diff --git a/src/EventSubscriber/CustomElementsFormatSubscriber.php b/src/EventSubscriber/CustomElementsFormatSubscriber.php
index e1a26d2dc33fac4ce01413873448a5d0c7d86133..62516f3829d26beced67c3edc20b6e8031f356f5 100644
--- a/src/EventSubscriber/CustomElementsFormatSubscriber.php
+++ b/src/EventSubscriber/CustomElementsFormatSubscriber.php
@@ -2,10 +2,10 @@
 
 namespace Drupal\lupus_ce_renderer\EventSubscriber;
 
-use Symfony\Component\HttpKernel\Event\ResponseEvent;
 use Drupal\Core\Site\Settings;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\HttpFoundation\RedirectResponse;
+use Symfony\Component\HttpKernel\Event\ResponseEvent;
 use Symfony\Component\HttpKernel\Exception\NotAcceptableHttpException;
 use Symfony\Component\HttpKernel\KernelEvents;
 
@@ -29,10 +29,15 @@ class CustomElementsFormatSubscriber implements EventSubscriberInterface {
     $response = $event->getResponse();
     $request = $event->getRequest();
 
-    // Dis-allow html format when custom elements rendering is enabled. Other
-    // formats like 'json' should be allowed, so custom APIs can be added.
+    // Dis-allow html responses when custom elements rendering is enabled.
+    // Other formats like 'json' are kept allowed for backwards compatibility.
     // @todo Make this alterable somehow.
-    if ($request->attributes->get('lupus_ce_renderer') && $request->getRequestFormat('custom_elements') == 'html') {
+    // Block responses that would result in HTML output. Check both the
+    // request format and whether the response has no explicit content type
+    // set (which defaults to text/html via Response::prepare()).
+    $is_html = $request->getRequestFormat('html') === 'html'
+      || !$response->headers->has('Content-Type');
+    if ($request->attributes->get('lupus_ce_renderer') && $is_html && !$response instanceof RedirectResponse) {
       // Throw status 406 http exception, but take care to not run in an
       // end-less loop here.
       if (!$event->getRequest()->attributes->get('exception') || !$event->getRequest()->attributes->get('exception') instanceof NotAcceptableHttpException) {
diff --git a/tests/modules/lupus_ce_renderer_test/lupus_ce_renderer_test.routing.yml b/tests/modules/lupus_ce_renderer_test/lupus_ce_renderer_test.routing.yml
index b0970b3f6ae896932d11914759b40dc60d800a9b..1f4daddbd44f1e812d08ffa038fe611223c4211d 100644
--- a/tests/modules/lupus_ce_renderer_test/lupus_ce_renderer_test.routing.yml
+++ b/tests/modules/lupus_ce_renderer_test/lupus_ce_renderer_test.routing.yml
@@ -29,6 +29,14 @@ lupus_ce_renderer_test.admin_render_array:
   options:
     _admin_route: TRUE
 
+lupus_ce_renderer_test.render_array:
+  path: '/lupus-ce-renderer-test/render-array'
+  defaults:
+    _controller: '\Drupal\lupus_ce_renderer_test\Controller\TestController::renderArray'
+    _title: 'Render array test page'
+  requirements:
+    _access: 'TRUE'
+
 lupus_ce_renderer_test.html_response_with_render:
   path: '/lupus-ce-renderer-test/html-response-with-render'
   defaults:
diff --git a/tests/src/Kernel/LupusCeRendererControllerTest.php b/tests/src/Kernel/LupusCeRendererControllerTest.php
index 8a6612365eeb4b7c876799585ec49b75b3a216b4..75e712df2ebc1b02ee9889643ce4b5e5b5f6bf51 100644
--- a/tests/src/Kernel/LupusCeRendererControllerTest.php
+++ b/tests/src/Kernel/LupusCeRendererControllerTest.php
@@ -2,7 +2,6 @@
 
 namespace Drupal\Tests\lupus_ce_renderer\Kernel;
 
-use Drupal\Core\Site\Settings;
 use Symfony\Component\HttpFoundation\Request;
 
 /**
@@ -32,35 +31,46 @@ class LupusCeRendererControllerTest extends LupusCeRendererKernelTestBase {
   ];
 
