diff --git a/core/modules/jsonapi/tests/src/Functional/CommentTest.php b/core/modules/jsonapi/tests/src/Functional/CommentTest.php
index 9a0e0eff57f..e972aca15f4 100644
--- a/core/modules/jsonapi/tests/src/Functional/CommentTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/CommentTest.php
@@ -377,8 +377,9 @@ class CommentTest extends ResourceTestBase {
 
     // Status should be FALSE when posting as anonymous.
     $response = $this->request('POST', $url, $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertResourceResponse(201, FALSE, $response);
-    $this->assertFalse(Json::decode((string) $response->getBody())['data']['attributes']['status']);
+    $this->assertFalse($document['data']['attributes']['status']);
     $this->assertFalse($this->entityStorage->loadUnchanged(2)->isPublished());
 
     // Grant anonymous permission to skip comment approval.
@@ -386,8 +387,9 @@ class CommentTest extends ResourceTestBase {
 
     // Status must be TRUE when posting as anonymous and skip comment approval.
     $response = $this->request('POST', $url, $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertResourceResponse(201, FALSE, $response);
-    $this->assertTrue(Json::decode((string) $response->getBody())['data']['attributes']['status']);
+    $this->assertTrue($document['data']['attributes']['status']);
     $this->assertTrue($this->entityStorage->loadUnchanged(3)->isPublished());
   }
 
@@ -441,14 +443,14 @@ class CommentTest extends ResourceTestBase {
     // ::doTestCollectionFilterAccessForPublishableEntities().
     $collection_filter_url = $collection_url->setOption('query', ["filter[spotlight.subject]" => $this->entity->label()]);
     $response = $this->request('GET', $collection_filter_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(1, $doc['data']);
     // Mark the commented entity as inaccessible.
     \Drupal::state()->set('jsonapi__entity_test_filter_access_blacklist', [$this->entity->getCommentedEntityId()]);
     Cache::invalidateTags(['state:jsonapi__entity_test_filter_access_blacklist']);
     // ?filter[spotlight.LABEL]: 0 results.
     $response = $this->request('GET', $collection_filter_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(0, $doc['data']);
   }
 
diff --git a/core/modules/jsonapi/tests/src/Functional/ConfigurableLanguageTest.php b/core/modules/jsonapi/tests/src/Functional/ConfigurableLanguageTest.php
index d9b1dedee07..f6ec17d1f2e 100644
--- a/core/modules/jsonapi/tests/src/Functional/ConfigurableLanguageTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/ConfigurableLanguageTest.php
@@ -4,7 +4,6 @@ declare(strict_types=1);
 
 namespace Drupal\Tests\jsonapi\Functional;
 
-use Drupal\Component\Serialization\Json;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Url;
@@ -134,7 +133,7 @@ class ConfigurableLanguageTest extends ConfigEntityResourceTestBase {
     $this->setUpAuthorization('GET');
     $response = $this->request('GET', $url, $request_options);
 
-    $normalization = Json::decode((string) $response->getBody());
+    $normalization = $this->getDocumentFromResponse($response);
     $this->assertArrayNotHasKey('_core', $normalization['data']['attributes']);
   }
 
diff --git a/core/modules/jsonapi/tests/src/Functional/EntryPointTest.php b/core/modules/jsonapi/tests/src/Functional/EntryPointTest.php
index 1e8360234cd..b4334085035 100644
--- a/core/modules/jsonapi/tests/src/Functional/EntryPointTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/EntryPointTest.php
@@ -4,9 +4,9 @@ declare(strict_types=1);
 
 namespace Drupal\Tests\jsonapi\Functional;
 
-use Drupal\Component\Serialization\Json;
 use Drupal\Core\Url;
 use Drupal\Tests\BrowserTestBase;
+use Drupal\Tests\jsonapi\Traits\GetDocumentFromResponseTrait;
 use Drupal\Tests\user\Traits\UserCreationTrait;
 use GuzzleHttp\RequestOptions;
 
@@ -19,6 +19,7 @@ use GuzzleHttp\RequestOptions;
  */
 class EntryPointTest extends BrowserTestBase {
 
+  use GetDocumentFromResponseTrait;
   use JsonApiRequestTestTrait;
   use UserCreationTrait;
 
