diff --git a/config/mercure.yaml b/config/mercure.yaml
index b9af31a96..1e9a071c0 100644
--- a/config/mercure.yaml
+++ b/config/mercure.yaml
@@ -13,7 +13,9 @@ services:
 
     Pimcore\Bundle\StudioBackendBundle\Mercure\Service\ServerTokenService: ~
 
-    Pimcore\Bundle\StudioBackendBundle\Mercure\Service\ClientTokenService: ~
+    Pimcore\Bundle\StudioBackendBundle\Mercure\Service\ClientTokenService:
+        arguments:
+            $cookieLifetime: '%pimcore_studio_backend.mercure_settings.cookie_lifetime%'
 
     Pimcore\Bundle\StudioBackendBundle\Mercure\Service\HubServiceInterface:
         class: Pimcore\Bundle\StudioBackendBundle\Mercure\Service\HubService
diff --git a/src/Mercure/Controller/JwtController.php b/src/Mercure/Controller/JwtController.php
index 77b9580c3..dc3fee694 100644
--- a/src/Mercure/Controller/JwtController.php
+++ b/src/Mercure/Controller/JwtController.php
@@ -13,8 +13,10 @@
 
 namespace Pimcore\Bundle\StudioBackendBundle\Mercure\Controller;
 
+use OpenApi\Attributes\JsonContent;
 use OpenApi\Attributes\Post;
 use Pimcore\Bundle\StudioBackendBundle\Controller\AbstractApiController;
+use Pimcore\Bundle\StudioBackendBundle\Mercure\Schema\Authorization;
 use Pimcore\Bundle\StudioBackendBundle\Mercure\Service\HubServiceInterface;
 use Pimcore\Bundle\StudioBackendBundle\OpenApi\Attribute\Response\DefaultResponses;
 use Pimcore\Bundle\StudioBackendBundle\OpenApi\Attribute\Response\SuccessResponse;
@@ -45,15 +47,21 @@ public function __construct(
     )]
     #[SuccessResponse(
         description: 'mercure_create_cookie_success_response',
+        content: new JsonContent(ref: Authorization::class)
     )]
     #[DefaultResponses]
     public function auth(): Response
     {
-        $res = new Response();
-        $res->headers->setCookie(
+        // The cookie authorises the subscription; the body tells the client when to come back for
+        // a new one. The hub checks authorisation once, at connect time, so a client that lets the
+        // cookie lapse reconnects anonymously and loses every private update without any error.
+        $response = $this->jsonResponse(
+            new Authorization($this->hubService->getCookieLifetime())
+        );
+        $response->headers->setCookie(
             $this->hubService->createCookie()
         );
 
-        return $res;
+        return $response;
     }
 }
diff --git a/src/Mercure/Schema/Authorization.php b/src/Mercure/Schema/Authorization.php
new file mode 100644
index 000000000..e08228efb
--- /dev/null
+++ b/src/Mercure/Schema/Authorization.php
@@ -0,0 +1,47 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * This source file is available under the terms of the
+ * Pimcore Open Core License (POCL)
+ * Full copyright and license information is available in
+ * LICENSE.md which is distributed with this source code.
+ *
+ *  @copyright  Copyright (c) Pimcore GmbH (https://www.pimcore.com)
+ *  @license    Pimcore Open Core License (POCL)
+ */
+
+namespace Pimcore\Bundle\StudioBackendBundle\Mercure\Schema;
+
+use OpenApi\Attributes\Property;
+use OpenApi\Attributes\Schema;
+
+/**
+ * @internal
+ */
+#[Schema(
+    title: 'MercureAuthorization',
+    required: [
+        'cookieLifetime',
+    ],
+    type: 'object'
+)]
+final readonly class Authorization
+{
+    public function __construct(
+        #[Property(
+            description: 'Lifetime of the authorization cookie in seconds. A client has to request a new ' .
+                'cookie before it elapses: the hub authorises a subscription once, at connect time, so an ' .
+                'expired cookie leaves every reconnect anonymous and silently drops all private updates.',
+            type: 'integer',
+            example: 3600
+        )]
+        private int $cookieLifetime
+    ) {
+    }
+
+    public function getCookieLifetime(): int
+    {
+        return $this->cookieLifetime;
+    }
+}
diff --git a/src/Mercure/Service/ClientTokenService.php b/src/Mercure/Service/ClientTokenService.php
index 2127cdf18..528a14b44 100644
--- a/src/Mercure/Service/ClientTokenService.php
+++ b/src/Mercure/Service/ClientTokenService.php
@@ -13,6 +13,7 @@
 
 namespace Pimcore\Bundle\StudioBackendBundle\Mercure\Service;
 
