diff --git a/apps/desktop/electron/event-dedupe.test.ts b/apps/desktop/electron/event-dedupe.test.ts
new file mode 100644
index 000000000000..3c0f905587e3
--- /dev/null
+++ b/apps/desktop/electron/event-dedupe.test.ts
@@ -0,0 +1,37 @@
+import assert from 'node:assert/strict'
+
+import { test } from 'vitest'
+
+import { createEventDeduper } from './event-dedupe'
+
+test('collapses the same key inside the window (two windows, one event)', () => {
+  const isDup = createEventDeduper(1000)
+
+  assert.equal(isDup('input:s1', 0), false, 'first window claims')
+  assert.equal(isDup('input:s1', 5), true, 'second window is deduped')
+})
+
+test('distinct keys are independent', () => {
+  const isDup = createEventDeduper(1000)
+
+  assert.equal(isDup('input:s1', 0), false)
+  assert.equal(isDup('approval:s1', 0), false, 'different kind')
+  assert.equal(isDup('input:s2', 0), false, 'different session')
+})
+
+test('re-fires once the window elapses', () => {
+  const isDup = createEventDeduper(1000)
+
+  assert.equal(isDup('turnDone:s1', 0), false)
+  assert.equal(isDup('turnDone:s1', 999), true, 'still within window')
+  assert.equal(isDup('turnDone:s1', 1000), false, 'window elapsed → fires again')
+})
+
+test('prunes stale keys so the map cannot grow unbounded', () => {
+  const isDup = createEventDeduper(1000)
+
+  for (let i = 0; i < 100; i += 1) {
+    // Each far-apart key is pruned before the next, so none linger as duplicates.
+    assert.equal(isDup(`turnDone:s${i}`, i * 2000), false)
+  }
+})
diff --git a/apps/desktop/electron/event-dedupe.ts b/apps/desktop/electron/event-dedupe.ts
new file mode 100644
index 000000000000..a2d12a8f2cba
--- /dev/null
+++ b/apps/desktop/electron/event-dedupe.ts
@@ -0,0 +1,32 @@
+// Cross-window de-dupe for one-shot side-effects (OS notifications, the turn-end
+// sound, spoken replies). Every desktop window is its own renderer process, so N
+// open windows each independently react to the same backend event. The main
+// process is the one place they all share and it handles IPC serially, so it's
+// the race-free owner: the first window to claim a key within the interval wins;
+// peers see it's taken and stay quiet. Pure + injectable clock, so it's
+// unit-testable without Electron.
+
+const DEDUPE_INTERVAL_MS = 1000
+
+// Returns true when `key` was already claimed within the interval (caller drops
+// this one). Self-evicting: stale keys are pruned on every call, so the map
+// can't grow unbounded.
+export function createEventDeduper(intervalMs = DEDUPE_INTERVAL_MS) {
+  const lastSeenAt = new Map<string, number>()
+
+  return function isDuplicate(key: string, now = Date.now()): boolean {
+    for (const [k, at] of lastSeenAt) {
+      if (now - at >= intervalMs) {
+        lastSeenAt.delete(k)
+      }
+    }
+
+    if (lastSeenAt.has(key)) {
+      return true
+    }
+
+    lastSeenAt.set(key, now)
+
+    return false
+  }
+}
diff --git a/apps/desktop/electron/main.ts b/apps/desktop/electron/main.ts
index b67e8b69f3a7..141497cf6b4d 100644
--- a/apps/desktop/electron/main.ts
+++ b/apps/desktop/electron/main.ts
@@ -67,6 +67,7 @@ import {
   uninstallArgsForMode
 } from './desktop-uninstall'
 import { installEmbedReferer } from './embed-referer'
+import { createEventDeduper } from './event-dedupe'
 import { readDirForIpc } from './fs-read-dir'
 import { probeGatewayWebSocket } from './gateway-ws-probe'
 import { scanGitRepos } from './git-repo-scan'
@@ -112,6 +113,7 @@ import {
   buildSessionWindowUrl,
   chatWindowWebPreferences,
   createSessionWindowRegistry,
+  instanceWindowBounds,
   SESSION_WINDOW_MIN_HEIGHT,
   SESSION_WINDOW_MIN_WIDTH
 } from './session-windows'
@@ -4613,9 +4615,9 @@ function getNativeOverlayWidth() {
   return computeNativeOverlayWidth({ isWindows: IS_WINDOWS, isWsl: IS_WSL, isMac: IS_MAC })
 }
 
-function getWindowState() {
+function getWindowState(win = mainWindow) {
   return {
-    isFullscreen: Boolean(mainWindow?.isFullScreen?.()),
+    isFullscreen: Boolean(win?.isFullScreen?.()),
     nativeOverlayWidth: getNativeOverlayWidth(),
     windowButtonPosition: getWindowButtonPosition()
   }
@@ -4716,18 +4718,21 @@ function sendOpenUpdatesRequested() {
   mainWindow.focus()
 }
 
