diff --git a/src/main/browser/browser-cookie-import.arc-ua.test.ts b/src/main/browser/browser-cookie-import.arc-ua.test.ts
new file mode 100644
index 000000000..6a7c7ff06
--- /dev/null
+++ b/src/main/browser/browser-cookie-import.arc-ua.test.ts
@@ -0,0 +1,81 @@
+import { describe, expect, it, vi, beforeEach, afterEach } from 'vitest'
+import type * as childProcessModule from 'node:child_process'
+
+describe('isAdvertisableChromiumEngineVersion', () => {
+  it('accepts real Chromium engine majors and rejects product versions', async () => {
+    const { isAdvertisableChromiumEngineVersion } = await import('./browser-session-ua')
+    expect(isAdvertisableChromiumEngineVersion('120.0.6099.71')).toBe(true)
+    expect(isAdvertisableChromiumEngineVersion('70.0.0.0')).toBe(true)
+    expect(isAdvertisableChromiumEngineVersion('1.158.1')).toBe(false)
+    expect(isAdvertisableChromiumEngineVersion('1.0.0')).toBe(false)
+    expect(isAdvertisableChromiumEngineVersion('not-a-version')).toBe(false)
+    // Malformed components with a valid major must not pass (would become Chrome/70.not-a-version).
+    expect(isAdvertisableChromiumEngineVersion('70.not-a-version')).toBe(false)
+    expect(isAdvertisableChromiumEngineVersion('120.0.invalid.1')).toBe(false)
+  })
+})
+
+describe('isUnadvertisableChromeUserAgent', () => {
+  it('flags stored Chrome/1.x UAs and leaves engine-scale ones alone', async () => {
+    const { isUnadvertisableChromeUserAgent } = await import('./browser-session-ua')
+    const ua = (version: string): string =>
+      `Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/${version} Safari/537.36`
+    expect(isUnadvertisableChromeUserAgent(ua('1.158.1'))).toBe(true)
+    expect(isUnadvertisableChromeUserAgent(ua('150.0.7871.47'))).toBe(false)
+    expect(isUnadvertisableChromeUserAgent(`${ua('151.0.0.0')} Edg/151.0.0.0`)).toBe(false)
+    // Why: non-Chrome UAs (Firefox/Safari imports) carry no engine claim to invalidate.
+    expect(isUnadvertisableChromeUserAgent('Mozilla/5.0 (Macintosh) Firefox/126.0')).toBe(false)
+  })
+})
+
+describe('getUserAgentForBrowser — Arc product version', () => {
+  const originalPlatform = process.platform
+
+  beforeEach(() => {
+    vi.resetModules()
+    Object.defineProperty(process, 'platform', { value: 'darwin' })
+  })
+
+  afterEach(() => {
+    Object.defineProperty(process, 'platform', { value: originalPlatform })
+    vi.restoreAllMocks()
+  })
+
+  it('does not persist Chrome/1.x when Arc reports its product version', async () => {
+    vi.doMock('node:child_process', async () => {
+      const actual = await vi.importActual<typeof childProcessModule>('node:child_process')
+      return {
+        ...actual,
+        execFileSync: (cmd: string, args: readonly string[]) => {
+          if (cmd === 'defaults' && args[1]?.includes('/Applications/Arc.app/Contents/Info')) {
+            return '1.158.1\n'
+          }
+          return actual.execFileSync(cmd, args as never)
+        }
+      }
+    })
+
+    const { getUserAgentForBrowser } = await import('./browser-cookie-import')
+    expect(getUserAgentForBrowser('arc')).toBeNull()
+  })
+
+  it('still builds a Chrome-shaped UA when Arc reports an engine-scale version', async () => {
+    vi.doMock('node:child_process', async () => {
+      const actual = await vi.importActual<typeof childProcessModule>('node:child_process')
+      return {
+        ...actual,
+        execFileSync: (cmd: string, args: readonly string[]) => {
+          if (cmd === 'defaults' && args[1]?.includes('/Applications/Arc.app/Contents/Info')) {
+            return '120.0.6099.71\n'
+          }
+          return actual.execFileSync(cmd, args as never)
+        }
+      }
+    })
+
+    const { getUserAgentForBrowser } = await import('./browser-cookie-import')
+    const ua = getUserAgentForBrowser('arc')
+    expect(ua).toContain('Chrome/120.0.6099.71')
+    expect(ua).not.toContain('Chrome/1.')
+  })
+})
diff --git a/src/main/browser/browser-cookie-import.ts b/src/main/browser/browser-cookie-import.ts
index e67c5470b..65ea11c1e 100644
--- a/src/main/browser/browser-cookie-import.ts
+++ b/src/main/browser/browser-cookie-import.ts
@@ -75,7 +75,7 @@ import type {
 } from '../../shared/types'
 import { browserSessionRegistry } from './browser-session-registry'
 import { getBrowserSessionUserAgentMode } from './browser-session-user-agent-mode'
-import { setupClientHintsOverride } from './browser-session-ua'
+import { isAdvertisableChromiumEngineVersion, setupClientHintsOverride } from './browser-session-ua'
 import {
   isGoogleSourceBoundCookie,
   normalizeCookieDomain,
@@ -804,32 +804,34 @@ export function getUserAgentForBrowser(
     }
   }
 
