diff --git a/src/platform/cloud/onboarding/CloudLoginView.vue b/src/platform/cloud/onboarding/CloudLoginView.vue
index fa7c2fb92fe..c32742b434c 100644
--- a/src/platform/cloud/onboarding/CloudLoginView.vue
+++ b/src/platform/cloud/onboarding/CloudLoginView.vue
@@ -96,31 +96,47 @@
 
 <script setup lang="ts">
 import Message from 'primevue/message'
-import { ref } from 'vue'
+import { ref, watch } from 'vue'
 import { useI18n } from 'vue-i18n'
 import { useRoute, useRouter } from 'vue-router'
 
 import Button from '@/components/ui/button/Button.vue'
 import { useAuthActions } from '@/composables/auth/useAuthActions'
+import { hasDesktopLoginRequest } from '@/platform/cloud/onboarding/desktopLoginBridge'
 import CloudSignInForm from '@/platform/cloud/onboarding/components/CloudSignInForm.vue'
 import { usePostAuthRedirect } from '@/platform/cloud/onboarding/composables/usePostAuthRedirect'
 import type { SignInData } from '@/schemas/signInSchema'
 import { getGoogleSsoBlockedReason } from '@/base/webviewDetection'
+import { useAuthStore } from '@/stores/authStore'
 
 const { t } = useI18n()
 const router = useRouter()
 const route = useRoute()
 const authActions = useAuthActions()
+const authStore = useAuthStore()
 const isSecureContext = globalThis.isSecureContext
 const authError = ref('')
 const showEmailForm = ref(false)
 const googleSsoBlockedReason = getGoogleSsoBlockedReason()
+let desktopLoginCompletionStarted = false
 const { onAuthSuccess } = usePostAuthRedirect({
   authError,
   successSummary: 'Login Completed',
   defaultRedirect: () => ({ name: 'cloud-user-check' })
 })
 
+watch(
+  () => [authStore.isInitialized, authStore.currentUser] as const,
+  ([isInitialized, user]) => {
+    if (!isInitialized || !user) return
+    if (!hasDesktopLoginRequest(route.query)) return
+    if (desktopLoginCompletionStarted) return
+    desktopLoginCompletionStarted = true
+    void onAuthSuccess()
+  },
+  { immediate: true }
+)
+
 function switchToEmailForm() {
   showEmailForm.value = true
 }
diff --git a/src/platform/cloud/onboarding/composables/usePostAuthRedirect.ts b/src/platform/cloud/onboarding/composables/usePostAuthRedirect.ts
index 19b768f1e1e..25e9624568a 100644
--- a/src/platform/cloud/onboarding/composables/usePostAuthRedirect.ts
+++ b/src/platform/cloud/onboarding/composables/usePostAuthRedirect.ts
@@ -4,8 +4,10 @@ import type { RouteLocationRaw } from 'vue-router'
 import { useRoute, useRouter } from 'vue-router'
 
 import { useOAuthPostLoginRedirect } from '@/platform/cloud/oauth/useOAuthPostLoginRedirect'
+import { completeDesktopLoginIfNeeded } from '@/platform/cloud/onboarding/desktopLoginBridge'
 import { getSafePreviousFullPath } from '@/platform/cloud/onboarding/utils/previousFullPath'
 import { useToastStore } from '@/platform/updates/common/toastStore'
+import { useAuthStore } from '@/stores/authStore'
 
 /**
  * Shared post-authentication redirect logic used by both CloudLoginView and
@@ -22,8 +24,28 @@ export function usePostAuthRedirect(options: {
   const route = useRoute()
   const toastStore = useToastStore()
   const { resumeOAuthIfNeeded } = useOAuthPostLoginRedirect()
+  const authStore = useAuthStore()
 