-function sendWindowStateChanged(nextIsFullscreen?: boolean) {
-  if (!mainWindow || mainWindow.isDestroyed()) {
+// Push titlebar/fullscreen chrome state to a window's renderer. Defaults to the
+// primary, but any full chat window (primary or a secondary "instance" peer)
+// passes itself so its own fullscreen toggle drives its own traffic-light inset.
+function sendWindowStateChanged(nextIsFullscreen?: boolean, target = mainWindow) {
+  if (!target || target.isDestroyed()) {
     return
   }
 
-  const { webContents } = mainWindow
+  const { webContents } = target
 
   if (!webContents || webContents.isDestroyed()) {
     return
   }
 
-  const state = getWindowState()
+  const state = getWindowState(target)
 
   if (typeof nextIsFullscreen === 'boolean') {
     state.isFullscreen = nextIsFullscreen
@@ -4765,6 +4770,11 @@ function buildApplicationMenu() {
   template.push({
     label: 'File',
     submenu: [
+      // No accelerator: ⌘⇧N is a rebindable renderer keybind (session.newWindow);
+      // a menu accelerator would fight the rebind panel and (on macOS) be
+      // swallowed before the renderer sees it. Here purely for discoverability.
+      { click: () => createInstanceWindow(), label: 'New Window' },
+      { type: 'separator' },
       IS_MAC
         ? {
             // NO accelerator: on macOS a registered ⌘W is consumed by the OS
@@ -7329,11 +7339,7 @@ function focusWindow(win) {
   win.focus()
 }
 
-function spawnSecondaryWindow({
-  sessionId,
-  watch,
-  newSession
-}: { sessionId?: string; watch?: boolean; newSession?: boolean } = {}) {
+function spawnSecondaryWindow({ sessionId, watch }: { sessionId?: string; watch?: boolean } = {}) {
   const icon = getAppIconPath()
 
   const win = new BrowserWindow({
@@ -7378,8 +7384,7 @@ function spawnSecondaryWindow({
     buildSessionWindowUrl(sessionId, {
       devServer: DEV_SERVER,
       rendererIndexPath: DEV_SERVER ? undefined : resolveRendererIndex(),
-      watch,
-      newSession
+      watch
     })
   )
 
@@ -7391,11 +7396,82 @@ function createSessionWindow(sessionId, { watch = false } = {}) {
   return sessionWindows.openOrFocus(sessionId, () => spawnSecondaryWindow({ sessionId, watch }))
 }
 
-// Open a fresh compact window on the new-session draft (#/). Not registry-keyed:
-// like ⌘N in a browser, every press opens a new window — and a draft window that
-// later converts to a real session must not get refocused as if it were blank.
-function createNewSessionWindow() {
-  return spawnSecondaryWindow({ newSession: true })
+// Additional full "instance" windows — peers of the primary that render the
+// COMPLETE app (sidebar, routing, its own draft) against the shared backend, so
+// a user can run multiple GUI windows at once (⌘⇧N / the "New Window" palette
+// command). Unlike the compact session windows they carry no `?win` flag. The
+// primary mainWindow stays the notification / deep-link / pet-overlay anchor and
+// is NOT tracked here. The set holds a strong reference so an open peer isn't
+// garbage-collected, and drops it on close.
+const instanceWindows = new Set<any>()
+
+// Cascade a new instance off whichever window spawned it so it doesn't land
+// exactly on top of its source. Falls back to the persisted primary geometry
+// when there's no live source window (e.g. all windows closed on macOS). The
+// pure cascade math lives in session-windows.ts (instanceWindowBounds).
+function nextInstanceBounds() {
+  const source = BrowserWindow.getFocusedWindow() || mainWindow
+  const fallback = computeWindowOptions(readWindowState(), screen.getAllDisplays())
+  const base = source && !source.isDestroyed() ? source.getBounds() : null
+
+  return instanceWindowBounds(base, fallback)
+}
+
+// Open a new full-chrome instance window. Mirrors createWindow()'s window
+// options (shared chatWindowWebPreferences keeps backgroundThrottling:false so a
+// streamed answer never stalls in the background) but is a peer, not the
+// primary: it never overwrites the mainWindow global, doesn't start the backend
+// (the renderer's getConnection() joins the already-running one), and loads the
+// plain renderer URL so the full app renders.
+function createInstanceWindow() {
+  const icon = getAppIconPath()
+
+  const win = new BrowserWindow({
+    ...nextInstanceBounds(),
+    minWidth: WINDOW_MIN_WIDTH,
+    minHeight: WINDOW_MIN_HEIGHT,
+    title: 'Hermes',
+    titleBarStyle: 'hidden',
+    titleBarOverlay: getTitleBarOverlayOptions(),
+    trafficLightPosition: IS_MAC ? WINDOW_BUTTON_POSITION : undefined,
+    vibrancy: IS_MAC ? 'sidebar' : undefined,
+    opacity: windowOpacity(),
+    icon,
+    show: false,
+    backgroundColor: getWindowBackgroundColor(),
+    webPreferences: chatWindowWebPreferences(PRELOAD_PATH)
+  })
+
+  instanceWindows.add(win)
+
+  if (IS_MAC) {
+    win.setWindowButtonPosition?.(WINDOW_BUTTON_POSITION)
+  }
+
+  win.once('ready-to-show', () => {
+    if (!win.isDestroyed()) {
+      win.show()
+    }
+  })
+
+  // Per-window fullscreen chrome: send this window its own titlebar inset so its
+  // traffic lights hide/show independently of the primary.
+  win.on('enter-full-screen', () => sendWindowStateChanged(true, win))
+  win.on('leave-full-screen', () => sendWindowStateChanged(false, win))
+
+  wireCommonWindowHandlers(win, zoomWiringForWindowKind('chat'))
+
+  win.on('closed', () => {
+    instanceWindows.delete(win)
+  })
+
+  if (DEV_SERVER) {
+    win.loadURL(DEV_SERVER)
+  } else {
+    win.loadURL(pathToFileURL(resolveRendererIndex()).toString())
+  }
+
+  return win
 }
 
