diff --git a/app/src/components/intelligence/ObsidianVaultSection.test.tsx b/app/src/components/intelligence/ObsidianVaultSection.test.tsx
new file mode 100644
index 000000000..c08aba693
--- /dev/null
+++ b/app/src/components/intelligence/ObsidianVaultSection.test.tsx
@@ -0,0 +1,77 @@
+/**
+ * Tests for the cross-host guard (#4278) in ObsidianVaultSection: when the
+ * vault lives on a different-OS core host, "View Vault" must surface guidance
+ * and disable the local-FS actions instead of firing a doomed deep link.
+ */
+import { fireEvent, screen, waitFor } from '@testing-library/react';
+import { describe, expect, it, type Mock, vi } from 'vitest';
+
+import { renderWithProviders } from '../../test/test-utils';
+import { ObsidianVaultSection } from './ObsidianVaultSection';
+
+vi.mock('../../utils/tauriCommands', () => ({ memoryTreeObsidianVaultStatus: vi.fn() }));
+
+vi.mock('../../utils/tauriCommands/workspacePaths', () => ({
+  resolveWorkspaceAbsolutePath: vi.fn().mockResolvedValue('/home/leigh/OHvault'),
+  revealWorkspacePath: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock('../../utils/openUrl', () => ({ openUrl: vi.fn().mockResolvedValue(undefined) }));
+
+vi.mock('./vaultHostMatch', () => ({
+  resolveVaultHostMatch: vi.fn().mockResolvedValue({ local: true }),
+}));
+
+const { memoryTreeObsidianVaultStatus } =
+  (await import('../../utils/tauriCommands')) as unknown as { memoryTreeObsidianVaultStatus: Mock };
+
+const { openUrl } = (await import('../../utils/openUrl')) as unknown as { openUrl: Mock };
+
+const { resolveVaultHostMatch } = (await import('./vaultHostMatch')) as unknown as {
+  resolveVaultHostMatch: Mock;
+};
+
+const CONTENT_ROOT = '/home/leigh/OHvault';
+
+describe('<ObsidianVaultSection /> cross-host (#4278)', () => {
+  it('surfaces guidance and disables local actions when the vault is on a different-OS core host', async () => {
+    memoryTreeObsidianVaultStatus.mockResolvedValueOnce({
+      registered: true,
+      config_found: true,
+      content_root_abs: CONTENT_ROOT,
+      host_os: 'linux',
+    });
+    resolveVaultHostMatch.mockResolvedValueOnce({ local: false, hostOs: 'linux' });
+
+    renderWithProviders(<ObsidianVaultSection contentRootAbs={CONTENT_ROOT} />);
+    fireEvent.click(screen.getByTestId('memory-open-in-obsidian'));
+
+    await waitFor(() => {
+      expect(screen.getByTestId('obsidian-vault-guidance')).toBeInTheDocument();
+    });
+    expect(screen.getByTestId('obsidian-vault-guidance')).toHaveTextContent('linux');
+    expect(screen.getByTestId('obsidian-reveal')).toBeDisabled();
+    expect(screen.getByTestId('obsidian-open-anyway')).toBeDisabled();
+    // Registered=true would normally fire the deep link; cross-host must NOT.
+    expect(openUrl).not.toHaveBeenCalled();
+  });
+
+  it('fires the deep link normally for a same-host registered vault (regression)', async () => {
+    memoryTreeObsidianVaultStatus.mockResolvedValueOnce({
+      registered: true,
+      config_found: true,
+      content_root_abs: CONTENT_ROOT,
+      host_os: 'macos',
+    });
+    resolveVaultHostMatch.mockResolvedValueOnce({ local: true, hostOs: 'macos' });
+
+    renderWithProviders(<ObsidianVaultSection contentRootAbs={CONTENT_ROOT} />);
+    fireEvent.click(screen.getByTestId('memory-open-in-obsidian'));
+
+    await waitFor(() => {
+      expect(openUrl).toHaveBeenCalledWith(
+        'obsidian://open?path=' + encodeURIComponent(CONTENT_ROOT)
+      );
+    });
+  });
+});
diff --git a/app/src/components/intelligence/ObsidianVaultSection.tsx b/app/src/components/intelligence/ObsidianVaultSection.tsx
index ce5398ffd..4c3410c9c 100644
--- a/app/src/components/intelligence/ObsidianVaultSection.tsx
+++ b/app/src/components/intelligence/ObsidianVaultSection.tsx
@@ -31,6 +31,7 @@ import {
 } from '../../utils/tauriCommands/workspacePaths';
 import Button from '../ui/Button';
 import { MEMORY_CONTENT_WORKSPACE_PATH } from './memoryWorkspacePaths';
+import { resolveVaultHostMatch } from './vaultHostMatch';
 
 /** localStorage key for the optional Obsidian config-dir override. */
 const CONFIG_DIR_KEY = 'openhuman.obsidian.configDir';
@@ -58,6 +59,9 @@ export function ObsidianVaultSection({ contentRootAbs, onToast }: ObsidianVaultS
   const [configFound, setConfigFound] = useState<boolean | null>(null);
   const [showAdvanced, setShowAdvanced] = useState(false);
   const [configDir, setConfigDir] = useState<string>(readConfigDirOverride);
+  // #4278: set to the core host OS when the vault lives on a different-OS core
+  // host, so local actions (Reveal / Open in Obsidian) are disabled + explained.
+  const [crossHostOs, setCrossHostOs] = useState<string | null>(null);
   const containerRef = useRef<HTMLDivElement>(null);
 