   async function onAuthSuccess() {
+    try {
+      const desktopLoginCompleted = await completeDesktopLoginIfNeeded(
+        route.query,
+        authStore.currentUser
+      )
+      if (desktopLoginCompleted) return
+    } catch (error) {
+      const message =
+        error instanceof Error ? error.message : 'Desktop login callback failed'
+      options.authError.value = message
+      toastStore.add({
+        severity: 'error',
+        summary: t('g.error'),
+        detail: message,
+        life: 4000
+      })
+      return
+    }
+
     toastStore.add({
       severity: 'success',
       summary: options.successSummary,
diff --git a/src/platform/cloud/onboarding/desktopLoginBridge.test.ts b/src/platform/cloud/onboarding/desktopLoginBridge.test.ts
new file mode 100644
index 00000000000..53a8e9fe6bc
--- /dev/null
+++ b/src/platform/cloud/onboarding/desktopLoginBridge.test.ts
@@ -0,0 +1,80 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import {
+  completeDesktopLoginIfNeeded,
+  getDesktopLoginRequest,
+  hasDesktopLoginRequest
+} from './desktopLoginBridge'
+
+const hoisted = vi.hoisted(() => ({
+  identifyPostHogUser: vi.fn()
+}))
+
+vi.mock('@/platform/telemetry/providers/cloud/posthogIdentity', () => ({
+  identifyPostHogUser: hoisted.identifyPostHogUser
+}))
+
+vi.mock('@/config/firebase', () => ({
+  getFirebaseConfig: () => ({ apiKey: 'firebase-api-key' })
+}))
+
+describe('desktopLoginBridge', () => {
+  beforeEach(() => {
+    hoisted.identifyPostHogUser.mockClear()
+    vi.unstubAllGlobals()
+  })
+
+  it('accepts localhost callback requests with state', () => {
+    const query = {
+      desktop_login_callback: 'http://localhost:9876/callback',
+      desktop_login_state: 'state-123'
+    }
+
+    expect(hasDesktopLoginRequest(query)).toBe(true)
+    expect(getDesktopLoginRequest(query)).toMatchObject({
+      callbackUrl: new URL('http://localhost:9876/callback'),
+      state: 'state-123'
+    })
+  })
+
+  it('rejects non-loopback callback requests', () => {
+    expect(
+      hasDesktopLoginRequest({
+        desktop_login_callback: 'https://evil.example/callback',
+        desktop_login_state: 'state-123'
+      })
+    ).toBe(false)
+  })
+
+  it('identifies the browser PostHog user and posts Firebase user payload to Desktop', async () => {
+    const fetchMock = vi.fn().mockResolvedValue({ ok: true, status: 204 })
+    vi.stubGlobal('fetch', fetchMock)
+
+    const completed = await completeDesktopLoginIfNeeded(
+      {
+        desktop_login_callback: 'http://localhost:9876/callback',
+        desktop_login_state: 'state-123'
+      },
+      {
+        uid: 'user-123',
+        toJSON: () => ({ uid: 'user-123', email: 'person@example.com' })
+      } as never
+    )
+
+    expect(completed).toBe(true)
+    expect(hoisted.identifyPostHogUser).toHaveBeenCalledWith('user-123')
+    expect(fetchMock).toHaveBeenCalledWith(
+      'http://localhost:9876/callback',
+      expect.objectContaining({
+        method: 'POST',
+        mode: 'cors',
+        credentials: 'omit',
+        body: JSON.stringify({
+          state: 'state-123',
+          apiKey: 'firebase-api-key',
+          user: { uid: 'user-123', email: 'person@example.com' }
+        })
+      })
+    )
+  })
+})
diff --git a/src/platform/cloud/onboarding/desktopLoginBridge.ts b/src/platform/cloud/onboarding/desktopLoginBridge.ts
new file mode 100644
index 00000000000..d716d5be260
--- /dev/null
+++ b/src/platform/cloud/onboarding/desktopLoginBridge.ts
@@ -0,0 +1,84 @@
+import type { User } from 'firebase/auth'
+import type { LocationQuery } from 'vue-router'
+
+import { getFirebaseConfig } from '@/config/firebase'
+import { identifyPostHogUser } from '@/platform/telemetry/providers/cloud/posthogIdentity'
+
+const CALLBACK_PARAM = 'desktop_login_callback'
+const STATE_PARAM = 'desktop_login_state'
+const MAX_STATE_LENGTH = 256
+
+function firstQueryValue(value: LocationQuery[string]): string | null {
+  if (Array.isArray(value)) return value[0] ?? null
+  return value ?? null
+}
+
+function parseLoopbackCallback(rawUrl: string): URL | null {
+  let url: URL
+  try {
+    url = new URL(rawUrl)
+  } catch {
+    return null
+  }
+
+  if (url.protocol !== 'http:') return null
+  if (!['localhost', '127.0.0.1'].includes(url.hostname)) return null
+  if (url.pathname !== '/callback') return null
+
+  return url
+}
+
+export function hasDesktopLoginRequest(query: LocationQuery): boolean {
+  return Boolean(getDesktopLoginRequest(query))
+}
+
+export function getDesktopLoginRequest(query: LocationQuery): {
+  callbackUrl: URL
+  state: string
+} | null {
+  const callback = firstQueryValue(query[CALLBACK_PARAM])
+  const state = firstQueryValue(query[STATE_PARAM])
+  if (!callback || !state || state.length > MAX_STATE_LENGTH) return null
+
+  const callbackUrl = parseLoopbackCallback(callback)
+  if (!callbackUrl) return null
+
+  return { callbackUrl, state }
+}
+
+export async function completeDesktopLoginIfNeeded(
+  query: LocationQuery,
+  user: User | null | undefined
+): Promise<boolean> {
+  const request = getDesktopLoginRequest(query)
+  if (!request || !user) return false
+
+  // Queue-safe: if PostHog has not finished initializing yet, the cloud
+  // provider flushes this identify call once the browser cookie store is ready.
+  identifyPostHogUser(user.uid)
+
+  const firebaseConfig = getFirebaseConfig()
+  if (!firebaseConfig.apiKey) {
+    throw new Error('Firebase API key missing')
+  }
+
+  const response = await fetch(request.callbackUrl.href, {
+    method: 'POST',
+    mode: 'cors',
+    credentials: 'omit',
+    headers: {
+      'Content-Type': 'application/json'
+    },
+    body: JSON.stringify({
+      state: request.state,
+      apiKey: firebaseConfig.apiKey,
+      user: user.toJSON()
+    })
+  })
+
+  if (!response.ok) {
+    throw new Error(`Desktop login callback returned ${response.status}`)
+  }
+
+  return true
+}
diff --git a/src/platform/cloud/onboarding/onboardingCloudRoutes.ts b/src/platform/cloud/onboarding/onboardingCloudRoutes.ts
index d9fd77577bb..cfe19d10348 100644
--- a/src/platform/cloud/onboarding/onboardingCloudRoutes.ts
+++ b/src/platform/cloud/onboarding/onboardingCloudRoutes.ts
@@ -1,5 +1,6 @@
 import type { RouteRecordRaw } from 'vue-router'
 