 // The pet overlay: a single transparent, frameless, always-on-top window that
@@ -7583,7 +7659,9 @@ function createWindow() {
     if (!nativeThemeListenerInstalled) {
       nativeThemeListenerInstalled = true
       nativeTheme.on('updated', () => {
-        applyTitleBarOverlay(mainWindow)
+        for (const win of BrowserWindow.getAllWindows()) {
+          applyTitleBarOverlay(win)
+        }
       })
     }
   }
@@ -7824,8 +7902,8 @@ ipcMain.handle('hermes:window:openSession', async (_event, sessionId, opts) => {
 
   return { ok: true }
 })
-ipcMain.handle('hermes:window:openNewSession', async () => {
-  createNewSessionWindow()
+ipcMain.handle('hermes:window:openInstance', async () => {
+  createInstanceWindow()
 
   return { ok: true }
 })
@@ -8439,11 +8517,28 @@ ipcMain.handle('hermes:api', async (_event, request) => {
   })
 })
 
+// One deduper per cross-window cue — the choke point every window shares. Main
+// handles IPC serially, so the first window to claim a key wins with no race.
+const isDuplicateNotification = createEventDeduper()
+const claimedAmbientCue = createEventDeduper()
+
+// A window asks "do I own this ambient cue (turn-end sound / spoken reply)?".
+// The first caller within the window gets true; peers get false and stay quiet.
+ipcMain.handle('hermes:ambient:claim', (_event, key) => !claimedAmbientCue(String(key ?? '')))
+
 ipcMain.handle('hermes:notify', (_event, payload) => {
   if (!Notification.isSupported()) {
     return false
   }
 
+  // Multiple full windows each run their own renderer throttle, so the same
+  // kind+session can arrive here twice. Collapse it at this single choke point.
+  // Return true (not false): a notification for the event IS being shown by the
+  // first caller, so the settings "send test" success probe stays honest.
+  if (isDuplicateNotification(`${payload?.kind ?? ''}:${payload?.sessionId ?? ''}`)) {
+    return true
+  }
+
   // Action buttons render only on signed macOS builds; elsewhere they're dropped
   // and the body click still works.
   const actions = Array.isArray(payload?.actions) ? payload.actions : []
@@ -8613,7 +8708,13 @@ ipcMain.on('hermes:titlebar-theme', (_event, payload) => {
     background: payload.background,
     foreground: payload.foreground
   }
-  applyTitleBarOverlay(mainWindow)
+
+  // Repaint the native (Windows/Linux) titlebar overlay on every open chat
+  // window, not just the primary — instance peers and session windows share the
+  // one app theme. applyTitleBarOverlay no-ops on the frameless pet overlay.
+  for (const win of BrowserWindow.getAllWindows()) {
+    applyTitleBarOverlay(win)
+  }
 })
 
 // Pin the native appearance to the app theme (see NATIVE_THEME_CONFIG_PATH).
diff --git a/apps/desktop/electron/preload.ts b/apps/desktop/electron/preload.ts
index 732d13a53668..37f068ca986f 100644
--- a/apps/desktop/electron/preload.ts
+++ b/apps/desktop/electron/preload.ts
@@ -6,7 +6,8 @@ contextBridge.exposeInMainWorld('hermesDesktop', {
   touchBackend: profile => ipcRenderer.invoke('hermes:backend:touch', profile),
   getGatewayWsUrl: profile => ipcRenderer.invoke('hermes:gateway:ws-url', profile),
   openSessionWindow: (sessionId, opts) => ipcRenderer.invoke('hermes:window:openSession', sessionId, opts),
-  openNewSessionWindow: () => ipcRenderer.invoke('hermes:window:openNewSession'),
+  openWindow: () => ipcRenderer.invoke('hermes:window:openInstance'),
+  claimAmbientCue: key => ipcRenderer.invoke('hermes:ambient:claim', key),
   petOverlay: {
     // Main renderer → main process: window lifecycle + drag. `request` is
     // `{ bounds, screen }`; resolves with the screen bounds it actually used.
diff --git a/apps/desktop/electron/session-windows.test.ts b/apps/desktop/electron/session-windows.test.ts
index fcfca8680730..5167593bfbf8 100644
--- a/apps/desktop/electron/session-windows.test.ts
+++ b/apps/desktop/electron/session-windows.test.ts
@@ -2,7 +2,12 @@ import assert from 'node:assert/strict'
 
 import { test } from 'vitest'
 
