diff --git a/api/http/utils.ts b/api/http/utils.ts
index 34717066..b96c51f3 100644
--- a/api/http/utils.ts
+++ b/api/http/utils.ts
@@ -1,3 +1,5 @@
+import { SUBPROTOCOL } from '../index.ts'
+
 export function isObject(body: unknown): body is object {
   return typeof body === 'object' && body !== null
 }
@@ -54,10 +56,12 @@ export async function fetchJSON<ResponseJSON = unknown>(
     body: JSON.stringify(body),
     credentials: 'include',
     headers: {
-      'Content-Type': 'application/json'
+      'Content-Type': 'application/json',
+      'X-Subprotocol': String(SUBPROTOCOL)
     },
     method
   })
+
   if (opts.response) opts.response(response)
   return response as HTTPResponse<ResponseJSON>
 }
diff --git a/api/index.ts b/api/index.ts
index 235ef378..5763b4b1 100644
--- a/api/index.ts
+++ b/api/index.ts
@@ -2,6 +2,7 @@
  * Client’s protocol version
  */
 export const SUBPROTOCOL = 0
+export const SUBPROTOCOL_ERROR_MESSAGE = 'Outdated client'
 
 /**
  * Project’s version to use in UI.
diff --git a/core/client.ts b/core/client.ts
index a0c67cb7..c43ddad1 100644
--- a/core/client.ts
+++ b/core/client.ts
@@ -51,6 +51,7 @@ function getServer(): ClientOptions['server'] {
 
 let prevClient: CrossTabClient | undefined
 export const client = atom<CrossTabClient | undefined>()
+export const isOutdatedClient = atom<boolean>(false)
 
 onEnvironment(({ logStoreCreator }) => {
   let unbindUser = effect(
@@ -71,6 +72,14 @@ onEnvironment(({ logStoreCreator }) => {
         encryptActions(logux, key, {
           ignore: [deleteUser.type]
         })
+
+        /* node:coverage disable */
+        logux.node.on('error', error => {
+          if (error.type === 'wrong-subprotocol') {
+            isOutdatedClient.set(true)
+          }
+        })
+        /* node:coverage enable */
         logux.start(connect)
         prevClient = logux
         client.set(logux)
diff --git a/core/lib/http.ts b/core/lib/http.ts
index 79108fcd..86f949b3 100644
--- a/core/lib/http.ts
+++ b/core/lib/http.ts
@@ -1,4 +1,4 @@
-import type { Requester } from '@slowreader/api'
+import { type Requester, SUBPROTOCOL_ERROR_MESSAGE } from '@slowreader/api'
 
 import { getEnvironment } from '../environment.ts'
 import {
@@ -6,6 +6,7 @@ import {
   HTTPStatusError,
   UserFacingError
 } from '../errors.ts'
+import { isOutdatedClient } from '../index.ts'
 
 /**
  * Takes fetch() wrapper from `@slowreader/api/http` and do the request
@@ -31,6 +32,9 @@ export async function checkErrors<Params extends object, ResponseJSON>(
   if (!response.ok) {
     let text = await response.text()
     if (response.status === 400 && text !== 'Invalid request') {
+      if (text === SUBPROTOCOL_ERROR_MESSAGE) {
+        isOutdatedClient.set(true)
+      }
       throw new UserFacingError(text)
     } else {
       throw new HTTPStatusError(
diff --git a/core/messages/index.ts b/core/messages/index.ts
index 01849f2f..6b43364c 100644
--- a/core/messages/index.ts
+++ b/core/messages/index.ts
@@ -12,4 +12,5 @@ export * from './post/en.ts'
 export * from './profile/en.ts'
 export * from './refresh/en.ts'
 export * from './settings/en.ts'
+export * from './update-client/en.ts'
 export * from './welcome/en.ts'
diff --git a/core/messages/update-client/en.ts b/core/messages/update-client/en.ts
new file mode 100644
index 00000000..dd13f3b3
--- /dev/null
+++ b/core/messages/update-client/en.ts
@@ -0,0 +1,8 @@
+import { i18n } from '../../i18n.ts'
+
+export const updateClientMessages = i18n('updateClient', {
+  pageText:
+    'You’re using an outdated client version. Please update the app to continue.',
+  pageTitle: 'Update Your App',
+  updateButton: 'Update Now'
+})
diff --git a/core/pages/index.ts b/core/pages/index.ts
index d887bba0..9889fe1c 100644
--- a/core/pages/index.ts
+++ b/core/pages/index.ts
@@ -14,6 +14,7 @@ import { importPage } from './import.ts'
 import { profilePage } from './profile.ts'
 import { signupPage } from './signup.ts'
 import { startPage } from './start.ts'
+import { updateClientPage } from './update-client.ts'
 
