@@ -72,6 +72,9 @@ mac:
     NSScreenCaptureUsageDescription: Mothership captures approved app windows to understand and verify computer-use actions.
     NSMicrophoneUsageDescription: Sim uses your microphone for voice input in Chat and for meetings you join in the built-in browser.
     NSCameraUsageDescription: Sim uses your camera for meetings you join in the built-in browser, such as Google Meet.
+    # macOS 15 blocks connections to private addresses (self-hosted servers on a
+    # LAN or VPN) until the user allows Sim; this string explains that prompt.
+    NSLocalNetworkUsageDescription: Sim connects to your self-hosted Sim server and to local addresses you open in the built-in browser.
   entitlements: build/entitlements.mac.plist
   entitlementsInherit: build/entitlements.mac.plist
   notarize: true
 
@@ -7,6 +7,11 @@ describe('classifyLoadError', () => {
     expect(classifyLoadError(-2)).toBe('unreachable')
     expect(classifyLoadError(-1)).toBe('unreachable')
   })
+
+  it('reports ERR_ADDRESS_UNREACHABLE as a local-network block only under Local Network privacy', () => {
+    expect(classifyLoadError(-109, true)).toBe('local_network')
+    expect(classifyLoadError(-109, false)).toBe('unreachable')
+  })
 })
 
 vi.mock('electron', () => import('@/test/electron-mock'))
 
@@ -1,3 +1,4 @@
+import { release } from 'node:os'
 import { createLogger } from '@sim/logger'
 import type { BrowserWindow } from 'electron'
 import { type EventRecorder, scrubUrl } from '@/main/observability'
@@ -6,14 +7,33 @@ const logger = createLogger('DesktopLoadHealth')
 
 const AUTO_RETRY_INTERVAL_MS = 5000
 const LOAD_WATCHDOG_MS = 30_000
+/** Darwin 24 is macOS 15, the first release that gates apps behind Local Network privacy. */
+const FIRST_LOCAL_NETWORK_PRIVACY_DARWIN = 24
 
-export type LoadErrorKind = 'offline' | 'dns' | 'tls' | 'timeout' | 'unreachable' | 'ignored'
+function hasLocalNetworkPrivacy(): boolean {
+  return (
+    process.platform === 'darwin' &&
+    Number.parseInt(release(), 10) >= FIRST_LOCAL_NETWORK_PRIVACY_DARWIN
+  )
+}
+
+export type LoadErrorKind =
+  | 'offline'
+  | 'dns'
+  | 'tls'
+  | 'timeout'
+  | 'local_network'
+  | 'unreachable'
+  | 'ignored'
 
 /**
  * Maps Chromium net error codes to recovery copy. -3 (ERR_ABORTED) is emitted
  * constantly by OAuth redirect chains and in-app aborts and must be ignored.
  */
-export function classifyLoadError(errorCode: number): LoadErrorKind {
+export function classifyLoadError(
+  errorCode: number,
+  localNetworkPrivacy: boolean = hasLocalNetworkPrivacy()
+): LoadErrorKind {
   // Only ERR_ABORTED (-3) and success (0) are ignored. ERR_FAILED (-2) and
   // ERR_IO_PENDING (-1) are real failures that must surface the offline page.
   if (errorCode === 0 || errorCode === -3) {
@@ -31,6 +51,12 @@ export function classifyLoadError(errorCode: number): LoadErrorKind {
   if (errorCode <= -200 && errorCode >= -213) {
     return 'tls'
   }
+  // macOS Local Network privacy fails the socket with EHOSTUNREACH, which
+  // Chromium reports as ERR_ADDRESS_UNREACHABLE (-109), so a self-hosted server
+  // on a private address looks like a routing failure until Sim is allowed.
+  if (errorCode === -109 && localNetworkPrivacy) {
+    return 'local_network'
+  }
   return 'unreachable'
 }
 
 
@@ -30,6 +30,11 @@ const ERROR_COPY = {
     title: 'The server isn’t responding',
     message: 'The connection timed out. The server may be down or your network may be blocking it.',
   },
+  local_network: {
+    title: 'Can’t reach the server',
+    message:
+      'If your Sim server is on a local or private network, macOS may be blocking Sim. Turn on Sim in System Settings > Privacy & Security > Local Network, then try again.',
+  },
   unreachable: {
     title: 'Can’t connect to Sim',
     message: 'Sim couldn’t reach the server. Check your internet connection, then try again.',