-import { buildSessionWindowUrl, chatWindowWebPreferences, createSessionWindowRegistry } from './session-windows'
+import {
+  buildSessionWindowUrl,
+  chatWindowWebPreferences,
+  createSessionWindowRegistry,
+  instanceWindowBounds
+} from './session-windows'
 
 // A minimal fake BrowserWindow: tracks listeners + destroyed state and lets a
 // test fire the 'closed' event, mirroring the slice of the Electron API the
@@ -83,10 +88,16 @@ test('buildSessionWindowUrl adds the watch flag for spectator windows, before th
   assert.equal(url, 'http://localhost:5173/?win=secondary&watch=1#/abc')
 })
 
-test('buildSessionWindowUrl routes new-session windows to the draft (#/)', () => {
-  const url = buildSessionWindowUrl(null, { devServer: 'http://localhost:5173', newSession: true })
+test('instanceWindowBounds cascades a new window off its source bounds', () => {
+  const bounds = instanceWindowBounds({ x: 100, y: 120, width: 1400, height: 900 }, { width: 1, height: 1 })
 
-  assert.equal(url, 'http://localhost:5173/?win=secondary&new=1#/')
+  assert.deepEqual(bounds, { width: 1400, height: 900, x: 132, y: 152 })
+})
+
+test('instanceWindowBounds falls back to the persisted geometry with no source window', () => {
+  const fallback = { width: 1280, height: 800 }
+
+  assert.equal(instanceWindowBounds(null, fallback), fallback)
 })
 
 test('registry opens one window per session and focuses on re-open', () => {
diff --git a/apps/desktop/electron/session-windows.ts b/apps/desktop/electron/session-windows.ts
index af55608b0f4e..48597ee9e0ea 100644
--- a/apps/desktop/electron/session-windows.ts
+++ b/apps/desktop/electron/session-windows.ts
@@ -38,13 +38,12 @@ function chatWindowWebPreferences(preloadPath: string) {
 // flag MUST sit in the query string BEFORE the '#': anything after the '#' is
 // treated as the route by HashRouter and would break routeSessionId(). The
 // renderer reads the flag from window.location.search to suppress the install /
-// onboarding overlays and the global session sidebar. `new=1` marks the compact
-// scratch window; `watch=1` marks a spectator window (e.g. a running subagent's
-// session): the renderer resumes it lazily so the gateway never builds an agent
-// just to stream into it.
-function buildSessionWindowUrl(sessionId: string, { devServer, rendererIndexPath, watch, newSession }: any = {}) {
-  const query = `?win=secondary${newSession ? '&new=1' : ''}${watch ? '&watch=1' : ''}`
-  const route = newSession ? '#/' : `#/${encodeURIComponent(sessionId)}`
+// onboarding overlays and the global session sidebar. `watch=1` marks a
+// spectator window (e.g. a running subagent's session): the renderer resumes it
+// lazily so the gateway never builds an agent just to stream into it.
+function buildSessionWindowUrl(sessionId: string, { devServer, rendererIndexPath, watch }: any = {}) {
+  const query = `?win=secondary${watch ? '&watch=1' : ''}`
+  const route = `#/${encodeURIComponent(sessionId)}`
 
   if (devServer) {
     const base = devServer.endsWith('/') ? devServer.slice(0, -1) : devServer
@@ -55,6 +54,28 @@ function buildSessionWindowUrl(sessionId: string, { devServer, rendererIndexPath
   return `${pathToFileURL(rendererIndexPath).toString()}${query}${route}`
 }
 
