diff --git a/apps/desktop/electron/desktop-profile.test.ts b/apps/desktop/electron/desktop-profile.test.ts
index af606fd637..4d6603eb27 100644
--- a/apps/desktop/electron/desktop-profile.test.ts
+++ b/apps/desktop/electron/desktop-profile.test.ts
@@ -8,6 +8,7 @@ import { test } from 'vitest'
 import {
   createDesktopProfilePreferences,
   resolveDesktopConnectionRequest,
+  resolveDesktopWindowLaunch,
   resolveDesktopWindowRoute
 } from './desktop-profile'
 import { WindowConnectionRouteRegistry } from './window-connection-route'
@@ -66,6 +67,15 @@ test('explicit routes are strict and never fall back to a different source windo
     connectionId: 'remote-a',
     profile: 'work'
   })
+  // Only an explicit route pins the window's New-session default; an inherited
+  // or fallback route seeds boot alone.
+  assert.deepEqual(resolveDesktopWindowLaunch(explicit, routes.get(1), fallback), { ...explicit, profileWindow: true })
+  assert.deepEqual(resolveDesktopWindowLaunch(undefined, routes.get(1), fallback), {
+    connectionId: 'remote-a',
+    profile: 'work',
+    profileWindow: false
+  })
+  assert.deepEqual(resolveDesktopWindowLaunch(undefined, null, fallback), { ...fallback, profileWindow: false })
   assert.deepEqual(routes.get(1), { connectionId: 'remote-a', profile: 'work', registryScoped: true })
   assert.throws(() => resolveDesktopWindowRoute({ profile: 'work' }, routes.get(1), fallback))
   assert.throws(() => resolveDesktopWindowRoute({ connectionId: null, profile: '../work' }, routes.get(1), fallback))
diff --git a/apps/desktop/electron/desktop-profile.ts b/apps/desktop/electron/desktop-profile.ts
index 08a97c0a9a..65c242280d 100644
--- a/apps/desktop/electron/desktop-profile.ts
+++ b/apps/desktop/electron/desktop-profile.ts
@@ -52,6 +52,21 @@ export function resolveDesktopWindowRoute(
     : { ...fallback }
 }
 
+/** A peer window's launch route plus whether the renderer asked for that
+ * route explicitly ("Open profile in new window"). Only an explicit route is
+ * the window's New-session default; an inherited one seeds boot alone. */
+export interface DesktopWindowLaunch extends DesktopProfileRoute {
+  profileWindow: boolean
+}
+
+export function resolveDesktopWindowLaunch(
+  explicit: unknown,
+  source: WindowConnectionRoute | null,
+  fallback: DesktopProfileRoute
+): DesktopWindowLaunch {
+  return { ...resolveDesktopWindowRoute(explicit, source, fallback), profileWindow: explicit !== undefined }
+}
+
 // A profile-less boot/reconnect belongs to its sender. An explicit profile
 // keeps the legacy route, even if the sender serves the same name remotely.
 export function resolveDesktopConnectionRequest(
diff --git a/apps/desktop/electron/main.ts b/apps/desktop/electron/main.ts
index 292f032356..fd83994d2e 100644
--- a/apps/desktop/electron/main.ts
+++ b/apps/desktop/electron/main.ts
@@ -171,7 +171,7 @@ import {
   DESKTOP_PROFILE_NAME_RE,
   type DesktopProfileRoute,
   resolveDesktopConnectionRequest,
-  resolveDesktopWindowRoute
+  resolveDesktopWindowLaunch
 } from './desktop-profile'
 import { resolveDesktopRemoteRoute, v1SshTerminalPoolKey } from './desktop-remote-route'
 import {
@@ -13569,7 +13569,7 @@ function createInstanceWindow(
   options?: DesktopProfileRoute,
   source: BrowserWindow | null = BrowserWindow.getFocusedWindow() || mainWindow
 ) {
-  const route = resolveDesktopWindowRoute(
+  const route = resolveDesktopWindowLaunch(
     options,
     source && !source.isDestroyed() ? windowConnectionRoutes.get(source.webContents.id) : null,
     { connectionId: null, profile: primaryProfileKey() }
@@ -13633,7 +13633,6 @@ function createInstanceWindow(
     win,
     buildInstanceWindowUrl({
       ...route,
-      profileWindow: options !== undefined,
       devServer: DEV_SERVER,
       rendererIndexPath: DEV_SERVER ? undefined : resolveRendererIndex()
     }),
diff --git a/apps/desktop/electron/session-windows.ts b/apps/desktop/electron/session-windows.ts
index 669aa1efc8..ff66425a1d 100644
--- a/apps/desktop/electron/session-windows.ts
+++ b/apps/desktop/electron/session-windows.ts
@@ -5,6 +5,8 @@
 
 import { pathToFileURL } from 'node:url'
 
+import type { DesktopWindowLaunch } from './desktop-profile'
+
 // Secondary windows open at the minimum usable size — a compact side panel for
 // subagent watch / cmd-click session pop-out, not a second full desktop.
 const SESSION_WINDOW_MIN_WIDTH = 420
@@ -87,7 +89,12 @@ function buildSessionWindowUrl(sessionId: string, { devServer, profile, renderer
 // separate marker lets the renderer distinguish a peer from the one primary
 // app window: app-launch source restoration belongs to the primary only, while
 // a peer keeps the already-running backend it joined during boot.
-function buildInstanceWindowUrl({ connectionId, devServer, profile, profileWindow, rendererIndexPath }: any = {}) {
+interface InstanceWindowUrlOptions extends Partial<DesktopWindowLaunch> {
+  devServer?: string
+  rendererIndexPath?: string
+}
+
+function buildInstanceWindowUrl({ connectionId, devServer, profile, profileWindow, rendererIndexPath }: InstanceWindowUrlOptions = {}) {
   const query = `?peer=1${profile ? `&profile=${encodeURIComponent(profile)}&connectionId=${encodeURIComponent(connectionId ?? '')}${profileWindow ? '&profileWindow=1' : ''}` : ''}`
 
   if (devServer) {