   const closePanel = useCallback(() => setExpanded(false), []);
@@ -175,10 +179,23 @@ export function ObsidianVaultSection({ contentRootAbs, onToast }: ObsidianVaultS
         const override = configDir.trim();
         const status = await memoryTreeObsidianVaultStatus(override || undefined);
         console.debug(
-          '[ui-flow][obsidian-vault] status registered=%s config_found=%s',
+          '[ui-flow][obsidian-vault] status registered=%s config_found=%s host_os=%s',
           status.registered,
-          status.config_found
+          status.config_found,
+          status.host_os
         );
+
+        // #4278: when the vault lives on a core host running a different OS, the
+        // path is not local — guide instead of firing a doomed deep link / reveal.
+        const match = await resolveVaultHostMatch(status.host_os);
+        if (!match.local) {
+          console.debug('[ui-flow][obsidian-vault] cross-host vault host_os=%s', match.hostOs);
+          setCrossHostOs(match.hostOs ?? status.host_os ?? '');
+          setConfigFound(status.config_found);
+          setExpanded(true);
+          return;
+        }
+        setCrossHostOs(null);
         setConfigFound(status.config_found);
 
         if (status.registered) {
@@ -216,8 +233,9 @@ export function ObsidianVaultSection({ contentRootAbs, onToast }: ObsidianVaultS
     handleViewVault();
   }, [configDir, handleViewVault]);
 
-  const helpText =
-    configFound === false
+  const helpText = crossHostOs
+    ? `${t('crossHostVault.title')} ${t('crossHostVault.message').replace('{os}', crossHostOs)}`
+    : configFound === false
       ? t('workspace.obsidianNotFoundHelp')
       : t('workspace.vaultNotRegisteredHelp');
 
@@ -266,15 +284,21 @@ export function ObsidianVaultSection({ contentRootAbs, onToast }: ObsidianVaultS
           </code>
 
           <div className="mt-3 flex flex-wrap gap-2">
-            <Button variant="secondary" size="sm" onClick={reveal} data-testid="obsidian-reveal">
+            <Button
+              variant="secondary"
+              size="sm"
+              onClick={reveal}
+              disabled={crossHostOs !== null}
+              data-testid="obsidian-reveal">
               {t('workspace.revealFolder')}
             </Button>
             <button
               type="button"
               onClick={openAnyway}
+              disabled={crossHostOs !== null}
               data-testid="obsidian-open-anyway"
               className="rounded-md border border-violet-300 bg-surface px-3 py-1.5 text-xs font-semibold
-                         text-violet-700 hover:bg-violet-50 dark:border-violet-500/40
+                         text-violet-700 hover:bg-violet-50 disabled:opacity-50 dark:border-violet-500/40
                          dark:bg-surface-muted dark:text-violet-300">
               {t('workspace.openAnyway')}
             </button>
diff --git a/app/src/components/intelligence/VaultHealthChecklist.test.tsx b/app/src/components/intelligence/VaultHealthChecklist.test.tsx
index a3e7bd20c..f54d2fe24 100644
--- a/app/src/components/intelligence/VaultHealthChecklist.test.tsx
+++ b/app/src/components/intelligence/VaultHealthChecklist.test.tsx
@@ -12,6 +12,10 @@ vi.mock('../../utils/openUrl', () => ({
   revealPath: vi.fn().mockResolvedValue(undefined),
 }));
 
+vi.mock('./vaultHostMatch', () => ({
+  resolveVaultHostMatch: vi.fn().mockResolvedValue({ local: true }),
+}));
+
 const { memoryTreeVaultHealthCheck } = (await import('../../utils/tauriCommands')) as unknown as {
   memoryTreeVaultHealthCheck: Mock;
 };
@@ -21,6 +25,10 @@ const { openUrl, revealPath } = (await import('../../utils/openUrl')) as unknown
   revealPath: Mock;
 };
 
+const { resolveVaultHostMatch } = (await import('./vaultHostMatch')) as unknown as {
+  resolveVaultHostMatch: Mock;
+};
+
 function health(overrides: Partial<VaultHealthCheck> = {}): VaultHealthCheck {
   return {
     content_root_abs: '/tmp/workspace/memory_tree/content',
@@ -89,4 +97,36 @@ describe('<VaultHealthChecklist />', () => {
       expect(revealPath).toHaveBeenCalledWith('/tmp/workspace/memory_tree/content');
     });
   });
+
+  // #4278: vault on a different-OS core host — surface a banner and disable the
+  // local-FS actions instead of letting them act on a foreign path.
+  it('warns and disables local actions when the vault is on a different-OS core host', async () => {
+    memoryTreeVaultHealthCheck.mockResolvedValueOnce(health({ host_os: 'linux' }));
+    resolveVaultHostMatch.mockResolvedValueOnce({ local: false, hostOs: 'linux' });
+    renderWithProviders(<VaultHealthChecklist />);
+
+    await waitFor(() => {
+      expect(screen.getByTestId('vault-health-cross-host')).toBeInTheDocument();
+    });
+    expect(screen.getByTestId('vault-health-cross-host')).toHaveTextContent('linux');
+    expect(screen.getByTestId('vault-health-reveal')).toBeDisabled();
+    expect(screen.getByTestId('vault-health-open-obsidian')).toBeDisabled();
+
+    fireEvent.click(screen.getByTestId('vault-health-reveal'));
+    fireEvent.click(screen.getByTestId('vault-health-open-obsidian'));
+    expect(revealPath).not.toHaveBeenCalled();
+    expect(openUrl).not.toHaveBeenCalled();
+  });
+
+  it('keeps local actions enabled for a same-host vault (regression)', async () => {
+    memoryTreeVaultHealthCheck.mockResolvedValueOnce(health({ host_os: 'macos' }));
+    resolveVaultHostMatch.mockResolvedValueOnce({ local: true, hostOs: 'macos' });
+    renderWithProviders(<VaultHealthChecklist />);
+
+    await waitFor(() => {
+      expect(screen.getByTestId('vault-health-reveal')).toBeInTheDocument();
+    });
+    expect(screen.queryByTestId('vault-health-cross-host')).not.toBeInTheDocument();
+    expect(screen.getByTestId('vault-health-reveal')).not.toBeDisabled();
+  });
 });
diff --git a/app/src/components/intelligence/VaultHealthChecklist.tsx b/app/src/components/intelligence/VaultHealthChecklist.tsx
index 4877a6d3c..c59274367 100644
--- a/app/src/components/intelligence/VaultHealthChecklist.tsx
+++ b/app/src/components/intelligence/VaultHealthChecklist.tsx
@@ -5,6 +5,7 @@ import type { ToastNotification } from '../../types/intelligence';
 import { openUrl, revealPath } from '../../utils/openUrl';
 import { memoryTreeVaultHealthCheck, type VaultHealthCheck } from '../../utils/tauriCommands';
 import Button from '../ui/Button';
+import { resolveVaultHostMatch, type VaultHostMatch } from './vaultHostMatch';
 