+// Full "instance" windows (⌘⇧N / the "New Window" command) open a complete app
+// peer, not a compact chat. Cascade each one off its source window's bounds so a
+// new window doesn't land exactly on top of the one it was spawned from. Pure so
+// it's unit-testable; the Electron glue (reading the focused window's bounds,
+// constructing the BrowserWindow) stays in main.ts. `base` is the source
+// window's current bounds, or null when there's no live source window — then the
+// persisted primary geometry (`fallback`) is used as-is.
+const INSTANCE_CASCADE_OFFSET = 32
+
+function instanceWindowBounds(base: { x: number; y: number; width: number; height: number } | null, fallback: any) {
+  if (!base) {
+    return fallback
+  }
+
+  return {
+    width: base.width,
+    height: base.height,
+    x: base.x + INSTANCE_CASCADE_OFFSET,
+    y: base.y + INSTANCE_CASCADE_OFFSET
+  }
+}
+
 // A small registry keyed by sessionId that guarantees one window per chat:
 // opening a session that already has a live window focuses it instead of
 // spawning a duplicate, and a window removes itself from the registry when it
@@ -119,6 +140,7 @@ export {
   buildSessionWindowUrl,
   chatWindowWebPreferences,
   createSessionWindowRegistry,
+  instanceWindowBounds,
   SESSION_WINDOW_MIN_HEIGHT,
   SESSION_WINDOW_MIN_WIDTH
 }
diff --git a/apps/desktop/src/app/chat/composer/hooks/use-auto-speak-replies.ts b/apps/desktop/src/app/chat/composer/hooks/use-auto-speak-replies.ts
index c3268bc9cbd3..949b8f1020c2 100644
--- a/apps/desktop/src/app/chat/composer/hooks/use-auto-speak-replies.ts
+++ b/apps/desktop/src/app/chat/composer/hooks/use-auto-speak-replies.ts
@@ -2,6 +2,7 @@ import { useStore } from '@nanostores/react'
 import { useEffect, useRef } from 'react'
 
 import { playSpeechText } from '@/lib/voice-playback'
+import { ownsAmbientCue } from '@/store/ambient'
 import { notifyError } from '@/store/notifications'
 import { $messages } from '@/store/session'
 import { $voicePlayback } from '@/store/voice-playback'
@@ -65,9 +66,16 @@ export function useAutoSpeakReplies({
       }
 
       markSpoken()
-      void playSpeechText(reply.text, { messageId: reply.id, source: 'read-aloud' }).catch(error =>
-        notifyError(error, failureLabel)
-      )
+      // Only one window voices a given reply when the same chat is open in
+      // several (reply.id is the shared backend message id). markSpoken already
+      // ran in every window, so peers just stay quiet.
+      void ownsAmbientCue(`speak:${reply.id}`).then(owns => {
+        if (owns) {
+          void playSpeechText(reply.text, { messageId: reply.id, source: 'read-aloud' }).catch(error =>
+            notifyError(error, failureLabel)
+          )
+        }
+      })
     }
 
     // Re-check on a reply completing ($messages) and on the prior clip ending
diff --git a/apps/desktop/src/app/command-palette/index.tsx b/apps/desktop/src/app/command-palette/index.tsx
index 05a4d804cb8c..e0a669bb300d 100644
--- a/apps/desktop/src/app/command-palette/index.tsx
+++ b/apps/desktop/src/app/command-palette/index.tsx
@@ -13,6 +13,7 @@ import { useI18n } from '@/i18n'
 import { sessionTitle } from '@/lib/chat-runtime'
 import {
   Activity,
+  AppWindow,
   Archive,
   BarChart3,
   ChevronLeft,
@@ -59,6 +60,7 @@ import { openPetGenerate } from '@/store/pet-generate'
 import { requestStartWorkSession } from '@/store/projects'
 import { runGatewayRestart } from '@/store/system-actions'
 import { applyBackendUpdate } from '@/store/updates'
+import { canOpenNewWindow, openNewWindow } from '@/store/windows'
 import { luminance } from '@/themes/color'
 import { type ThemeMode, useTheme } from '@/themes/context'
 import { isUserTheme, resolveTheme } from '@/themes/user-themes'
@@ -413,6 +415,18 @@ export function CommandPalette() {
             label: cc.nav.newChat.title,
             run: go(NEW_CHAT_ROUTE)
           },
+          ...(canOpenNewWindow()
+            ? [
+                {
+                  action: 'session.newWindow',
+                  icon: AppWindow,
+                  id: 'nav-new-window',
+                  keywords: ['window', 'instance', 'open', 'new'],
+                  label: t.keybinds.actions['session.newWindow'],
+                  run: () => void openNewWindow()
+                }
+              ]
+            : []),
           {
             action: 'view.showTerminal',
             icon: Terminal,
diff --git a/apps/desktop/src/app/hooks/use-keybinds.ts b/apps/desktop/src/app/hooks/use-keybinds.ts
index 34a6bfa1e1c6..4df1edb202c4 100644
--- a/apps/desktop/src/app/hooks/use-keybinds.ts
+++ b/apps/desktop/src/app/hooks/use-keybinds.ts
@@ -40,7 +40,7 @@ import {
   switcherActive,
   switcherJustClosed
 } from '@/store/session-switcher'
-import { openNewSessionInNewWindow } from '@/store/windows'
+import { openNewWindow } from '@/store/windows'
 import { useTheme } from '@/themes/context'
 
