_KEY;
+
 const readInjectedApiBaseUrl = (): string => {
   if (typeof window === 'undefined') return '';
   const injected = (window as typeof window & { __OPENCHAMBER_API_BASE_URL__?: string }).__OPENCHAMBER_API_BASE_URL__;