 const OBSIDIAN_DOWNLOAD_URL = 'https://obsidian.md/download';
 
@@ -41,6 +42,7 @@ export function VaultHealthChecklist({ onToast, title }: VaultHealthChecklistPro
   const { t } = useT();
   const resolvedTitle = title ?? t('vaultHealth.title');
   const [health, setHealth] = useState<VaultHealthCheck | null>(null);
+  const [hostMatch, setHostMatch] = useState<VaultHostMatch | null>(null);
   const [loading, setLoading] = useState(true);
   const [refreshing, setRefreshing] = useState(false);
   const [error, setError] = useState<string | null>(null);
@@ -50,6 +52,7 @@ export function VaultHealthChecklist({ onToast, title }: VaultHealthChecklistPro
     try {
       const next = await memoryTreeVaultHealthCheck();
       setHealth(next);
+      setHostMatch(await resolveVaultHostMatch(next.host_os));
       setError(null);
     } catch (err) {
       const message = err instanceof Error ? err.message : String(err);
@@ -69,6 +72,12 @@ export function VaultHealthChecklist({ onToast, title }: VaultHealthChecklistPro
     return health.exists ? health.content_root_abs : dirname(health.content_root_abs);
   }, [health]);
 
+  // #4278: the vault lives on the core host's filesystem. When this frontend
+  // runs on a different OS than the core, the path can't be opened/registered
+  // locally — disable the local-FS actions and explain why instead of firing a
+  // doomed reveal / deep link.
+  const crossHost = hostMatch ? !hostMatch.local : false;
+
   const openObsidian = useCallback(() => {
     if (!health?.content_root_abs) return;
     void (async () => {
@@ -164,19 +173,28 @@ export function VaultHealthChecklist({ onToast, title }: VaultHealthChecklistPro
         </code>
       ) : null}
 