 import { requestComposerFocus, requestVoiceToggle } from '../chat/composer/focus'
@@ -145,7 +145,7 @@ export function useKeybinds(deps: KeybindRuntimeDeps): void {
       window.dispatchEvent(new CustomEvent('hermes:new-session-shortcut'))
     },
     'session.newTab': () => deps.openNewSessionTab(),
-    'session.newWindow': () => void openNewSessionInNewWindow(),
+    'session.newWindow': () => void openNewWindow(),
     // ⌃Tab cycles the focused session/main tab strip; only a non-tabbed focus
     // falls through to the recent-session switcher.
     'session.next': () => void (cycleTreeTabInFocusedZone(1) || stepSession(1)),
diff --git a/apps/desktop/src/app/session/hooks/use-message-stream/gateway-event.ts b/apps/desktop/src/app/session/hooks/use-message-stream/gateway-event.ts
index 09ba6bdb52ad..9d2ded2e5f6c 100644
--- a/apps/desktop/src/app/session/hooks/use-message-stream/gateway-event.ts
+++ b/apps/desktop/src/app/session/hooks/use-message-stream/gateway-event.ts
@@ -483,7 +483,8 @@ export function useGatewayEventHandler(deps: GatewayEventDeps) {
 
         flushQueuedDeltas(sessionId)
 
-        playCompletionSound()
+        // Keyed by session so only one window beeps when several are open.
+        playCompletionSound(sessionId)
 
         const finalText = coerceGatewayText(payload?.text) || coerceGatewayText(payload?.rendered)
         completeAssistantMessage(sessionId, finalText, payload?.response_previewed)
diff --git a/apps/desktop/src/global.d.ts b/apps/desktop/src/global.d.ts
index 9202c28e7fc6..fe2a3eaa6dc0 100644
--- a/apps/desktop/src/global.d.ts
+++ b/apps/desktop/src/global.d.ts
@@ -31,8 +31,14 @@ declare global {
       // a spectator window (lazy resume — no agent build) for live-streaming
       // a running subagent's session.
       openSessionWindow: (sessionId: string, opts?: { watch?: boolean }) => Promise<{ ok: boolean; error?: string }>
-      // Open (or focus) a compact secondary window on the new-session draft.
-      openNewSessionWindow: () => Promise<{ ok: boolean; error?: string }>
+      // Open a new full-chrome app window — a peer instance of the primary that
+      // renders the complete app against the shared backend, so the user can run
+      // multiple GUI windows at once.
+      openWindow: () => Promise<{ ok: boolean; error?: string }>
+      // Claim a one-shot cross-window ambient cue (turn-end sound / spoken
+      // reply). Resolves true for the first window to claim a key, false for
+      // peers — so N open windows don't all fire the same cue.
+      claimAmbientCue: (key: string) => Promise<boolean>
       // The pop-out pet overlay: a transparent always-on-top window hosting only
       // the mascot. The main renderer drives it (open/close/drag + state push);
       // the overlay sends control messages back (pop-in, composer submit).
diff --git a/apps/desktop/src/i18n/en.ts b/apps/desktop/src/i18n/en.ts
index a17ecbb3bd60..049d5da5e74c 100644
--- a/apps/desktop/src/i18n/en.ts
+++ b/apps/desktop/src/i18n/en.ts
@@ -225,7 +225,7 @@ export const en: Translations = {
       'nav.agents': 'Open agents',
       'session.new': 'New session',
       'session.newTab': 'New session tab',
-      'session.newWindow': 'New session in window',
+      'session.newWindow': 'New window',
       'session.next': 'Next session',
       'session.prev': 'Previous session',
       'session.slot.1': 'Switch to recent session 1',
diff --git a/apps/desktop/src/i18n/zh.ts b/apps/desktop/src/i18n/zh.ts
index 61d6ff602aab..5e165f6be0a5 100644
--- a/apps/desktop/src/i18n/zh.ts
+++ b/apps/desktop/src/i18n/zh.ts
@@ -220,7 +220,7 @@ export const zh: Translations = {
       'nav.agents': '打开智能体',
       'session.new': '新建会话',
       'session.newTab': '新建会话标签',
-      'session.newWindow': '在新窗口中新建会话',
+      'session.newWindow': '新建窗口',
       'session.next': '下一个会话',
       'session.prev': '上一个会话',
       'session.slot.1': '切换到最近会话 1',
diff --git a/apps/desktop/src/lib/completion-sound.ts b/apps/desktop/src/lib/completion-sound.ts
index 4457d912b78d..1557c58e419e 100644
--- a/apps/desktop/src/lib/completion-sound.ts
+++ b/apps/desktop/src/lib/completion-sound.ts
@@ -1,6 +1,7 @@
 // Completion sound bank for agent turn-end cues.
 // Fourteen curated presets for A/B in Settings → Appearance. Default is variant 1.
 