+import { hasDesktopLoginRequest } from '@/platform/cloud/onboarding/desktopLoginBridge'
 import { getOAuthRequestId } from '@/platform/cloud/oauth/oauthState'
 
 // `oauth_request_id` capture lives in the global router.beforeEach guard
@@ -53,7 +54,7 @@ export const cloudOnboardingRoutes: RouteRecordRaw[] = [
           import('@/platform/cloud/onboarding/CloudLoginView.vue'),
         beforeEnter: async (to, _from, next) => {
           // Only redirect if not explicitly switching accounts
-          if (!to.query.switchAccount) {
+          if (!to.query.switchAccount && !hasDesktopLoginRequest(to.query)) {
             const { useCurrentUser } =
               await import('@/composables/auth/useCurrentUser')
             const { isLoggedIn } = useCurrentUser()
@@ -71,7 +72,7 @@ export const cloudOnboardingRoutes: RouteRecordRaw[] = [
         component: () =>
           import('@/platform/cloud/onboarding/CloudSignupView.vue'),
         beforeEnter: async (to, _from, next) => {
-          if (!to.query.switchAccount) {
+          if (!to.query.switchAccount && !hasDesktopLoginRequest(to.query)) {
             const { useCurrentUser } =
               await import('@/composables/auth/useCurrentUser')
             const { isLoggedIn } = useCurrentUser()
diff --git a/src/platform/telemetry/providers/cloud/PostHogTelemetryProvider.ts b/src/platform/telemetry/providers/cloud/PostHogTelemetryProvider.ts
index 55bbcf08182..94b43b75549 100644
--- a/src/platform/telemetry/providers/cloud/PostHogTelemetryProvider.ts
+++ b/src/platform/telemetry/providers/cloud/PostHogTelemetryProvider.ts
@@ -9,6 +9,10 @@ import { useSubscription } from '@/platform/cloud/subscription/composables/useSu
 import { remoteConfig } from '@/platform/remoteConfig/remoteConfig'
 import type { RemoteConfig } from '@/platform/remoteConfig/types'
 