+      {crossHost ? (
+        <div
+          className="rounded-md border border-amber-200 dark:border-amber-500/30 bg-amber-50 dark:bg-amber-500/10 px-3 py-2 text-xs text-amber-800 dark:text-amber-200"
+          data-testid="vault-health-cross-host">
+          <span className="font-semibold">{t('crossHostVault.title')}</span>{' '}
+          {t('crossHostVault.message').replace('{os}', hostMatch?.hostOs ?? '')}
+        </div>
+      ) : null}
+
       <div className="flex flex-wrap gap-2">
         <Button
           variant="secondary"
           size="sm"
           onClick={revealVault}
-          disabled={!health?.content_root_abs}
+          disabled={!health?.content_root_abs || crossHost}
           data-testid="vault-health-reveal">
           {t('vaultHealth.revealFolder')}
         </Button>
         <button
           type="button"
           onClick={openObsidian}
-          disabled={!health?.content_root_abs}
+          disabled={!health?.content_root_abs || crossHost}
           className="rounded-md border border-violet-300 dark:border-violet-500/40 bg-surface px-3 py-1.5 text-xs font-semibold text-violet-700 dark:text-violet-300 disabled:opacity-50"
           data-testid="vault-health-open-obsidian">
           {t('vaultHealth.openInObsidian')}
diff --git a/app/src/components/intelligence/vaultHostMatch.test.ts b/app/src/components/intelligence/vaultHostMatch.test.ts
new file mode 100644
index 000000000..ba230a35e
--- /dev/null
+++ b/app/src/components/intelligence/vaultHostMatch.test.ts
@@ -0,0 +1,88 @@
+/**
+ * Unit tests for cross-host vault detection (#4278).
+ */
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+import { isVaultLocalToThisDevice, normalizeOs, resolveVaultHostMatch } from './vaultHostMatch';
+
+const isTauriMock = vi.fn();
+const platformMock = vi.fn();
+
+vi.mock('../../utils/tauriCommands/common', () => ({ isTauri: () => isTauriMock() }));
+vi.mock('@tauri-apps/plugin-os', () => ({ platform: () => platformMock() }));
+
+describe('normalizeOs', () => {
+  it('folds known aliases to canonical tokens', () => {
+    expect(normalizeOs('macos')).toBe('macos');
+    expect(normalizeOs('Darwin')).toBe('macos');
+    expect(normalizeOs('win32')).toBe('windows');
+    expect(normalizeOs('Windows')).toBe('windows');
+    expect(normalizeOs('linux')).toBe('linux');
+  });
+
+  it('returns undefined for empty / unknown values', () => {
+    expect(normalizeOs(undefined)).toBeUndefined();
+    expect(normalizeOs(null)).toBeUndefined();
+    expect(normalizeOs('')).toBeUndefined();
+    expect(normalizeOs('plan9')).toBeUndefined();
+  });
+});
+
+describe('isVaultLocalToThisDevice', () => {
+  it('is true only when both OSes are known and equal', () => {
+    expect(isVaultLocalToThisDevice('macos', 'macos')).toBe(true);
+    expect(isVaultLocalToThisDevice('linux', 'linux')).toBe(true);
+  });
+
+  it('is false when the OSes differ', () => {
+    expect(isVaultLocalToThisDevice('macos', 'linux')).toBe(false);
+    expect(isVaultLocalToThisDevice('linux', 'windows')).toBe(false);
+  });
+
+  it('defaults to local (true) when either OS is unknown — never block on missing signal', () => {
+    expect(isVaultLocalToThisDevice(undefined, 'macos')).toBe(true);
+    expect(isVaultLocalToThisDevice('macos', undefined)).toBe(true);
+    expect(isVaultLocalToThisDevice(undefined, undefined)).toBe(true);
+  });
+});
+
+describe('resolveVaultHostMatch', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('treats a vault as local when the core reports no host_os (older core)', async () => {
+    isTauriMock.mockReturnValue(true);
+    const m = await resolveVaultHostMatch(undefined);
+    expect(m).toEqual({ local: true });
+    expect(platformMock).not.toHaveBeenCalled();
+  });
+
+  it('treats a vault as local outside Tauri', async () => {
+    isTauriMock.mockReturnValue(false);
+    const m = await resolveVaultHostMatch('linux');
+    expect(m).toEqual({ local: true, hostOs: 'linux' });
+    expect(platformMock).not.toHaveBeenCalled();
+  });
+
+  it('flags cross-host when the core OS differs from this device', async () => {
+    isTauriMock.mockReturnValue(true);
+    platformMock.mockResolvedValue('macos');
+    const m = await resolveVaultHostMatch('linux');
+    expect(m).toEqual({ local: false, hostOs: 'linux' });
+  });
+
+  it('reports local when the core OS matches this device', async () => {
+    isTauriMock.mockReturnValue(true);
+    platformMock.mockResolvedValue('linux');
+    const m = await resolveVaultHostMatch('linux');
+    expect(m).toEqual({ local: true, hostOs: 'linux' });
+  });
+
+  it('falls back to local when the platform probe throws', async () => {
+    isTauriMock.mockReturnValue(true);
+    platformMock.mockRejectedValue(new Error('no os'));
+    const m = await resolveVaultHostMatch('windows');
+    expect(m).toEqual({ local: true, hostOs: 'windows' });
+  });
+});
diff --git a/app/src/components/intelligence/vaultHostMatch.ts b/app/src/components/intelligence/vaultHostMatch.ts
new file mode 100644
index 000000000..9bd4508a7
--- /dev/null
+++ b/app/src/components/intelligence/vaultHostMatch.ts
@@ -0,0 +1,86 @@
+/**
+ * Cross-host vault awareness (issue #4278).
+ *
+ * `openhuman-core` owns a single local filesystem. The memory-tree / Obsidian
+ * vault physically lives under the CORE host's filesystem, and the vault RPCs