+import { ownsAmbientCue } from '@/store/ambient'
 import { $completionSoundVariantId, resolveCompletionSoundVariantId } from '@/store/completion-sound'
 import { $hapticsMuted } from '@/store/haptics'
 
@@ -452,13 +453,22 @@ export function previewCompletionSound(variantId?: number) {
   playVariant(resolveCompletionSoundVariantId(variantId ?? $completionSoundVariantId.get()))
 }
 
-// Plays the selected completion cue on any `message.complete`.
-export function playCompletionSound() {
+// Plays the selected completion cue on any `message.complete`. Pass a dedupeKey
+// (the session id) so only one window beeps when several are open — the mute
+// check runs first, so a muted window never claims the cue out from under an
+// audible peer.
+export function playCompletionSound(dedupeKey?: string) {
   if ($hapticsMuted.get()) {
     return
   }
 
-  playVariant($completionSoundVariantId.get())
+  const play = () => playVariant($completionSoundVariantId.get())
+
+  if (!dedupeKey) {
+    return play()
+  }
+
+  void ownsAmbientCue(`sound:${dedupeKey}`).then(owns => owns && play())
 }
 
 interface AirPuffSpec {
diff --git a/apps/desktop/src/lib/icons.ts b/apps/desktop/src/lib/icons.ts
index 0260ce82c6c7..fae9e8d76c09 100644
--- a/apps/desktop/src/lib/icons.ts
+++ b/apps/desktop/src/lib/icons.ts
@@ -2,6 +2,7 @@ import {
   IconActivity as Activity,
   IconAlertCircle as AlertCircle,
   IconAlertTriangle as AlertTriangle,
+  IconAppWindow as AppWindow,
   IconArchive as Archive,
   IconArchiveOff as ArchiveOff,
   IconArrowUp as ArrowUp,
@@ -120,6 +121,7 @@ export {
   Activity,
   AlertCircle,
   AlertTriangle,
+  AppWindow,
   Archive,
   ArchiveOff,
   ArrowUp,
diff --git a/apps/desktop/src/store/ambient.ts b/apps/desktop/src/store/ambient.ts
new file mode 100644
index 000000000000..0c7a8ee31d6d
--- /dev/null
+++ b/apps/desktop/src/store/ambient.ts
@@ -0,0 +1,18 @@
+// One window owns each cross-window ambient cue (turn-end sound, spoken reply)
+// so N open full windows don't all fire it for the same backend event. The main
+// process is the race-free owner (see electron/event-dedupe.ts). Off Electron —
+// or when the bridge/claim fails — every window emits, preserving the
+// single-window behavior rather than going silent.
+export async function ownsAmbientCue(key: string): Promise<boolean> {
+  const claim = window.hermesDesktop?.claimAmbientCue
+
+  if (!claim) {
+    return true
+  }
+
+  try {
+    return await claim(key)
+  } catch {
+    return true
+  }
+}
diff --git a/apps/desktop/src/store/windows.test.ts b/apps/desktop/src/store/windows.test.ts
index 28ae3cc39c9f..e6c3f4974a56 100644
--- a/apps/desktop/src/store/windows.test.ts
+++ b/apps/desktop/src/store/windows.test.ts
@@ -1,6 +1,6 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 
-import { canOpenSessionWindow, openNewSessionInNewWindow, openSessionInNewWindow } from './windows'
+import { canOpenNewWindow, canOpenSessionWindow, openNewWindow, openSessionInNewWindow } from './windows'
 
 const desktopWindow = window as unknown as { hermesDesktop?: Window['hermesDesktop'] }
 const initialHermesDesktop = desktopWindow.hermesDesktop
@@ -13,11 +13,11 @@ vi.mock('./notifications', () => ({
 
 function installBridge(
   openSessionWindow?: Window['hermesDesktop']['openSessionWindow'],
-  openNewSessionWindow?: Window['hermesDesktop']['openNewSessionWindow']
+  openWindow?: Window['hermesDesktop']['openWindow']
 ) {
   desktopWindow.hermesDesktop = {
     ...(openSessionWindow ? { openSessionWindow } : {}),
-    ...(openNewSessionWindow ? { openNewSessionWindow } : {})
+    ...(openWindow ? { openWindow } : {})
   } as unknown as Window['hermesDesktop']
 }
 
@@ -106,37 +106,54 @@ describe('openSessionInNewWindow', () => {
   })
 })
 
-describe('openNewSessionInNewWindow', () => {
+describe('canOpenNewWindow', () => {
+  it('is false when the desktop bridge is absent', () => {
+    delete desktopWindow.hermesDesktop
+    expect(canOpenNewWindow()).toBe(false)
+  })
+
+  it('is false when the bridge lacks openWindow', () => {
+    installBridge(vi.fn().mockResolvedValue({ ok: true }))
+    expect(canOpenNewWindow()).toBe(false)
+  })
+
+  it('is true when the bridge exposes openWindow', () => {
+    installBridge(undefined, vi.fn().mockResolvedValue({ ok: true }))
+    expect(canOpenNewWindow()).toBe(true)
+  })
+})
+
+describe('openNewWindow', () => {
   it('no-ops gracefully when the bridge is absent (web fallback)', async () => {
     delete desktopWindow.hermesDesktop
 