+  function chromeShapedUa(version: string | null, edgeSuffix = false): string | null {
+    if (!version || !isAdvertisableChromiumEngineVersion(version)) {
+      return null
+    }
+    const base = `Mozilla/5.0 (${platform}) ${chromeBase} Chrome/${version} Safari/537.36`
+    return edgeSuffix ? `${base} Edg/${version}` : base
+  }
+
   switch (family) {
     case 'chrome': {
-      const v = readBrowserVersion('/Applications/Google Chrome.app')
-      return v ? `Mozilla/5.0 (${platform}) ${chromeBase} Chrome/${v} Safari/537.36` : null
+      return chromeShapedUa(readBrowserVersion('/Applications/Google Chrome.app'))
     }
     case 'edge': {
-      const v = readBrowserVersion('/Applications/Microsoft Edge.app')
-      return v ? `Mozilla/5.0 (${platform}) ${chromeBase} Chrome/${v} Safari/537.36 Edg/${v}` : null
+      return chromeShapedUa(readBrowserVersion('/Applications/Microsoft Edge.app'), true)
     }
     case 'arc': {
-      const v = readBrowserVersion('/Applications/Arc.app')
-      return v ? `Mozilla/5.0 (${platform}) ${chromeBase} Chrome/${v} Safari/537.36` : null
+      return chromeShapedUa(readBrowserVersion('/Applications/Arc.app'))
     }
     case 'chromium': {
-      const v = readBrowserVersion('/Applications/Brave Browser.app')
-      return v ? `Mozilla/5.0 (${platform}) ${chromeBase} Chrome/${v} Safari/537.36` : null
+      return chromeShapedUa(readBrowserVersion('/Applications/Brave Browser.app'))
     }
     case 'comet': {
       // Why: Comet is Chromium-based; use Chrome's UA shape so Google-bound auth cookies survive import.
-      const v = readBrowserVersion('/Applications/Comet.app')
-      return v ? `Mozilla/5.0 (${platform}) ${chromeBase} Chrome/${v} Safari/537.36` : null
+      return chromeShapedUa(readBrowserVersion('/Applications/Comet.app'))
     }
     case 'helium': {
       // Why: Helium is Chromium-based; use Chrome's UA shape so Google-bound auth cookies survive import.
-      const v = readBrowserVersion('/Applications/Helium.app')
-      return v ? `Mozilla/5.0 (${platform}) ${chromeBase} Chrome/${v} Safari/537.36` : null
+      return chromeShapedUa(readBrowserVersion('/Applications/Helium.app'))
     }
     case 'firefox':
     case 'safari':
diff --git a/src/main/browser/browser-session-registry.persistence.test.ts b/src/main/browser/browser-session-registry.persistence.test.ts
index f7792903b..0d84a47fd 100644
--- a/src/main/browser/browser-session-registry.persistence.test.ts
+++ b/src/main/browser/browser-session-registry.persistence.test.ts
@@ -1,4 +1,5 @@
 import { beforeEach, describe, expect, it, vi } from 'vitest'
+import type * as browserSessionUaModule from './browser-session-ua'
 
 const USER_DATA = '/user-data'
 const META_PATH = `${USER_DATA}/browser-session-meta.json`
@@ -118,10 +119,15 @@ function installModuleMocks(
     hasSystemMediaAccess: vi.fn(() => true),
     requestSystemMediaAccess: requestSystemMediaAccessMock
   }))
-  vi.doMock('./browser-session-ua', () => ({
-    cleanElectronUserAgent: vi.fn((ua: string) => ua.replace(/\s*Electron\/\S+/, '')),
-    setupClientHintsOverride: setupClientHintsOverrideMock
-  }))
+  vi.doMock('./browser-session-ua', async () => {
+    // Why: the version gate is the behavior under test, so use the real predicate here.
+    const actual = await vi.importActual<typeof browserSessionUaModule>('./browser-session-ua')
+    return {
+      cleanElectronUserAgent: vi.fn((ua: string) => ua.replace(/\s*Electron\/\S+/, '')),
+      isUnadvertisableChromeUserAgent: actual.isUnadvertisableChromeUserAgent,
+      setupClientHintsOverride: setupClientHintsOverrideMock
+    }
+  })
 
   return {
     sessionFromPartitionMock,
@@ -405,6 +411,39 @@ describe('BrowserSessionRegistry persistence', () => {
     ).toBe(true)
   })
 