+ * return that host's absolute path (`content_root_abs`) plus the core's OS
+ * (`host_os`). When a frontend attaches from a DIFFERENT OS than the core, that
+ * path cannot be opened or registered locally — "Reveal Folder" / "Open in
+ * Obsidian" would act on a path that does not exist on this machine.
+ *
+ * These helpers compare the core's OS against this frontend's OS so the local
+ * file integrations can be disabled with a clear message instead of silently
+ * firing a doomed deep link or a cryptic opener failure.
+ *
+ * Backward compatibility: when `host_os` is absent (older core) or this
+ * frontend's OS cannot be determined, treat the vault as local — never block a
+ * single-host user on missing signal.
+ */
+import { platform } from '@tauri-apps/plugin-os';
+
+import { isTauri } from '../../utils/tauriCommands/common';
+
+/** Canonical OS tokens shared by Rust `std::env::consts::OS` and plugin-os. */
+export type OsToken = 'macos' | 'linux' | 'windows';
+
+/**
+ * Normalize an OS string to a canonical token. Rust's `std::env::consts::OS`
+ * and `@tauri-apps/plugin-os` both emit `"macos"`/`"linux"`/`"windows"`, but we
+ * defensively fold common aliases (`darwin`, `win32`, …) so a future source
+ * change can't silently break the comparison.
+ */
+export function normalizeOs(os: string | undefined | null): OsToken | undefined {
+  if (!os) return undefined;
+  const v = os.trim().toLowerCase();
+  if (v === 'macos' || v === 'darwin' || v === 'mac' || v === 'osx') return 'macos';
+  if (v === 'windows' || v === 'win' || v === 'win32') return 'windows';
+  if (v === 'linux') return 'linux';
+  return undefined;
+}
+
+/**
+ * Pure decision: is a vault whose path lives on `hostOs` local to a frontend
+ * running on `clientOs`?
+ *
+ * Returns `true` (local — safe to open) when either OS is unknown, so a missing
+ * signal never disables local actions for a genuine single-host setup.
+ */
+export function isVaultLocalToThisDevice(
+  hostOs: string | undefined | null,
+  clientOs: string | undefined | null
+): boolean {
+  const host = normalizeOs(hostOs);
+  const client = normalizeOs(clientOs);
+  if (!host || !client) return true;
+  return host === client;
+}
+
+/** Resolved cross-host state for a vault response. */
+export interface VaultHostMatch {
+  /** True when the vault path is openable on this device (same OS, or unknown). */
+  local: boolean;
+  /** Normalized core host OS, when known. */
+  hostOs?: OsToken;
+}
+
+/**
+ * Resolve the cross-host state for a vault response carrying `host_os`.
+ *
+ * Reads this frontend's OS via `@tauri-apps/plugin-os`. Outside Tauri, or if
+ * the platform probe throws, falls back to "local" so non-desktop / preview
+ * contexts are never blocked.
+ */
+export async function resolveVaultHostMatch(
+  hostOs: string | undefined | null
+): Promise<VaultHostMatch> {
+  const normalizedHost = normalizeOs(hostOs);
+  if (!normalizedHost) return { local: true };
+  if (!isTauri()) return { local: true, hostOs: normalizedHost };
+  try {
+    const clientOs = await platform();
+    return { local: isVaultLocalToThisDevice(normalizedHost, clientOs), hostOs: normalizedHost };
+  } catch {
+    // Can't determine this device's OS — don't block local actions.
+    return { local: true, hostOs: normalizedHost };
+  }
+}
diff --git a/app/src/lib/i18n/ar.ts b/app/src/lib/i18n/ar.ts
index 7f850db83..bddb61162 100644
--- a/app/src/lib/i18n/ar.ts
+++ b/app/src/lib/i18n/ar.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // Arabic (العربية) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': 'الخزنة موجودة على مضيف النواة.',
+  'crossHostVault.message':
+    'يتم تخزين خزنة الذاكرة هذه على مضيف openhuman-core ({os}). لا يمكن فتحها أو عرضها إلا على ذلك الجهاز، وليس من هذا الجهاز.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'شارك ملاحظاتك',
   'feedback.board': 'لوحة الملاحظات',
diff --git a/app/src/lib/i18n/bn.ts b/app/src/lib/i18n/bn.ts
index 557e86974..5c1e18f09 100644
--- a/app/src/lib/i18n/bn.ts
+++ b/app/src/lib/i18n/bn.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // Bengali (বাংলা) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': 'ভল্টটি কোর হোস্টে রয়েছে।',
+  'crossHostVault.message':
+    'এই মেমরি ভল্টটি openhuman-core হোস্টে ({os}) সংরক্ষিত আছে। এটি কেবল সেই মেশিনেই খোলা বা দেখানো যায়, এই ডিভাইস থেকে নয়।',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'মতামত দিন',
   'feedback.board': 'মতামত বোর্ড',
diff --git a/app/src/lib/i18n/de.ts b/app/src/lib/i18n/de.ts
index fa572e995..573aa4e95 100644
--- a/app/src/lib/i18n/de.ts
+++ b/app/src/lib/i18n/de.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // German (Deutsch) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': 'Der Vault liegt auf dem Core-Host.',
+  'crossHostVault.message':
+    'Dieser Memory-Vault wird auf dem openhuman-core-Host ({os}) gespeichert. Er kann nur auf diesem Rechner geöffnet oder angezeigt werden, nicht von diesem Gerät.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Feedback geben',
   'feedback.board': 'Feedback-Board',
diff --git a/app/src/lib/i18n/en.ts b/app/src/lib/i18n/en.ts
index 205971076..71186f555 100644
--- a/app/src/lib/i18n/en.ts
+++ b/app/src/lib/i18n/en.ts
@@ -2904,6 +2904,11 @@ const en: TranslationMap = {
   'vaultHealth.timeDayAgo': '{n} day ago',
   'vaultHealth.timeDaysAgo': '{n} days ago',
 