@@ -43,7 +44,7 @@ class EntryPointTest extends BrowserTestBase {
     $request_options = [];
     $request_options[RequestOptions::HEADERS]['Accept'] = 'application/vnd.api+json';
     $response = $this->request('GET', Url::fromUri('base://jsonapi'), $request_options);
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response);
     $expected_cache_contexts = [
       'url.site',
       'user.roles:authenticated',
@@ -61,7 +62,7 @@ class EntryPointTest extends BrowserTestBase {
     $user = $this->createUser();
     $request_options[RequestOptions::HEADERS]['Authorization'] = 'Basic ' . base64_encode($user->name->value . ':' . $user->passRaw);
     $response = $this->request('GET', Url::fromUri('base://jsonapi'), $request_options);
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response);
     $this->assertArrayHasKey('meta', $document);
     $this->assertStringEndsWith('/jsonapi/user/user/' . $user->uuid(), $document['meta']['links']['me']['href']);
   }
diff --git a/core/modules/jsonapi/tests/src/Functional/ExternalNormalizersTest.php b/core/modules/jsonapi/tests/src/Functional/ExternalNormalizersTest.php
index 3efaea6791f..c0b9131aff6 100644
--- a/core/modules/jsonapi/tests/src/Functional/ExternalNormalizersTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/ExternalNormalizersTest.php
@@ -10,6 +10,7 @@ use Drupal\entity_test\Entity\EntityTest;
 use Drupal\field\Entity\FieldConfig;
 use Drupal\field\Entity\FieldStorageConfig;
 use Drupal\Tests\BrowserTestBase;
+use Drupal\Tests\jsonapi\Traits\GetDocumentFromResponseTrait;
 use Drupal\user\Entity\Role;
 use Drupal\user\RoleInterface;
 use GuzzleHttp\RequestOptions;
@@ -23,6 +24,8 @@ use GuzzleHttp\RequestOptions;
  */
 class ExternalNormalizersTest extends BrowserTestBase {
 
+  use GetDocumentFromResponseTrait;
+
   /**
    * {@inheritdoc}
    */
@@ -152,7 +155,7 @@ class ExternalNormalizersTest extends BrowserTestBase {
     // $url = $this->entity->toUrl('jsonapi');
     $client = $this->getSession()->getDriver()->getClient()->getClient();
     $response = $client->request('GET', $url->setAbsolute(TRUE)->toString());
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame($expected_value_jsonapi_normalization, $document['data']['attributes']['field_test']);
 
     // Asserts the expected JSON:API denormalization.
@@ -167,7 +170,7 @@ class ExternalNormalizersTest extends BrowserTestBase {
     ]);
     $request_options[RequestOptions::HEADERS]['Content-Type'] = 'application/vnd.api+json';
     $response = $client->request('POST', Url::fromRoute('jsonapi.entity_test--entity_test.collection.post')->setAbsolute(TRUE)->toString(), $request_options);
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(static::VALUE_OVERRIDDEN, $document['data']['attributes']['field_test']);
     $entity_type_manager = $this->container->get('entity_type.manager');
     $uuid_key = $entity_type_manager->getDefinition('entity_test')->getKey('uuid');
diff --git a/core/modules/jsonapi/tests/src/Functional/FileTest.php b/core/modules/jsonapi/tests/src/Functional/FileTest.php
index b24dbc73f06..b4a7b71676f 100644
--- a/core/modules/jsonapi/tests/src/Functional/FileTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/FileTest.php
@@ -4,7 +4,6 @@ declare(strict_types=1);
 
 namespace Drupal\Tests\jsonapi\Functional;
 
-use Drupal\Component\Serialization\Json;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Url;
 use Drupal\file\Entity\File;
@@ -241,7 +240,7 @@ class FileTest extends ResourceTestBase {
     $this->entity->setOwner($this->account);
     $this->entity->save();
     $response = $this->request('GET', $collection_filter_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(1, $doc['data']);
 