   /**
-   * Tests that controllers calling render() work with custom_elements format.
+   * Tests non-entity controllers with custom_elements format.
    *
-   * Regression test: CustomElementsRouteSubscriber previously stripped
-   * EarlyRenderingControllerWrapperSubscriber for all non-entity routes when
-   * the format was custom_elements. This removed the render context, causing
-   * controllers that call render() (like Canvas) to fail with "Render context
-   * is empty" LogicException.
+   * Render array controllers should work and return CE JSON responses.
+   * Controllers returning HtmlResponse should be blocked with 406.
+   * Other formats like JSON should still be allowed for custom APIs.
    */
   public function testControllerWithRenderCallWorksWithCustomElements() {
-    // Test with lupus_ce_renderer enabled globally.
-    new Settings(Settings::getAll() + ['lupus_ce_renderer_enable' => TRUE]);
-
-    $request = Request::create('/lupus-ce-renderer-test/html-response-with-render');
+    // Render array controller works with custom_elements format.
+    $request = Request::create('/lupus-ce-renderer-test/render-array', 'GET', [
+      '_format' => 'custom_elements',
+    ]);
     $response = $this->container->get('http_kernel')->handle($request);
-
-    // The controller returns an HtmlResponse, which should pass through
-    // without error. The key assertion is that no LogicException about
-    // empty render context is thrown.
     $this->assertEquals(200, $response->getStatusCode());
-    $this->assertStringContainsString('Test content from render call', $response->getContent());
+    $this->assertStringContainsString('application/json', $response->headers->get('Content-Type'));
+    $data = $this->decodeResponse($response);
+    $this->assertNotEmpty($data['content']);
 
-    // Also test with explicit _format parameter.
+    // HtmlResponse controller is blocked with 406 when lupus_ce_renderer
+    // is active (as it would be via /ce-api).
     $request = Request::create('/lupus-ce-renderer-test/html-response-with-render', 'GET', [
       '_format' => 'custom_elements',
     ]);
+    $request->attributes->set('lupus_ce_renderer', TRUE);
     $response = $this->container->get('http_kernel')->handle($request);
+    $this->assertEquals(406, $response->getStatusCode());
 
+    // Without lupus_ce_renderer, the HTML route works normally.
+    $request = Request::create('/lupus-ce-renderer-test/html-response-with-render');
+    $response = $this->container->get('http_kernel')->handle($request);
     $this->assertEquals(200, $response->getStatusCode());
     $this->assertStringContainsString('Test content from render call', $response->getContent());
+
+    // JSON responses should still be allowed (BC for custom APIs).
+    $request = Request::create('/user/login_status', 'GET', [
+      '_format' => 'json',
+    ]);
+    $request->attributes->set('lupus_ce_renderer', TRUE);
+    $response = $this->container->get('http_kernel')->handle($request);
+    $this->assertEquals(200, $response->getStatusCode());
+    $this->assertEquals('0', $response->getContent());
   }
 
   /**
@@ -72,9 +82,9 @@ class LupusCeRendererControllerTest extends LupusCeRendererKernelTestBase {
    * pipeline should then include this metadata in the final JSON response.
    */
   public function testEarlyRenderingCacheMetadataPreservedForRenderArray() {
-    new Settings(Settings::getAll() + ['lupus_ce_renderer_enable' => TRUE]);
-
-    $request = Request::create('/lupus-ce-renderer-test/render-array-with-early-rendering');
+    $request = Request::create('/lupus-ce-renderer-test/render-array-with-early-rendering', 'GET', [
+      '_format' => 'custom_elements',
+    ]);
     $response = $this->container->get('http_kernel')->handle($request);
 
     $this->assertEquals(200, $response->getStatusCode());
@@ -96,9 +106,9 @@ class LupusCeRendererControllerTest extends LupusCeRendererKernelTestBase {
    * Without the decorator, core would throw a LogicException.
    */
   public function testEarlyRenderingCacheMetadataPreservedForCustomElement() {
-    new Settings(Settings::getAll() + ['lupus_ce_renderer_enable' => TRUE]);
-
-    $request = Request::create('/lupus-ce-renderer-test/custom-element-with-early-rendering');
+    $request = Request::create('/lupus-ce-renderer-test/custom-element-with-early-rendering', 'GET', [
+      '_format' => 'custom_elements',
+    ]);
     $response = $this->container->get('http_kernel')->handle($request);
 
     $this->assertEquals(200, $response->getStatusCode());
@@ -121,8 +131,8 @@ class LupusCeRendererControllerTest extends LupusCeRendererKernelTestBase {
    *
    * Entity canonical routes have their controller replaced by
    * CustomElementsControllerSubscriber. The decorated early rendering
-   * subscriber should still
-   * provide a render context and handle any leaked metadata.
+   * subscriber should still provide a render context and handle any leaked
+   * metadata.
    */
   public function testEntityRouteEarlyRenderingHasRenderContext() {
     $response = $this->request($this->nodePath, ['_format' => 'custom_elements']);