+  // Cross-host vault (#4278) — shared by VaultHealthChecklist + ObsidianVaultSection
+  'crossHostVault.title': 'Vault is on the core host.',
+  'crossHostVault.message':
+    'This memory vault is stored on the openhuman-core host ({os}). It can only be opened or revealed on that machine, not from this device.',
+
   // Memory data panel (storage explainer)
   'memoryData.howItWorks': 'How memory storage works',
   'memoryData.workspaceVault': 'Workspace vault · write',
diff --git a/app/src/lib/i18n/es.ts b/app/src/lib/i18n/es.ts
index 5b74a42f6..97da7e5ee 100644
--- a/app/src/lib/i18n/es.ts
+++ b/app/src/lib/i18n/es.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // Spanish (Español) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': 'El vault está en el host del core.',
+  'crossHostVault.message':
+    'Este vault de memoria se almacena en el host de openhuman-core ({os}). Solo se puede abrir o mostrar en esa máquina, no desde este dispositivo.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Compartir opiniones',
   'feedback.board': 'Tablero de opiniones',
diff --git a/app/src/lib/i18n/fr.ts b/app/src/lib/i18n/fr.ts
index 3c9e34f1e..85321c979 100644
--- a/app/src/lib/i18n/fr.ts
+++ b/app/src/lib/i18n/fr.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // French (Français) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': "Le coffre se trouve sur l'hôte du cœur.",
+  'crossHostVault.message':
+    "Ce coffre de mémoire est stocké sur l'hôte openhuman-core ({os}). Il ne peut être ouvert ou affiché que sur cette machine, pas depuis cet appareil.",
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Donner mon avis',
   'feedback.board': 'Tableau des suggestions',
diff --git a/app/src/lib/i18n/hi.ts b/app/src/lib/i18n/hi.ts
index 3b3757144..bba631c70 100644
--- a/app/src/lib/i18n/hi.ts
+++ b/app/src/lib/i18n/hi.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // Hindi (हिन्दी) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': 'वॉल्ट कोर होस्ट पर है।',
+  'crossHostVault.message':
+    'यह मेमोरी वॉल्ट openhuman-core होस्ट ({os}) पर संग्रहीत है। इसे केवल उसी मशीन पर खोला या दिखाया जा सकता है, इस डिवाइस से नहीं।',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'फ़ीडबैक साझा करें',
   'feedback.board': 'फ़ीडबैक बोर्ड',
diff --git a/app/src/lib/i18n/id.ts b/app/src/lib/i18n/id.ts
index 6187f98e9..7e18e7861 100644
--- a/app/src/lib/i18n/id.ts
+++ b/app/src/lib/i18n/id.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // Indonesian (Bahasa Indonesia) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': 'Vault berada di host core.',
+  'crossHostVault.message':
+    'Vault memori ini disimpan di host openhuman-core ({os}). Hanya dapat dibuka atau ditampilkan di mesin tersebut, bukan dari perangkat ini.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Bagikan masukan',
   'feedback.board': 'Papan masukan',
diff --git a/app/src/lib/i18n/it.ts b/app/src/lib/i18n/it.ts
index 2b61615ac..8451b81e8 100644
--- a/app/src/lib/i18n/it.ts
+++ b/app/src/lib/i18n/it.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // Italian (Italiano) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': "Il vault è sull'host del core.",
+  'crossHostVault.message':
+    "Questo vault di memoria è archiviato sull'host openhuman-core ({os}). Può essere aperto o mostrato solo su quella macchina, non da questo dispositivo.",
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Condividi feedback',
   'feedback.board': 'Bacheca dei feedback',
diff --git a/app/src/lib/i18n/ko.ts b/app/src/lib/i18n/ko.ts
index 0cb3b706e..62683a6e0 100644
--- a/app/src/lib/i18n/ko.ts
+++ b/app/src/lib/i18n/ko.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // Korean (한국어) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': '보관소가 코어 호스트에 있습니다.',
+  'crossHostVault.message':
+    '이 메모리 보관소는 openhuman-core 호스트({os})에 저장되어 있습니다. 해당 컴퓨터에서만 열거나 표시할 수 있으며 이 기기에서는 불가능합니다.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': '피드백 보내기',
   'feedback.board': '피드백 보드',
diff --git a/app/src/lib/i18n/pl.ts b/app/src/lib/i18n/pl.ts
index e4aaf36b1..4aeac42dc 100644
--- a/app/src/lib/i18n/pl.ts
+++ b/app/src/lib/i18n/pl.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // Polish (Polski) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': 'Skarbiec znajduje się na hoście rdzenia.',
+  'crossHostVault.message':
+    'Ten skarbiec pamięci jest przechowywany na hoście openhuman-core ({os}). Można go otworzyć lub pokazać tylko na tym komputerze, a nie z tego urządzenia.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Podziel się opinią',
   'feedback.board': 'Tablica opinii',
diff --git a/app/src/lib/i18n/pt.ts b/app/src/lib/i18n/pt.ts
index 5c67c9f52..965889a9d 100644
--- a/app/src/lib/i18n/pt.ts
+++ b/app/src/lib/i18n/pt.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // Portuguese (Português) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': 'O vault está no host do core.',
+  'crossHostVault.message':
+    'Este vault de memória fica armazenado no host openhuman-core ({os}). Só pode ser aberto ou exibido nessa máquina, não a partir deste dispositivo.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Enviar feedback',
   'feedback.board': 'Quadro de feedback',
diff --git a/app/src/lib/i18n/ru.ts b/app/src/lib/i18n/ru.ts
index e46f59de3..c493d116e 100644
--- a/app/src/lib/i18n/ru.ts
+++ b/app/src/lib/i18n/ru.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // Russian (Русский) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': 'Хранилище находится на хосте ядра.',
+  'crossHostVault.message':
+    'Это хранилище памяти размещено на хосте openhuman-core ({os}). Его можно открыть или показать только на той машине, но не с этого устройства.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Поделиться отзывом',
   'feedback.board': 'Доска отзывов',
diff --git a/app/src/lib/i18n/zh-CN.ts b/app/src/lib/i18n/zh-CN.ts
index a3812d248..b56d2f7ce 100644
--- a/app/src/lib/i18n/zh-CN.ts
+++ b/app/src/lib/i18n/zh-CN.ts
@@ -3,6 +3,10 @@ import type { TranslationMap } from './types';
 // Simplified Chinese (简体中文) translations. Keys mirror en.ts; missing/
 // English-identical values fall back to English via I18nContext.resolveEn().
 const messages: TranslationMap = {
+  // Cross-host vault (#4278)
+  'crossHostVault.title': '记忆库位于核心主机上。',
+  'crossHostVault.message':
+    '此记忆库存储在 openhuman-core 主机（{os}）上。只能在该机器上打开或显示，无法从本设备访问。',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': '分享反馈',
   'feedback.board': '反馈板',
diff --git a/app/src/utils/openUrl.test.ts b/app/src/utils/openUrl.test.ts
index 05b661da2..cc6da9d40 100644
--- a/app/src/utils/openUrl.test.ts
+++ b/app/src/utils/openUrl.test.ts
@@ -11,6 +11,7 @@ const isTauriMock = vi.fn();
 const tauriOpenUrlMock = vi.fn();
 const revealItemInDirMock = vi.fn();
 const addBreadcrumbMock = vi.fn();
+const platformMock = vi.fn();
 