+use DateTimeImmutable;
 use Pimcore\Bundle\StudioBackendBundle\Mercure\Model\TopicCollection;
 use Pimcore\Bundle\StudioBackendBundle\Mercure\Service\Loader\TopicLoaderInterface;
 use Symfony\Component\Mercure\Jwt\TokenFactoryInterface;
@@ -25,7 +26,8 @@
 {
     public function __construct(
         private TopicLoaderInterface $topicLoader,
-        private TokenFactoryInterface $tokenFactory
+        private TokenFactoryInterface $tokenFactory,
+        private int $cookieLifetime = 3600
     ) {
     }
 
@@ -39,6 +41,12 @@ public function getJwt(): string
         return $this->tokenFactory->create(
             $this->getTopicCollection()->getClientSubscribableTopics(),
             $this->getTopicCollection()->getClientPublishableTopics(),
+            // Without an explicit claim the factory derives `exp` from `session.cookie_lifetime`
+            // (or 3600), which has nothing to do with the lifetime the cookie is stamped with and
+            // the client is told to renew on. Configuring a longer `cookie_lifetime` would then
+            // leave a window where the browser still sends a cookie the hub already rejects, which
+            // is the dead-authorization state this whole mechanism exists to avoid.
+            ['exp' => new DateTimeImmutable('+' . $this->cookieLifetime . ' seconds')]
         );
     }
 }
diff --git a/src/Mercure/Service/HubService.php b/src/Mercure/Service/HubService.php
index 717362bf1..b9efed9e1 100644
--- a/src/Mercure/Service/HubService.php
+++ b/src/Mercure/Service/HubService.php
@@ -31,6 +31,11 @@ public function __construct(
     ) {
     }
 
