fix(desktop): report a stock Chrome user agent in the browser tab (#6695)

Electron's default user agent carries Sim/<version> and Electron/<version> tokens, and the detection libraries sites gate on test for Electron before Chrome — so the browser read as "Electron", which is on no site's supported list. Ashby warned "Ashby does not support this browser"; stricter sites refuse to render.

Rebuild the string as the desktop form Chrome's user-agent reduction specifies — same platform token and Chromium major version, the rest zeroed, no application or Electron token — and apply it to both the browser partition session and each tab's WebContents. Service workers do not inherit a tab's user agent, so without the session-level call a worker's script request still announced Electron.

Scoped to the browser partition: app.userAgentFallback is left alone so the Sim shell's own user agent is unchanged.

apps/desktop/src/main/browser-agent/session.test.ts

@@ -21,6 +21,7 @@ interface MockView {
       setPermissionCheckHandler: ReturnType<typeof vi.fn>
     }
     on: ReturnType<typeof vi.fn>
+    setUserAgent: ReturnType<typeof vi.fn>
     setWindowOpenHandler: ReturnType<typeof vi.fn>
     loadURL: ReturnType<typeof vi.fn>
     reload: ReturnType<typeof vi.fn>
@@ -169,6 +170,18 @@ describe('browser-agent session', () => {
     expect(onTabNavigated).toHaveBeenCalledWith(contents, true)
   })
 
+  it('gives every tab a user agent with no Electron token in 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\//)
+    }
+  })
+
   it('settles the tab spinner when only subresources are still loading', () => {
     const tab = session.ensureTab()
     const contents = (tab.view as unknown as MockView).webContents

apps/desktop/src/main/browser-agent/session.ts

@@ -62,6 +62,7 @@ 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 {
@@ -819,6 +820,11 @@ function configureAgentPartition(ses: Session): void {
   configuredPartitions.add(ses)
   ses.setPermissionRequestHandler((_wc, _permission, callback) => callback(false))
   ses.setPermissionCheckHandler(() => false)
+  // 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
@@ -1101,6 +1107,10 @@ function createTabView(): WebContentsView {
   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, false)),

apps/desktop/src/main/browser-agent/user-agent.test.ts

@@ -0,0 +1,43 @@
+import { app } from 'electron'
+import { describe, expect, it, vi } from 'vitest'
+import { browserUserAgent, stockChromeUserAgent } from '@/main/browser-agent/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'
+
+describe('stockChromeUserAgent', () => {
+  it('drops the application and Electron tokens a browser allowlist rejects', () => {
+    const agent = stockChromeUserAgent(ELECTRON_DEFAULT)
+    expect(agent).not.toMatch(/Electron/)
+    expect(agent).not.toMatch(/Sim\//)
+  })
+
+  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'
+    )
+  })
+
+  it('keeps the platform token of the machine it is running on', () => {
+    const windowsDefault =
+      'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Sim/1.0.0 Chrome/140.0.7339.207 Electron/43.1.1 Safari/537.36'
+    expect(stockChromeUserAgent(windowsDefault)).toContain('(Windows NT 10.0; Win64; x64)')
+  })
+
+  it('passes through a string that is not a Chromium user agent', () => {
+    expect(stockChromeUserAgent('curl/8.4.0')).toBe('curl/8.4.0')
+    expect(stockChromeUserAgent('')).toBe('')
+  })
+})
+
+describe('browserUserAgent', () => {
+  it('derives from the string Electron would otherwise have sent', () => {
+    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'
+    )
+  })
+})

apps/desktop/src/main/browser-agent/user-agent.ts

@@ -0,0 +1,46 @@
+/**
+ * The user agent the browser resource presents to sites.
+ *
+ * Electron's default string carries two tokens no browser sends —
+ * `Sim/<version>` and `Electron/<version>`. Chromium's own token sits right
+ * beside them, but that does not save it: the detection libraries sites gate on
+ * test for Electron BEFORE Chrome (bowser matches `/electron/i` several
+ * descriptors ahead of its Chrome one, ua-parser-js reports `Electron` as the
+ * browser name), so the browser reads as "Electron", which is on nobody's
+ * supported list. Ashby warns "Ashby does not support this browser"; stricter
+ * sites refuse to render at all.
+ *
+ * Reporting stock Chrome is accurate rather than a disguise — the engine is the
+ * Chromium build the token already names, and Electron's user-agent client
+ * hints (`Sec-CH-UA`, `navigator.userAgentData`) only ever carried a Chromium
+ * brand, so dropping the token makes the header and the hints agree instead of
+ * contradicting each other.
+ */
+import { app } from 'electron'
+
+/** Platform token, then the Chromium major version, in the order a Chromium user agent lists them. */
+const CHROMIUM_USER_AGENT = /^Mozilla\/5\.0 \(([^)]*)\).* Chrome\/(\d+)\./
+
+/**
+ * Rebuilds the default user agent as the string Chrome itself sends. Chrome's
+ * user-agent reduction fixes the desktop form at
+ * `Mozilla/5.0 (<platform>) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/<major>.0.0.0 Safari/537.36`,
+ * so keeping the platform token and the Chromium major version — and zeroing
+ * the rest — reproduces it exactly, with no room left for an application or
+ * Electron token. A string that is not a Chromium user agent is returned
+ * unchanged rather than replaced with a guess.
+ */
+export function stockChromeUserAgent(defaultUserAgent: string): string {
+  const match = defaultUserAgent.match(CHROMIUM_USER_AGENT)
+  if (!match) return defaultUserAgent
+  const [, platform, chromeMajor] = match
+  return `Mozilla/5.0 (${platform}) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/${chromeMajor}.0.0.0 Safari/537.36`
+}
+
+/**
+ * Derived from the string Electron would otherwise have sent, so the reported
+ * Chromium version tracks whatever Chromium the app actually ships.
+ */
+export function browserUserAgent(): string {
+  return stockChromeUserAgent(app.userAgentFallback)
+}

apps/desktop/src/test/electron-mock.ts

@@ -11,6 +11,8 @@ import { vi } from 'vitest'
 export const app = {
   name: 'Sim',
   isPackaged: false,
+  userAgentFallback:
+    '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',
   getVersion: vi.fn(() => '1.0.0'),
   getName: vi.fn(() => 'Sim'),
   setName: vi.fn(),
@@ -152,6 +154,7 @@ function createWebContentsMock() {
     findInPage: vi.fn(() => 1),
     stopFindInPage: vi.fn(),
     setBackgroundThrottling: vi.fn(),
+    setUserAgent: vi.fn(),
     setIgnoreMenuShortcuts: vi.fn(),
     getZoomFactor: vi.fn(() => 1),
     setZoomFactor: vi.fn(),
@@ -185,6 +188,7 @@ function createWebContentsMock() {
     session: {
       setPermissionRequestHandler: vi.fn(),
       setPermissionCheckHandler: vi.fn(),
+      setUserAgent: vi.fn(),
       webRequest: { onBeforeRequest: vi.fn() },
       on: vi.fn(),
     },