+  it('drops a persisted fork product-version UA instead of replaying Chrome/1.x', async () => {
+    const brokenUa =
+      'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/1.158.1 Safari/537.36'
+    const fsState = createFsState()
+    seedMeta(fsState, {
+      defaultSource: { browserFamily: 'arc', importedAt: 1 },
+      userAgent: brokenUa,
+      userAgentByPartition: { 'persist:orca-browser': brokenUa },
+      pendingCookieDbPath: null,
+      pendingCookieImports: {},
+      profiles: []
+    })
+
+    const { sessionFromPartitionMock, setupClientHintsOverrideMock } = installModuleMocks(fsState)
+    const { browserSessionRegistry } = await import('./browser-session-registry')
+
+    browserSessionRegistry.initializeBrowserSessionsFromPersistedState()
+
+    const appliedUas = sessionFromPartitionMock.mock.results.flatMap((r) =>
+      r.value.setUserAgent.mock.calls.map((c: unknown[]) => c[0])
+    )
+    expect(appliedUas).not.toContain(brokenUa)
+    // Why: with the broken UA gone the profile must fall back to Orca's own cleaned engine UA.
+    expect(appliedUas).toContain('Mozilla/5.0 Orca')
+    expect(setupClientHintsOverrideMock.mock.calls.some((c: unknown[]) => c[1] === brokenUa)).toBe(
+      false
+    )
+
+    const persisted = JSON.parse(fsState.files.get(META_PATH) ?? '{}')
+    expect(persisted.userAgentByPartition).toEqual({})
+    expect(persisted.userAgent).toBeNull()
+  })
+
   it('preserves native mode across hydration when no source UA was imported', async () => {
     const importedPartition = 'persist:orca-browser-session-12121212-1212-4121-8121-121212121212'
     const fsState = createFsState()
diff --git a/src/main/browser/browser-session-registry.ts b/src/main/browser/browser-session-registry.ts
index d6188bf88..175c81ffa 100644
--- a/src/main/browser/browser-session-registry.ts
+++ b/src/main/browser/browser-session-registry.ts
@@ -25,7 +25,11 @@ import type {
 } from '../../shared/types'
 import { browserManager } from './browser-manager'
 import { hasSystemMediaAccess, requestSystemMediaAccess } from './browser-media-access'
-import { cleanElectronUserAgent, setupClientHintsOverride } from './browser-session-ua'
+import {
+  cleanElectronUserAgent,
+  isUnadvertisableChromeUserAgent,
+  setupClientHintsOverride
+} from './browser-session-ua'
 import {
   clearBrowserSessionUserAgentMode,
   setBrowserSessionUserAgentMode
@@ -198,11 +202,17 @@ class BrowserSessionRegistry {
         setBrowserSessionUserAgentMode(sess, userAgentMode)
         const persistedUa = meta.userAgentByPartition[partition]
         if (persistedUa) {
-          sess.setUserAgent(persistedUa)
-          setupClientHintsOverride(sess, persistedUa, {
-            googleAuthOverride: userAgentMode !== 'native'
-          })
-          continue
+          // Why: imports before the engine-version gate stored a fork's product version (Chrome/1.x);
+          // it is reapplied every launch, so drop it here or the profile stays blocked forever.
+          if (isUnadvertisableChromeUserAgent(persistedUa)) {
+            this.persistUserAgent(partition, null)
+          } else {
+            sess.setUserAgent(persistedUa)
+            setupClientHintsOverride(sess, persistedUa, {
+              googleAuthOverride: userAgentMode !== 'native'
+            })
+            continue
+          }
         }
 
         if (profile.userAgentMode === 'native') {
diff --git a/src/main/browser/browser-session-ua.ts b/src/main/browser/browser-session-ua.ts
index 081e1a5de..86adef919 100644
--- a/src/main/browser/browser-session-ua.ts
+++ b/src/main/browser/browser-session-ua.ts
@@ -21,6 +21,26 @@ export function cleanElectronUserAgent(ua: string): string {
   )
 }
 
+// Why: Chromium forks (Arc, Brave, …) report a product version (1.x) in
+// CFBundleShortVersionString. Advertising that as Chrome/1.x makes version-gating
+// sites treat the session as ancient Chromium, so only engine-scale majors are safe.
+export function isAdvertisableChromiumEngineVersion(version: string): boolean {
+  const normalizedVersion = version.trim()
+  // Reject malformed tokens (e.g. 70.not-a-version) so they never become Chrome/… in the UA.
+  if (!/^\d+(?:\.\d+)*$/.test(normalizedVersion)) {
+    return false
+  }
+  // Chrome 70+ covers every Chromium engine we still support; product versions stay below.
+  return Number(normalizedVersion.split('.')[0]) >= 70
+}
+
+// Why: builds without the version gate persisted Chrome/1.x for fork imports, and a stored
+// UA is reapplied on every launch — so the profile stays blocked until the value is dropped.
+export function isUnadvertisableChromeUserAgent(ua: string): boolean {
+  const chromeVersion = /Chrome\/(\S+)/.exec(ua)?.[1]
+  return chromeVersion !== undefined && !isAdvertisableChromiumEngineVersion(chromeVersion)
+}
+
 // Why: Electron's actual Chromium version (e.g. 134) differs from the source
 // browser's version (e.g. Edge 147). The sec-ch-ua Client Hints headers
 // reveal the real version, creating a mismatch that Google's anti-fraud