+import {
+  identifyPostHogUser,
+  setPostHogIdentityClient
+} from './posthogIdentity'
 import type {
   AuthMetadata,
   DefaultViewSetMetadata,
@@ -139,6 +143,7 @@ export class PostHogTelemetryProvider implements TelemetryProvider {
               // to clear localStorage on other subdomains, causing identity bleed on logout.
               before_send: createPostHogBeforeSend()
             })
+            setPostHogIdentityClient(this.posthog)
             this.isInitialized = true
             this.flushEventQueue()
             this.registerDesktopEntryProps()
@@ -146,7 +151,7 @@ export class PostHogTelemetryProvider implements TelemetryProvider {
             const currentUser = useCurrentUser()
             currentUser.onUserResolved((user) => {
               if (this.posthog && user.id) {
-                this.posthog.identify(user.id)
+                identifyPostHogUser(user.id)
                 this.setDesktopEntryPersonProperties()
                 this.setSubscriptionProperties()
               }
@@ -166,14 +171,17 @@ export class PostHogTelemetryProvider implements TelemetryProvider {
           })
           .catch((error) => {
             console.error('Failed to load PostHog:', error)
+            setPostHogIdentityClient(null)
             this.isEnabled = false
           })
       } catch (error) {
         console.error('Failed to initialize PostHog:', error)
+        setPostHogIdentityClient(null)
         this.isEnabled = false
       }
     } else {
       console.warn('PostHog API key not provided in runtime config')
+      setPostHogIdentityClient(null)
       this.isEnabled = false
     }
   }
diff --git a/src/platform/telemetry/providers/cloud/posthogIdentity.test.ts b/src/platform/telemetry/providers/cloud/posthogIdentity.test.ts
new file mode 100644
index 00000000000..bdcc0a2af9a
--- /dev/null
+++ b/src/platform/telemetry/providers/cloud/posthogIdentity.test.ts
@@ -0,0 +1,39 @@
+import type { PostHog } from 'posthog-js'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+import {
+  identifyPostHogUser,
+  setPostHogIdentityClient
+} from './posthogIdentity'
+
+describe('posthogIdentity', () => {
+  beforeEach(() => {
+    setPostHogIdentityClient(null)
+  })
+
+  afterEach(() => {
+    setPostHogIdentityClient(null)
+  })
+
+  it('queues identify calls until the PostHog client is ready', () => {
+    identifyPostHogUser('user-123')
+
+    const posthog = {
+      identify: vi.fn()
+    } as unknown as PostHog
+    setPostHogIdentityClient(posthog)
+
+    expect(posthog.identify).toHaveBeenCalledWith('user-123')
+  })
+
+  it('identifies immediately when the PostHog client is ready', () => {
+    const posthog = {
+      identify: vi.fn()
+    } as unknown as PostHog
+    setPostHogIdentityClient(posthog)
+
+    identifyPostHogUser('user-123')
+
+    expect(posthog.identify).toHaveBeenCalledWith('user-123')
+  })
+})
diff --git a/src/platform/telemetry/providers/cloud/posthogIdentity.ts b/src/platform/telemetry/providers/cloud/posthogIdentity.ts
new file mode 100644
index 00000000000..821bb62f9e2
--- /dev/null
+++ b/src/platform/telemetry/providers/cloud/posthogIdentity.ts
@@ -0,0 +1,36 @@
+import type { PostHog } from 'posthog-js'
+
+let posthogClient: PostHog | null = null
+const pendingUserIds = new Set<string>()
+
+function tryIdentify(userId: string): void {
+  if (!posthogClient) {
+    pendingUserIds.add(userId)
+    return
+  }
+
+  try {
+    posthogClient.identify(userId)
+  } catch (error) {
+    console.error('Failed to identify PostHog user:', error)
+  }
+}
+
+export function setPostHogIdentityClient(client: PostHog | null): void {
+  posthogClient = client
+  if (!client) {
+    pendingUserIds.clear()
+    return
+  }
+
+  const queuedUserIds = Array.from(pendingUserIds)
+  pendingUserIds.clear()
+  for (const userId of queuedUserIds) {
+    tryIdentify(userId)
+  }
+}
+
+export function identifyPostHogUser(userId: string | null | undefined): void {
+  if (!userId) return
+  tryIdentify(userId)
+}