 export type { AboutPage } from './about.ts'
 export type { AddPage } from './add.ts'
@@ -26,6 +27,7 @@ export type { ImportPage } from './import.ts'
 export type { ProfilePage } from './profile.ts'
 export type { SignupPage } from './signup.ts'
 export type { StartPage } from './start.ts'
+export type { UpdateClientPage } from './update-client.ts'
 
 export const pages = {
   about: aboutPage,
@@ -45,6 +47,7 @@ export const pages = {
   signup: signupPage,
   slow: slowPage,
   start: startPage,
+  updateClient: updateClientPage,
   welcome: createSimplePage('welcome')
 } satisfies {
   [Name in RouteName]: Name extends 'fast' | 'slow'
diff --git a/core/pages/update-client.ts b/core/pages/update-client.ts
new file mode 100644
index 00000000..64731134
--- /dev/null
+++ b/core/pages/update-client.ts
@@ -0,0 +1,17 @@
+import { createPage } from './common.ts'
+
+export const updateClientPage = createPage('updateClient', () => {
+  /* node:coverage disable */
+  function handleUpdateClient(): void {
+    location.reload()
+  }
+  /* node:coverage enable */
+
+  return {
+    exit() {},
+    handleUpdateClient,
+    params: {}
+  }
+})
+
+export type UpdateClientPage = ReturnType<typeof updateClientPage>
diff --git a/core/router.ts b/core/router.ts
index a5bcc59e..a04e78cf 100644
--- a/core/router.ts
+++ b/core/router.ts
@@ -28,6 +28,7 @@ export interface Routes {
     from?: number
   }
   start: {}
+  updateClient: {}
   welcome: {}
 }
 
diff --git a/core/test/pages/update-client.test.ts b/core/test/pages/update-client.test.ts
new file mode 100644
index 00000000..0bc2899e
--- /dev/null
+++ b/core/test/pages/update-client.test.ts
@@ -0,0 +1,28 @@
+import { equal } from 'node:assert'
+import { afterEach, beforeEach, test } from 'node:test'
+
+import { currentPage } from '../../index.ts'
+import {
+  cleanClientTest,
+  enableClientTest,
+  openPage,
+  setBaseTestRoute
+} from '../utils.ts'
+
+beforeEach(() => {
+  enableClientTest()
+  setBaseTestRoute({ params: {}, route: 'updateClient' })
+})
+
+afterEach(async () => {
+  await cleanClientTest()
+})
+
+test('shows update client page', () => {
+  let page = openPage({
+    params: {},
+    route: 'updateClient'
+  })
+
+  equal(currentPage.get().route, page.route)
+})
diff --git a/server/lib/http.ts b/server/lib/http.ts
index 8b38d7f5..f4500135 100644
--- a/server/lib/http.ts
+++ b/server/lib/http.ts
@@ -1,5 +1,5 @@
 import type { BaseServer } from '@logux/server'
-import type { Endpoint } from '@slowreader/api'
+import { type Endpoint, SUBPROTOCOL_ERROR_MESSAGE } from '@slowreader/api'
 import type { IncomingMessage, ServerResponse } from 'node:http'
 
 import { config } from './config.ts'
@@ -37,7 +37,7 @@ function allowCors(res: ServerResponse, origin: string): void {
     'Access-Control-Allow-Methods',
     'OPTIONS, POST, GET, PUT, DELETE'
   )
-  res.setHeader('Access-Control-Allow-Headers', 'Content-Type')
+  res.setHeader('Access-Control-Allow-Headers', 'Content-Type, X-Subprotocol')
 }
 