-    await openNewSessionInNewWindow()
+    await openNewWindow()
 
     expect(notifyError).not.toHaveBeenCalled()
   })
 
-  it('no-ops when openNewSessionWindow is missing', async () => {
+  it('no-ops when openWindow is missing', async () => {
     installBridge(vi.fn().mockResolvedValue({ ok: true }))
 
-    await openNewSessionInNewWindow()
+    await openNewWindow()
 
     expect(notifyError).not.toHaveBeenCalled()
   })
 
   it('invokes the bridge', async () => {
-    const openNew = vi.fn().mockResolvedValue({ ok: true })
-    installBridge(vi.fn().mockResolvedValue({ ok: true }), openNew)
+    const openWindow = vi.fn().mockResolvedValue({ ok: true })
+    installBridge(undefined, openWindow)
 
-    await openNewSessionInNewWindow()
+    await openNewWindow()
 
-    expect(openNew).toHaveBeenCalledTimes(1)
+    expect(openWindow).toHaveBeenCalledTimes(1)
     expect(notifyError).not.toHaveBeenCalled()
   })
 
   it('notifies on an ok:false result', async () => {
-    installBridge(vi.fn().mockResolvedValue({ ok: true }), vi.fn().mockResolvedValue({ ok: false, error: 'nope' }))
+    installBridge(undefined, vi.fn().mockResolvedValue({ ok: false, error: 'nope' }))
 
-    await openNewSessionInNewWindow()
+    await openNewWindow()
 
     expect(notifyError).toHaveBeenCalledTimes(1)
   })
diff --git a/apps/desktop/src/store/windows.ts b/apps/desktop/src/store/windows.ts
index a881aa4794c6..349b29d12ba0 100644
--- a/apps/desktop/src/store/windows.ts
+++ b/apps/desktop/src/store/windows.ts
@@ -6,7 +6,6 @@ import { notifyError } from './notifications'
 // never from the router. A "secondary" window renders a single chat without the
 // global session sidebar or the install / onboarding overlays.
 const SECONDARY_WINDOW_FLAG = 'secondary'
-const NEW_SESSION_WINDOW_FLAG = '1'
 
 let secondaryWindowCache: boolean | null = null
 
@@ -28,26 +27,6 @@ export function isSecondaryWindow(): boolean {
   return result
 }
 
-let newSessionWindowCache: boolean | null = null
-
-export function isNewSessionWindow(): boolean {
-  if (newSessionWindowCache !== null) {
-    return newSessionWindowCache
-  }
-
-  let result = false
-
-  try {
-    result = new URLSearchParams(window.location.search).get('new') === NEW_SESSION_WINDOW_FLAG
-  } catch {
-    result = false
-  }
-
-  newSessionWindowCache = result
-
-  return result
-}
-
 let watchWindowCache: boolean | null = null
 
 // A "watch" window spectates a session that is being driven elsewhere (a
@@ -78,11 +57,16 @@ export function canOpenSessionWindow(): boolean {
   return typeof window !== 'undefined' && typeof window.hermesDesktop?.openSessionWindow === 'function'
 }
 
+// True when the shell can open a full peer app window (⌘⇧N / "New Window").
+export function canOpenNewWindow(): boolean {
+  return typeof window !== 'undefined' && typeof window.hermesDesktop?.openWindow === 'function'
+}
+
 type WindowOpenResult = { ok: boolean; error?: string } | undefined
 
 // Run a window-open bridge call, surfacing any failure as a toast. Shared by the
-// session pop-out and the new-session pop-out.
-async function openWindow(call: () => Promise<WindowOpenResult>, failMessage: string): Promise<void> {
+// session pop-out and the new-window opener.
+async function runWindowOpen(call: () => Promise<WindowOpenResult>, failMessage: string): Promise<void> {
   try {
     const result = await call()
 
@@ -102,14 +86,18 @@ export async function openSessionInNewWindow(sessionId: string, opts?: { watch?:
     return
   }
 
-  await openWindow(() => window.hermesDesktop.openSessionWindow(sessionId, opts), 'Could not open chat in a new window')
+  await runWindowOpen(
+    () => window.hermesDesktop.openSessionWindow(sessionId, opts),
+    'Could not open chat in a new window'
+  )
 }
 
-// Open a fresh compact window on the new-session draft.
-export async function openNewSessionInNewWindow(): Promise<void> {
-  if (!canOpenSessionWindow() || typeof window.hermesDesktop.openNewSessionWindow !== 'function') {
+// Open a new full-chrome app window — a peer instance of the primary that
+// renders the complete app against the shared backend. No-ops outside Electron.
+export async function openNewWindow(): Promise<void> {
+  if (!canOpenNewWindow()) {
     return
   }
 
-  await openWindow(() => window.hermesDesktop.openNewSessionWindow(), 'Could not open new session window')
+  await runWindowOpen(() => window.hermesDesktop.openWindow(), 'Could not open a new window')
 }