 vi.mock('./tauriCommands/common', () => ({ isTauri: () => isTauriMock() }));
 
@@ -19,6 +20,8 @@ vi.mock('@tauri-apps/plugin-opener', () => ({
   revealItemInDir: (path: string) => revealItemInDirMock(path),
 }));
 
+vi.mock('@tauri-apps/plugin-os', () => ({ platform: () => platformMock() }));
+
 vi.mock('@sentry/react', () => ({
   addBreadcrumb: (...args: unknown[]) => addBreadcrumbMock(...args),
 }));
@@ -29,6 +32,9 @@ describe('openUrl', () => {
 
   beforeEach(() => {
     vi.clearAllMocks();
+    // Default this device to macOS so existing POSIX-path reveal tests pass; the
+    // #4278 cross-host tests override per-case.
+    platformMock.mockResolvedValue('macos');
     originalWindowOpen = window.open;
     windowOpenMock = vi.fn();
     window.open = windowOpenMock as unknown as typeof window.open;
@@ -154,6 +160,46 @@ describe('openUrl', () => {
     await expect(revealPath('/Users/me/Vault')).rejects.toThrow('reveal failed');
   });
 
+  // #4278: a shared openhuman-core on a different OS serves its own absolute
+  // path; revealing it locally must fail with a clear error, not cryptically.
+  it('revealPath rejects a foreign-OS path instead of revealing it (#4278)', async () => {
+    isTauriMock.mockReturnValue(true);
+    platformMock.mockResolvedValue('windows'); // core served a POSIX path to a Windows frontend
+    revealItemInDirMock.mockResolvedValue(undefined);
+
+    const { revealPath } = await import('./openUrl');
+    await expect(revealPath('/home/leigh/OHvault')).rejects.toThrow(/openhuman-core host/);
+    expect(revealItemInDirMock).not.toHaveBeenCalled();
+  });
+
+  it('revealPath still reveals a path native to this device (#4278 regression)', async () => {
+    isTauriMock.mockReturnValue(true);
+    platformMock.mockResolvedValue('windows');
+    revealItemInDirMock.mockResolvedValue(undefined);
+
+    const { revealPath } = await import('./openUrl');
+    await revealPath('C:\\Users\\me\\Vault');
+
+    expect(revealItemInDirMock).toHaveBeenCalledWith('C:\\Users\\me\\Vault');
+  });
+
+  describe('isForeignFsPath', () => {
+    it('flags a POSIX path on Windows and a Windows path on POSIX', async () => {
+      const { isForeignFsPath } = await import('./openUrl');
+      expect(isForeignFsPath('/home/leigh/OHvault', 'windows')).toBe(true);
+      expect(isForeignFsPath('C:\\Users\\me\\Vault', 'macos')).toBe(true);
+      expect(isForeignFsPath('\\\\server\\share', 'linux')).toBe(true);
+    });
+
+    it('treats same-family paths as local and is permissive on unknown OS', async () => {
+      const { isForeignFsPath } = await import('./openUrl');
+      expect(isForeignFsPath('/Users/me/Vault', 'macos')).toBe(false);
+      expect(isForeignFsPath('C:\\x', 'windows')).toBe(false);
+      expect(isForeignFsPath('/home/x', undefined)).toBe(false);
+      expect(isForeignFsPath('', 'windows')).toBe(false);
+    });
+  });
+
   it('trims surrounding whitespace before classifying an http URL for fallback', async () => {
     isTauriMock.mockReturnValue(true);
     tauriOpenUrlMock.mockRejectedValue(
diff --git a/app/src/utils/openUrl.ts b/app/src/utils/openUrl.ts
index 0d8c553ac..a55ef1d4f 100644
--- a/app/src/utils/openUrl.ts
+++ b/app/src/utils/openUrl.ts
@@ -1,5 +1,6 @@
 import * as Sentry from '@sentry/react';
 import { revealItemInDir, openUrl as tauriOpenUrl } from '@tauri-apps/plugin-opener';
+import { platform } from '@tauri-apps/plugin-os';
 
 import { isTauri } from './tauriCommands/common';
 
@@ -70,6 +71,27 @@ export const openUrl = async (url: string): Promise<void> => {
   window.open(normalizedUrl, '_blank', 'noopener,noreferrer');
 };
 
+/**
+ * Detects a filesystem path that belongs to a different OS family than the one
+ * running this frontend — a Windows path (`C:\…` or a `\\UNC` share) on a POSIX
+ * host, or a POSIX absolute path (`/…`) on Windows.
+ *
+ * This is the cross-host guard for issue #4278: `openhuman-core` can serve a
+ * path that lives on its own (possibly different-OS) host, and revealing such a
+ * path locally would fail with a cryptic opener error. Returns `false` when the
+ * OS is unknown so we never block a legitimate same-host reveal.
+ */
+export const isForeignFsPath = (path: string, clientOs: string | undefined): boolean => {
+  const p = path.trim();
+  if (!p) return false;
+  const looksWindows = /^[a-zA-Z]:[\\/]/.test(p) || /^\\\\/.test(p);
+  const looksPosixAbs = p.startsWith('/');
+  const os = (clientOs ?? '').toLowerCase();
+  if (os === 'windows') return looksPosixAbs && !looksWindows;
+  if (os === 'macos' || os === 'linux') return looksWindows;
+  return false;
+};
+
 /**
  * Reveals a filesystem path in the host OS file manager
  * (Finder on macOS, Explorer on Windows, the default file manager on
@@ -78,8 +100,23 @@ export const openUrl = async (url: string): Promise<void> => {
  * target app isn't installed.
  *
  * Outside Tauri this is a no-op — there's no OS shell to drive.
+ *
+ * Rejects with a clear error when `path` belongs to a different OS than this
+ * device (issue #4278) — e.g. a shared `openhuman-core` running on another OS
+ * served its own absolute path — instead of letting the opener fail cryptically.
  */
 export const revealPath = async (path: string): Promise<void> => {
   if (!isTauri()) return;
+  let clientOs: string | undefined;
+  try {
+    clientOs = await platform();
+  } catch {
+    clientOs = undefined;
+  }
+  if (isForeignFsPath(path, clientOs)) {
+    throw new Error(
+      `Cannot reveal "${path}" on this device — it is a path on the openhuman-core host's filesystem (a different OS). Open it on the machine running the core.`
+    );
+  }
   await revealItemInDir(path);
 };
diff --git a/app/src/utils/tauriCommands/memoryTree.ts b/app/src/utils/tauriCommands/memoryTree.ts
index 697bb4214..719170e00 100644
--- a/app/src/utils/tauriCommands/memoryTree.ts
+++ b/app/src/utils/tauriCommands/memoryTree.ts
@@ -638,6 +638,13 @@ export interface ObsidianVaultStatus {
   config_found: boolean;
   /** Absolute filesystem path to `<workspace>/memory_tree/content/`. */
   content_root_abs: string;
+  /**
+   * OS of the core host (`"macos"` / `"linux"` / `"windows"`). `content_root_abs`
+   * is a path on that host; when the frontend runs on a different OS the vault
+   * cannot be opened/registered locally (issue #4278). Optional for backward
+   * compatibility with cores that predate this field.
+   */
+  host_os?: string;
 }
 