 const LOCALHOST = /:\/\/localhost:/
@@ -67,6 +67,18 @@ export function jsonApi<Response, Request extends object>(
         allowCors(res, req.headers.origin)
       }
     }
+
+    if (req.headers['x-subprotocol'] && server.options.minSubprotocol) {
+      let clientSubprotocol = Number(req.headers['x-subprotocol'])
+
+      if (
+        isNaN(clientSubprotocol) ||
+        clientSubprotocol < server.options.minSubprotocol
+      ) {
+        return badRequest(res, SUBPROTOCOL_ERROR_MESSAGE)
+      }
+    }
+
     if (req.method === 'OPTIONS') {
       res.writeHead(200)
       res.end()
diff --git a/server/test/auth.test.ts b/server/test/auth.test.ts
index 450329cb..3d5a7175 100644
--- a/server/test/auth.test.ts
+++ b/server/test/auth.test.ts
@@ -4,7 +4,8 @@ import {
   setPassword,
   signIn,
   signOut,
-  signUp
+  signUp,
+  SUBPROTOCOL_ERROR_MESSAGE
 } from '@slowreader/api'
 import { eq } from 'drizzle-orm'
 import { deepEqual, equal, notEqual, ok } from 'node:assert/strict'
@@ -253,3 +254,19 @@ test('supports CORS', async () => {
     'https://dev.slowreader.app'
   )
 })
+
+test('rejects old clients', async () => {
+  server = buildTestServer()
+  server.options.minSubprotocol = 2
+
+  let response = await server.fetch('/users/1', {
+    headers: {
+      'Content-Type': 'application/json',
+      'X-Subprotocol': '0'
+    },
+    method: 'POST'
+  })
+  equal(response.status, 400)
+  let text = await response.text()
+  equal(text, SUBPROTOCOL_ERROR_MESSAGE)
+})
diff --git a/web/.size-limit.json b/web/.size-limit.json
index 46f2ef92..0f6ddce4 100644
--- a/web/.size-limit.json
+++ b/web/.size-limit.json
@@ -16,7 +16,7 @@
       "!dist/assets/playpen-sans-*.woff2",
       "!dist/assets/*-{italic,math,latin-ext}-*.woff2"
     ],
-    "limit": "185 KB"
+    "limit": "190 KB"
   },
   {
     "name": "Core scripts to execute",
diff --git a/web/main/main.svelte b/web/main/main.svelte
index 0e387a0f..5e12f7af 100644
--- a/web/main/main.svelte
+++ b/web/main/main.svelte
@@ -2,8 +2,10 @@
   import {
     busy,
     currentPage,
+    isOutdatedClient,
     layoutType,
     notFound,
+    pages,
     popupsStatus,
     signOut,
     subscribeUntil,
@@ -23,6 +25,7 @@
   import ProfilePage from '../pages/profile.svelte'
   import SignupPage from '../pages/signup.svelte'
   import StartPage from '../pages/start.svelte'
+  import UpdateClientPage from '../pages/update-client.svelte'
   import FeedPopup from '../popups/feed.svelte'
   import LoadingPopup from '../popups/loading.svelte'
   import NotFoundPopup from '../popups/not-found.svelte'
@@ -59,6 +62,8 @@
   {#if !globalLoader}
     <BusyPage />
   {/if}
+{:else if $isOutdatedClient}
+  <UpdateClientPage page={pages.updateClient()} />
 {:else if $currentPage.route === 'notFound'}
   <NotFoundPage />
 {:else if $currentPage.route === 'fast'}
diff --git a/web/pages/update-client.svelte b/web/pages/update-client.svelte
new file mode 100644
index 00000000..bb105ba2
--- /dev/null
+++ b/web/pages/update-client.svelte
@@ -0,0 +1,34 @@
+<script lang="ts">
+  import { mdiSyncOff } from '@mdi/js'
+  import {
+    updateClientMessages as t,
+    type UpdateClientPage
+  } from '@slowreader/core'
+
+  import Button from '../ui/button.svelte'
+  import PageIcon from '../ui/page-icon.svelte'
+  import Stack from '../ui/stack.svelte'
+  import ThinPage from '../ui/thin-page.svelte'
+  import Title from '../ui/title.svelte'
+
+  let { page }: { page: UpdateClientPage } = $props()
+</script>
+
+<ThinPage align="center" bottomOnMobile={false} title={$t.pageTitle}>
+  <Stack align="center" gap="xl">
+    <PageIcon path={mdiSyncOff} />
+    <Title>{$t.pageTitle}</Title>
+    <p class="update-client_message">{$t.pageText}</p>
+    <Button onclick={page.handleUpdateClient} variant="main"
+      >{$t.updateButton}</Button
+    >
+  </Stack>
+</ThinPage>
+
+<style>
+  :global {
+    .update-client_message {
+      text-align: center;
+    }
+  }
+</style>
diff --git a/web/stores/url-router.ts b/web/stores/url-router.ts
index 6e3d2af6..35e55288 100644
--- a/web/stores/url-router.ts
+++ b/web/stores/url-router.ts
@@ -34,6 +34,7 @@ export const pathRouter = createRouter({
   signup: '/signup',
   slow: '/slow/:feed?',
   start: '/start',
+  updateClient: '/update-client',
   welcome: '/welcome'
 })
 
diff --git a/web/stories/pages/update-client.stories.svelte b/web/stories/pages/update-client.stories.svelte
new file mode 100644
index 00000000..54fb7821
--- /dev/null
+++ b/web/stories/pages/update-client.stories.svelte
@@ -0,0 +1,18 @@
+<script context="module" lang="ts">
+  import { pages } from '@slowreader/core'
+  import { defineMeta } from '@storybook/addon-svelte-csf'
+
+  import UpdateClientPage from '../../pages/update-client.svelte'
+  import Scene from '../scene.svelte'
+
+  let { Story } = defineMeta({
+    component: UpdateClientPage,
+    title: 'Pages/Update Client'
+  })
+</script>
+
+<Story name="Light" asChild parameters={{ layout: 'fullscreen' }}>
+  <Scene route="updateClient" user={false}>
+    <UpdateClientPage page={pages.updateClient()} />
+  </Scene>
+</Story>