+    public function getCookieLifetime(): int
+    {
+        return $this->cookieLifetime;
+    }
+
     public function createCookie(): Cookie
     {
         $urlParts = parse_url($this->urlService->getClientSideUrl());
diff --git a/src/Mercure/Service/HubServiceInterface.php b/src/Mercure/Service/HubServiceInterface.php
index 0c8e6a09b..e671472f9 100644
--- a/src/Mercure/Service/HubServiceInterface.php
+++ b/src/Mercure/Service/HubServiceInterface.php
@@ -21,4 +21,10 @@
 interface HubServiceInterface
 {
     public function createCookie(): Cookie;
+
+    /**
+     * Lifetime of the cookie returned by createCookie(), in seconds. Clients need it to
+     * renew their authorization before the hub stops accepting it.
+     */
+    public function getCookieLifetime(): int;
 }
diff --git a/tests/Unit/Mercure/Service/HubServiceTest.php b/tests/Unit/Mercure/Service/HubServiceTest.php
new file mode 100644
index 000000000..4bb10390e
--- /dev/null
+++ b/tests/Unit/Mercure/Service/HubServiceTest.php
@@ -0,0 +1,115 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * This source file is available under the terms of the
+ * Pimcore Open Core License (POCL)
+ * Full copyright and license information is available in
+ * LICENSE.md which is distributed with this source code.
+ *
+ *  @copyright  Copyright (c) Pimcore GmbH (https://www.pimcore.com)
+ *  @license    Pimcore Open Core License (POCL)
+ */
+
+namespace Pimcore\Bundle\StudioBackendBundle\Tests\Unit\Mercure\Service;
+
+use Codeception\Test\Unit;
+use Pimcore\Bundle\StudioBackendBundle\Mercure\Model\TopicCollection;
+use Pimcore\Bundle\StudioBackendBundle\Mercure\Service\ClientTokenService;
+use Pimcore\Bundle\StudioBackendBundle\Mercure\Service\HubService;
+use Pimcore\Bundle\StudioBackendBundle\Mercure\Service\Loader\TopicLoaderInterface;
+use Pimcore\Bundle\StudioBackendBundle\Mercure\Service\UrlServiceInterface;
+use Symfony\Component\Mercure\Jwt\LcobucciFactory;
+use Symfony\Component\Mercure\Jwt\TokenProviderInterface;
+
+final class HubServiceTest extends Unit
+{
+    private const int CUSTOM_LIFETIME = 900;
+
+    private const string JWT_KEY = 'a-test-secret-that-is-long-enough-for-hmac-sha256';
+
+    public function testGetCookieLifetimeReturnsConfiguredValue(): void
+    {
+        $this->assertSame(
+            self::CUSTOM_LIFETIME,
+            $this->createHubService(self::CUSTOM_LIFETIME)->getCookieLifetime()
+        );
+    }
+
+    public function testGetCookieLifetimeDefaultsToOneHour(): void
+    {
+        $service = new HubService(
+            $this->makeEmpty(TokenProviderInterface::class, ['getJwt' => 'jwt']),
+            $this->makeEmpty(UrlServiceInterface::class, ['getClientSideUrl' => 'https://example.com/hub']),
+        );
+
+        $this->assertSame(3600, $service->getCookieLifetime());
+    }
+
+    /**
+     * The lifetime a client renews on and the lifetime the cookie actually expires on must be the
+     * same number. If they drift apart, a client that renews "in time" still reconnects with an
+     * expired cookie, which the hub accepts as anonymous, silently dropping every private update.
+     */
+    public function testCookieExpiryMatchesTheAdvertisedLifetime(): void
+    {
+        $service = $this->createHubService(self::CUSTOM_LIFETIME);
+
+        $before = time();
+        $expiresAt = $service->createCookie()->getExpiresTime();
+        $after = time();
+
+        $this->assertGreaterThanOrEqual($before + $service->getCookieLifetime(), $expiresAt);
+        $this->assertLessThanOrEqual($after + $service->getCookieLifetime(), $expiresAt);
+    }
+
+    /**
+     * The cookie carries a JWT with its own `exp`, and the hub rejects the subscription as soon as
+     * that claim has passed - regardless of how long the browser keeps sending the cookie. So the
+     * advertised lifetime has to match the TOKEN, not just the outer cookie: `LcobucciFactory`
+     * otherwise derives `exp` from `session.cookie_lifetime` (or 3600), and a `cookie_lifetime`
+     * configured above that would leave the client renewing long after the hub stopped accepting
+     * its token.
+     */
+    public function testAdvertisedLifetimeMatchesTheTokenExpiry(): void
+    {
+        $lifetime = 7200;
+        $service = new HubService(
+            new ClientTokenService(
+                $this->makeEmpty(TopicLoaderInterface::class, [
+                    'loadTopics' => new TopicCollection([], [], [], ['studio-backend-default']),
+                ]),
+                new LcobucciFactory(self::JWT_KEY),
+                $lifetime
+            ),
+            $this->makeEmpty(UrlServiceInterface::class, ['getClientSideUrl' => 'https://example.com/hub']),
+            $lifetime
+        );
+
+        $cookie = $service->createCookie();
+        $claims = $this->decodeClaims((string) $cookie->getValue());
+
+        $this->assertSame($lifetime, $service->getCookieLifetime());
+        $this->assertEqualsWithDelta($lifetime, $claims['exp'] - time(), 5);
+        $this->assertEqualsWithDelta($claims['exp'], $cookie->getExpiresTime(), 5);
+    }
+
+    /**
+     * @return array<string, mixed>
+     */
+    private function decodeClaims(string $jwt): array
+    {
+        $payload = explode('.', $jwt)[1];
+
+        return json_decode(base64_decode(strtr($payload, '-_', '+/')), true, 512, JSON_THROW_ON_ERROR);
+    }
+
+    private function createHubService(int $cookieLifetime): HubService
+    {
+        return new HubService(
+            $this->makeEmpty(TokenProviderInterface::class, ['getJwt' => 'jwt']),
+            $this->makeEmpty(UrlServiceInterface::class, ['getClientSideUrl' => 'https://example.com/hub']),
+            $cookieLifetime
+        );
+    }
+}