big_pipe.module                                 |  8 +--
 big_pipe.services.yml                           |  7 +++
 src/Cache/Context/SessionExistsCacheContext.php | 70 +++++++++++++++++++++++++
 src/Render/Placeholder/BigPipeStrategy.php      | 12 +++--
 4 files changed, 89 insertions(+), 8 deletions(-)

diff --git a/big_pipe.module b/big_pipe.module
index 516b29b..4cacef4 100644
--- a/big_pipe.module
+++ b/big_pipe.module
@@ -14,15 +14,17 @@ use Drupal\Core\Url;
  * @see \Drupal\big_pipe\Controller\BigPipeController::setNoJsCookie()
  */
 function big_pipe_page_attachments(array &$page) {
+  $request = \Drupal::request();
   // BigPipe is only used when there is an actual session, so only add the no-JS
   // detection when there actually is a session.
   // @see \Drupal\big_pipe\Render\Placeholder\BigPipeStrategy.
-  $page['#cache']['contexts'][] = 'cookies:' . session_name();
+  $session_exists = \Drupal::service('session_configuration')->hasSession($request);
+  $page['#cache']['contexts'][] = 'session.exists';
   // Only do the no-JS detection while we don't know if there's no JS support:
   // avoid endless redirect loops.
+  $has_big_pipe_nojs_cookie = $request->cookies->has(BigPipeStrategy::NOJS_COOKIE);
   $page['#cache']['contexts'][] = 'cookies:' . BigPipeStrategy::NOJS_COOKIE;
-  $cookies = \Drupal::request()->cookies;
-  if ($cookies->has(session_name()) && !$cookies->has(BigPipeStrategy::NOJS_COOKIE)) {
+  if ($session_exists && !$has_big_pipe_nojs_cookie) {
     $page['#attached']['html_head'][] = [
       [
         // Redirect through a 'Refresh' meta tag if JavaScript is disabled.
diff --git a/big_pipe.services.yml b/big_pipe.services.yml
index b8ff9d4..cf5ca80 100644
--- a/big_pipe.services.yml
+++ b/big_pipe.services.yml
@@ -18,3 +18,10 @@ services:
     decorates: html_response.attachments_processor
     decoration_inner_name: html_response.attachments_processor.original
     arguments: ['@html_response.attachments_processor.original', '@asset.resolver', '@config.factory', '@asset.css.collection_renderer', '@asset.js.collection_renderer', '@request_stack', '@renderer', '@module_handler']
+
+  # @todo Move this into Drupal 8 core.
+  cache_context.session.exists:
+    class: Drupal\big_pipe\Cache\Context\SessionExistsCacheContext
+    arguments: ['@session_configuration', '@request_stack']
+    tags:
+      - { name: cache.context}
diff --git a/src/Cache/Context/SessionExistsCacheContext.php b/src/Cache/Context/SessionExistsCacheContext.php
new file mode 100644
index 0000000..eaf02e1
--- /dev/null
+++ b/src/Cache/Context/SessionExistsCacheContext.php
@@ -0,0 +1,70 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\big_pipe\Cache\Context\SessionExistsCacheContext.
+ */
+
+namespace Drupal\big_pipe\Cache\Context;
+
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Cache\Context\CacheContextInterface;
+use Drupal\Core\Session\SessionConfigurationInterface;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * Defines the SessionCacheContext service, for "per session" caching.
+ *
+ * Cache context ID: 'session'.
+ */
+class SessionExistsCacheContext implements CacheContextInterface {
+
+  /**
+   * The session configuration.
+   *
+   * @var \Drupal\Core\Session\SessionConfigurationInterface
+   */
+  protected $sessionConfiguration;
+
+  /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * Constructs a new SessionExistsCacheContext class.
+   *
+   * @param \Drupal\Core\Session\SessionConfigurationInterface $session_configuration
+   *   The session configuration.
+   * @param \Symfony\Component\HttpFoundation\RequestStack
+   *   The request stack.
+   */
+  public function __construct(SessionConfigurationInterface $session_configuration, RequestStack $request_stack) {
+    $this->sessionConfiguration = $session_configuration;
+    $this->requestStack = $request_stack;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getLabel() {
+    return t('Session exists');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContext() {
+    return $this->sessionConfiguration->hasSession($this->requestStack->getCurrentRequest()) ? '1' : '0';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheableMetadata() {
+    return new CacheableMetadata();
+  }
+
+}
diff --git a/src/Render/Placeholder/BigPipeStrategy.php b/src/Render/Placeholder/BigPipeStrategy.php
index d5b6c4c..96524be 100644
--- a/src/Render/Placeholder/BigPipeStrategy.php
+++ b/src/Render/Placeholder/BigPipeStrategy.php
@@ -116,9 +116,6 @@ class BigPipeStrategy implements PlaceholderStrategyInterface {
    *   The BigPipe placeholders.
    */
   protected function doProcessPlaceholders(array $placeholders) {
-    $non_html_placeholder_cache_contexts = ['cookies:' . session_name()];
-    $html_placeholder_cache_contexts = ['cookies:' . session_name(), 'cookies:' . static::NOJS_COOKIE];
-
     $overridden_placeholders = [];
     foreach ($placeholders as $placeholder => $placeholder_elements) {
       // BigPipe uses JavaScript and the DOM to find the placeholder to replace.
@@ -136,7 +133,6 @@ class BigPipeStrategy implements PlaceholderStrategyInterface {
       // @see \Drupal\Core\Form\FormBuilder::renderPlaceholderFormAction()
       if ($placeholder[0] !== '<' || $placeholder !== Html::normalize($placeholder)) {
         $overridden_placeholders[$placeholder] = static::createBigPipeNoJsPlaceholder($placeholder, $placeholder_elements);
-        $overridden_placeholders[$placeholder]['#cache']['contexts'] = $non_html_placeholder_cache_contexts;
       }
       else {
         // If the current request/session doesn't have JavaScript, fall back to
@@ -147,7 +143,7 @@ class BigPipeStrategy implements PlaceholderStrategyInterface {
         else {
           $overridden_placeholders[$placeholder] = static::createBigPipeJsPlaceholder($placeholder, $placeholder_elements);
         }
-        $overridden_placeholders[$placeholder]['#cache']['contexts'] = $html_placeholder_cache_contexts;
+        $overridden_placeholders[$placeholder]['#cache']['contexts'][] = 'cookies:' . static::NOJS_COOKIE;
       }
     }
 
@@ -186,6 +182,9 @@ class BigPipeStrategy implements PlaceholderStrategyInterface {
       '#markup' => '<div data-big-pipe-selector="' . Html::escape($big_pipe_js_selector) . '"></div>',
       '#cache' => [
         'max-age' => 0,
+        'contexts' => [
+          'session.exists',
+        ],
       ],
       '#attached' => [
         'library' => [
@@ -222,6 +221,9 @@ class BigPipeStrategy implements PlaceholderStrategyInterface {
       '#markup' => '<div data-big-pipe-selector-nojs="' . $html_placeholder . '"></div>',
       '#cache' => [
         'max-age' => 0,
+        'contexts' => [
+          'session.exists',
+        ],
       ],
       '#attached' => [
         'big_pipe_nojs_placeholders' => [