@@ -37,6 +37,7 @@ interface MockView {
     session: {
       setPermissionRequestHandler: ReturnType<typeof vi.fn>
       setPermissionCheckHandler: ReturnType<typeof vi.fn>
+      setUserAgent: ReturnType<typeof vi.fn>
       webRequest: { onBeforeRequest: ReturnType<typeof vi.fn> }
     }
     on: ReturnType<typeof vi.fn>
@@ -358,15 +359,14 @@ describe('browser-agent session', () => {
     expect(onTabNavigated).toHaveBeenCalledWith(contents, true)
   })
 
-  it('gives every tab a user agent with no Electron token in it', () => {
+  it('leaves every tab on the process-wide user agent instead of overriding it', () => {
     const first = session.ensureTab()
     const second = session.addTab()
 
     for (const tab of [first, second]) {
       const contents = (tab.view as unknown as MockView).webContents
-      const agent = contents.setUserAgent.mock.calls.at(-1)?.[0] as string | undefined
-      expect(agent).toMatch(/^Mozilla\/5\.0 \(.+\) .*Chrome\/\d+\.0\.0\.0 Safari\/537\.36$/)
-      expect(agent).not.toMatch(/Electron|Sim\//)
+      expect(contents.setUserAgent).not.toHaveBeenCalled()
+      expect(contents.session.setUserAgent).not.toHaveBeenCalled()
     }
   })
 
 
@@ -67,7 +67,6 @@ import {
   isBlockedSubresourceUrl,
   subresourceNeedsResolution,
 } from '@/main/browser-agent/url-guard'
-import { browserUserAgent } from '@/main/browser-agent/user-agent'
 import type { BrowserSessionSnapshot } from '@/main/desktop-chat-session-store'
 import { suggestedFilename, uniqueDownloadPath } from '@/main/downloads'
 import {
@@ -1413,11 +1412,6 @@ function configureAgentPartition(ses: Session): void {
     }
     return ALLOWED_SITE_PERMISSIONS.has(permission)
   })
-  // Service workers do not inherit a tab's user agent. With only the tab's set,
-  // the document request carries the browser string while the worker's own
-  // script request still announces Electron — and on a site that routes its
-  // fetches through a worker, that is the one the server sees.
-  ses.setUserAgent(browserUserAgent())
   // SSRF choke point for the agent partition. Document navigations (top-level +
   // iframes) get the full DNS-resolving check — the one seam every navigation
   // passes through, including page-initiated ones the driver never sees (server
@@ -1965,10 +1959,6 @@ function initializeTabView(view: WebContentsView, scopeId: string): WebContentsV
   const contents = view.webContents
   registerAgentWebContents(contents)
   configureAgentPartition(contents.session)
-  // The session default does not reach a WebContents that already exists, and
-  // the first tab is what brings the session into being, so each tab sets its
-  // own as well — otherwise tab one browses as Electron and the rest as Chrome.
-  contents.setUserAgent(browserUserAgent())
   attachAgentContextMenu(contents, {
     addToChat: (text) => withBrowserScope(scopeId, () => addPageSelectionToChat(contents, text)),
     openTab: (url) => withBrowserScope(scopeId, () => openTabWithUrl(url, { agentOwned: false })),
 
@@ -83,6 +83,7 @@ import { attachTelemetryPolicy } from '@/main/telemetry-policy'
 import { TerminalRegistry } from '@/main/terminal/registry'
 import { installTray, type TrayHandle } from '@/main/tray'
 import { checkForUpdatesInteractive, initUpdater, type UpdaterHandle } from '@/main/updater'
+import { installBrowserUserAgent } from '@/main/user-agent'
 import { createMainWindow, setupPermissionHandlers } from '@/main/window'
 import { attachWindowOpenPolicy, isPopupContents } from '@/main/windows'
 
@@ -899,6 +900,7 @@ app.setName(APP_NAME_FOR_CHANNEL[channelForOrigin(DEFAULT_ORIGIN)])
 if (process.env.SIM_DESKTOP_USER_DATA) {
   app.setPath('userData', process.env.SIM_DESKTOP_USER_DATA)
 }
+installBrowserUserAgent()
 
 // The scheme the offline page and server picker load from must be declared
 // before the app is ready; the per-session handlers attach later.
 
@@ -1,11 +1,13 @@
 import { app } from 'electron'
 import { describe, expect, it, vi } from 'vitest'
-import { browserUserAgent, stockChromeUserAgent } from '@/main/browser-agent/user-agent'
+import { installBrowserUserAgent, stockChromeUserAgent } from '@/main/user-agent'
 
 vi.mock('electron', () => import('@/test/electron-mock'))
 
 const ELECTRON_DEFAULT =
   'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Sim/1.0.0 Chrome/140.0.7339.207 Electron/43.1.1 Safari/537.36'
+const STOCK_CHROME =
+  'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/140.0.0.0 Safari/537.36'
 
 describe('stockChromeUserAgent', () => {
   it('drops the application and Electron tokens a browser allowlist rejects', () => {
@@ -15,9 +17,7 @@ describe('stockChromeUserAgent', () => {
   })
 
   it('reproduces the desktop string Chrome sends under user-agent reduction', () => {
-    expect(stockChromeUserAgent(ELECTRON_DEFAULT)).toBe(
-      'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/140.0.0.0 Safari/537.36'
-    )
+    expect(stockChromeUserAgent(ELECTRON_DEFAULT)).toBe(STOCK_CHROME)
   })
 
   it('keeps the platform token of the machine it is running on', () => {
@@ -32,12 +32,13 @@ describe('stockChromeUserAgent', () => {
   })
 })
 
-describe('browserUserAgent', () => {
-  it('derives from the string Electron would otherwise have sent', () => {
+describe('installBrowserUserAgent', () => {
+  it('idempotently makes stock Chrome the process-wide fallback every request path uses', () => {
     app.userAgentFallback = ELECTRON_DEFAULT
 
-    expect(browserUserAgent()).toBe(
-      'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/140.0.0.0 Safari/537.36'
-    )
+    installBrowserUserAgent()
+    installBrowserUserAgent()
+
+    expect(app.userAgentFallback).toBe(STOCK_CHROME)
   })
 })
@@ -1,5 +1,6 @@
 /**
- * The user agent the browser resource presents to sites.
+ * The user agent the whole desktop process presents: the embedded browser and
+ * the app's own windows alike (desktop identity travels in `X-Sim-Client-Info`).
  *
  * Electron's default string carries two tokens no browser sends —
  * `Sim/<version>` and `Electron/<version>`. Chromium's own token sits right
@@ -38,9 +39,11 @@ export function stockChromeUserAgent(defaultUserAgent: string): string {
 }
 
 /**
- * Derived from the string Electron would otherwise have sent, so the reported
- * Chromium version tracks whatever Chromium the app actually ships.
+ * Sets the stock Chrome identity as `app.userAgentFallback` before any session
+ * exists. Session and per-tab overrides miss some request paths (a cross-origin
+ * challenge frame still sends the process default), and a site that sees two
+ * user agents in one challenge rejects it as a spoof. Idempotent.
  */
-export function browserUserAgent(): string {
-  return stockChromeUserAgent(app.userAgentFallback)
+export function installBrowserUserAgent(): void {
+  app.userAgentFallback = stockChromeUserAgent(app.userAgentFallback)
 }