src/Routing/Routes.php | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/Routing/Routes.php b/src/Routing/Routes.php
index d54a5ca..87bd6d3 100644
--- a/src/Routing/Routes.php
+++ b/src/Routing/Routes.php
@@ -116,6 +116,7 @@ class Routes implements ContainerInjectionInterface {
 
       // Collection endpoint, like /jsonapi/file/photo.
       $route_collection = (new Route($route_base_path, $defaults))
+        ->setRequirement('_content_type_format', 'api_json')
         ->setRequirement('_entity_type', $resource_type->getEntityTypeId())
         ->setRequirement('_bundle', $resource_type->getBundle())
         ->setRequirement('_permission', 'access content')
@@ -129,6 +130,7 @@ class Routes implements ContainerInjectionInterface {
       $parameters = [$resource_type->getEntityTypeId() => ['type' => 'entity:' . $resource_type->getEntityTypeId()]];
       $route_individual = (new Route(sprintf('%s/{%s}', $route_base_path, $resource_type->getEntityTypeId())))
         ->addDefaults($defaults)
+        ->setRequirement('_content_type_format', 'api_json')
         ->setRequirement('_entity_type', $resource_type->getEntityTypeId())
         ->setRequirement('_bundle', $resource_type->getBundle())
         ->setRequirement('_permission', 'access content')
@@ -154,6 +156,7 @@ class Routes implements ContainerInjectionInterface {
 
       // Related endpoint, like /jsonapi/file/photo/123/relationships/comments.
       $route_relationship = (new Route(sprintf('%s/{%s}/relationships/{related}', $route_base_path, $resource_type->getEntityTypeId()), $defaults + ['_on_relationship' => TRUE]))
+        ->setRequirement('_content_type_format', 'api_json')
         ->setRequirement('_entity_type', $resource_type->getEntityTypeId())
         ->setRequirement('_bundle', $resource_type->getBundle())
         ->setRequirement('_permission', 'access content')