     // 0 results because the current user is no longer the file owner and the
@@ -249,7 +248,7 @@ class FileTest extends ResourceTestBase {
     $this->entity->setOwner(User::load(0));
     $this->entity->save();
     $response = $this->request('GET', $collection_filter_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(0, $doc['data']);
   }
 
diff --git a/core/modules/jsonapi/tests/src/Functional/FileUploadTest.php b/core/modules/jsonapi/tests/src/Functional/FileUploadTest.php
index bc566701d7a..127dd9e73a2 100644
--- a/core/modules/jsonapi/tests/src/Functional/FileUploadTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/FileUploadTest.php
@@ -905,7 +905,7 @@ class FileUploadTest extends ResourceTestBase {
    */
   protected function assertResponseData(array $expected, ResponseInterface $response): void {
     static::recursiveKSort($expected);
-    $actual = Json::decode((string) $response->getBody());
+    $actual = $this->getDocumentFromResponse($response);
     static::recursiveKSort($actual);
 
     $this->assertSame($expected, $actual);
diff --git a/core/modules/jsonapi/tests/src/Functional/JsonApiFilterRegressionTest.php b/core/modules/jsonapi/tests/src/Functional/JsonApiFilterRegressionTest.php
index 442a43b6fb0..9a0faaac582 100644
--- a/core/modules/jsonapi/tests/src/Functional/JsonApiFilterRegressionTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/JsonApiFilterRegressionTest.php
@@ -7,7 +7,6 @@ namespace Drupal\Tests\jsonapi\Functional;
 use Drupal\comment\Entity\Comment;
 use Drupal\comment\Plugin\Field\FieldType\CommentItemInterface;
 use Drupal\comment\Tests\CommentTestTrait;
-use Drupal\Component\Serialization\Json;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\Core\Url;
 use Drupal\node\Entity\Node;
@@ -109,8 +108,8 @@ class JsonApiFilterRegressionTest extends JsonApiFunctionalTestBase {
         $user->pass_raw,
       ],
     ]);
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $doc = Json::decode((string) $response->getBody());
     $this->assertNotEmpty($doc['data']);
     $this->assertSame($doc['data'][0]['id'], $shortcut->uuid());
     $this->assertSame($doc['data'][0]['attributes']['drupal_internal__id'], (int) $shortcut->id());
@@ -157,20 +156,24 @@ class JsonApiFilterRegressionTest extends JsonApiFunctionalTestBase {
       RequestOptions::AUTH => [$user->getAccountName(), $user->pass_raw],
     ];
     $response = $this->request('GET', $url, $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode(), (string) $response->getBody());
-    $this->assertSame($node->uuid(), Json::decode((string) $response->getBody())['data'][0]['id']);
+    $this->assertSame($node->uuid(), $document['data'][0]['id']);
     $response = $this->request('GET', $url->setOption('query', [
       'filter[test][condition][path]' => 'field_parent_folder',
       'filter[test][condition][operator]' => 'IS NULL',
     ]), $request_options);
+    $document = $this->getDocumentFromResponse($response);
+
     $this->assertSame(200, $response->getStatusCode(), (string) $response->getBody());
-    $this->assertSame($node->uuid(), Json::decode((string) $response->getBody())['data'][0]['id']);
+    $this->assertSame($node->uuid(), $document['data'][0]['id']);
     $response = $this->request('GET', $url->setOption('query', [
       'filter[test][condition][path]' => 'field_parent_folder',
       'filter[test][condition][operator]' => 'IS NOT NULL',
     ]), $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode(), (string) $response->getBody());
-    $this->assertEmpty(Json::decode((string) $response->getBody())['data']);
+    $this->assertEmpty($document['data']);
   }
 
   /**
@@ -213,13 +216,13 @@ class JsonApiFilterRegressionTest extends JsonApiFunctionalTestBase {
 
     // Ensure that an entity can be filtered by a target machine name.
     $response = $this->request('GET', Url::fromUri('internal:/jsonapi/user/user?filter[roles.meta.drupal_internal__target_id]=llamalovers'), $request_options);
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode(), var_export($document, TRUE));
     // Only one user should have the first role.
     $this->assertCount(1, $document['data']);
     $this->assertSame($users[0]->uuid(), $document['data'][0]['id']);
     $response = $this->request('GET', Url::fromUri('internal:/jsonapi/user/user?sort=drupal_internal__uid&filter[roles.meta.drupal_internal__target_id]=catcuddlers'), $request_options);
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode(), var_export($document, TRUE));
     // Two users should have the second role. A sort is used on this request to
     // ensure a consistent ordering with different databases.
@@ -229,7 +232,7 @@ class JsonApiFilterRegressionTest extends JsonApiFunctionalTestBase {
 