 /**
@@ -687,6 +694,13 @@ export interface VaultHealthCheck {
   pipeline_healthy: boolean;
   /** Epoch ms of newest chunk timestamp; zero when no chunks exist yet. */
   last_sync_ms: number;
+  /**
+   * OS of the core host (`"macos"` / `"linux"` / `"windows"`). `exists` /
+   * `readable` / `writable` describe that host's filesystem; a frontend on a
+   * different OS cannot open `content_root_abs` locally even when they report
+   * healthy (issue #4278). Optional for backward compatibility.
+   */
+  host_os?: string;
 }
 
 /**
diff --git a/src/openhuman/memory/read_rpc/types.rs b/src/openhuman/memory/read_rpc/types.rs
index 173487096..d2e9b03b4 100644
--- a/src/openhuman/memory/read_rpc/types.rs
+++ b/src/openhuman/memory/read_rpc/types.rs
@@ -160,6 +160,11 @@ pub struct ObsidianVaultStatusResponse {
     pub registered: bool,
     pub config_found: bool,
     pub content_root_abs: String,
+    /// OS of the host the core runs on (`std::env::consts::OS`:
+    /// `"macos"` / `"linux"` / `"windows"`). `content_root_abs` is a path on
+    /// THIS host's filesystem; a frontend attached from a different OS must not
+    /// treat it as a local path (issue #4278).
+    pub host_os: String,
 }
 
 /// Response shape for [`vault_health_check_rpc`].
@@ -172,4 +177,9 @@ pub struct VaultHealthCheckResponse {
     pub obsidian_registered: bool,
     pub pipeline_healthy: bool,
     pub last_sync_ms: i64,
+    /// OS of the host the core runs on (`std::env::consts::OS`). The
+    /// `exists` / `readable` / `writable` probes describe THIS host's
+    /// filesystem; a frontend on a different OS cannot open `content_root_abs`
+    /// locally even when these report healthy (issue #4278).
+    pub host_os: String,
 }
diff --git a/src/openhuman/memory/read_rpc/vault.rs b/src/openhuman/memory/read_rpc/vault.rs
index 0934a32c7..10b5930d4 100644
--- a/src/openhuman/memory/read_rpc/vault.rs
+++ b/src/openhuman/memory/read_rpc/vault.rs
@@ -23,6 +23,7 @@ pub async fn obsidian_vault_status_rpc(
             registered: reg.registered,
             config_found: reg.config_found,
             content_root_abs: content_root.to_string_lossy().to_string(),
+            host_os: std::env::consts::OS.to_string(),
         }
     })
     .await
@@ -84,6 +85,7 @@ pub async fn vault_health_check_rpc(
         obsidian_registered,
         pipeline_healthy,
         last_sync_ms,
+        host_os: std::env::consts::OS.to_string(),
     };
 
     let log = format!(
diff --git a/src/openhuman/memory/read_rpc_tests.rs b/src/openhuman/memory/read_rpc_tests.rs
index b353e3130..07ed603e6 100644
--- a/src/openhuman/memory/read_rpc_tests.rs
+++ b/src/openhuman/memory/read_rpc_tests.rs
@@ -913,6 +913,24 @@ async fn vault_health_check_reports_missing_content_root_for_fresh_workspace() {
     assert_eq!(outcome.value.last_sync_ms, 0);
 }
 
+/// #4278: both vault RPCs stamp the core host's OS so a frontend attached
+/// from a different OS can tell `content_root_abs` is a foreign-host path and
+/// must not open/reveal it locally.
+#[tokio::test]
+async fn vault_rpcs_report_core_host_os() {
+    let (_tmp, cfg) = test_config();
+
+    let status = obsidian_vault_status_rpc(&cfg, None).await.unwrap();
+    assert_eq!(status.value.host_os, std::env::consts::OS);
+
+    let health = vault_health_check_rpc(&cfg, None).await.unwrap();
+    assert_eq!(health.value.host_os, std::env::consts::OS);
+    assert!(
+        !health.value.host_os.is_empty(),
+        "host_os must be populated"
+    );
+}
+
 #[tokio::test]
 async fn vault_health_check_reports_writable_and_obsidian_registered_when_ready() {
     let (_tmp, cfg) = test_config();