     // Ensure that an entity can be filtered by an target entity integer ID.
     $response = $this->request('GET', Url::fromUri('internal:/jsonapi/node/article?filter[uid.meta.drupal_internal__target_id]=' . $users[1]->id()), $request_options);
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode(), var_export($document, TRUE));
     // Only the node authored by the filtered user should be returned.
     $this->assertCount(1, $document['data']);
diff --git a/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalMultilingualTest.php b/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalMultilingualTest.php
index e305a74803f..d60a0e3df15 100644
--- a/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalMultilingualTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalMultilingualTest.php
@@ -158,20 +158,20 @@ class JsonApiFunctionalMultilingualTest extends JsonApiFunctionalTestBase {
       ->save();
     $response = $this->request('PATCH', Url::fromUri('base:/ca/jsonapi/node/article/' . $this->nodes[0]->uuid()), $request_options);
     $this->assertSame(500, $response->getStatusCode());
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertSame('The translation language cannot be changed (ca).', $document['errors'][0]['detail']);
 
     // Changing the langcode of the default ('en') translation is possible:
     // first verify that it currently is 'en', then change it to 'ca-fr', and
     // verify that the title is unchanged, but the langcode is updated.
     $response = $this->request('GET', Url::fromUri('base:/jsonapi/node/article/' . $this->nodes[0]->uuid()), $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $document = Json::decode((string) $response->getBody());
     $this->assertSame($node->getTitle(), $document['data']['attributes']['title']);
     $this->assertSame('en', $document['data']['attributes']['langcode']);
     $response = $this->request('PATCH', Url::fromUri('base:/jsonapi/node/article/' . $this->nodes[0]->uuid()), $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $document = Json::decode((string) $response->getBody());
     $this->assertSame($node->getTitle(), $document['data']['attributes']['title']);
     $this->assertSame('ca-fr', $document['data']['attributes']['langcode']);
 
@@ -179,18 +179,18 @@ class JsonApiFunctionalMultilingualTest extends JsonApiFunctionalTestBase {
     // - When GETting the 'en' translation, we get 'ca-fr', since the 'en'
     //   translation doesn't exist anymore.
     $response = $this->request('GET', Url::fromUri('base:/jsonapi/node/article/' . $this->nodes[0]->uuid()), $request_options);
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame('ca-fr', $document['data']['attributes']['langcode']);
     $this->assertSame($node->getTitle(), $document['data']['attributes']['title']);
     // - When GETting the 'ca' translation, we still get the 'ca' one.
     $response = $this->request('GET', Url::fromUri('base:/ca/jsonapi/node/article/' . $this->nodes[0]->uuid()), $request_options);
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame('ca', $document['data']['attributes']['langcode']);
     $this->assertSame($node->getTitle() . ' (ca) UPDATED', $document['data']['attributes']['title']);
     // - When GETting the 'ca-fr' translation, we now get the default
     //   translation.
     $response = $this->request('GET', Url::fromUri('base:/ca-fr/jsonapi/node/article/' . $this->nodes[0]->uuid()), $request_options);
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame('ca-fr', $document['data']['attributes']['langcode']);
     $this->assertSame($node->getTitle(), $document['data']['attributes']['title']);
   }
@@ -232,7 +232,7 @@ class JsonApiFunctionalMultilingualTest extends JsonApiFunctionalTestBase {
     ]);
     $response = $this->request('PATCH', Url::fromUri('base:/ca-fr/jsonapi/node/article/' . $this->nodes[0]->uuid()), $request_options);
     $this->assertSame(405, $response->getStatusCode());
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertSame('The requested translation of the resource object does not exist, instead modify one of the translations that do exist: ca, en.', $document['errors'][0]['detail']);
   }
 
@@ -263,7 +263,7 @@ class JsonApiFunctionalMultilingualTest extends JsonApiFunctionalTestBase {
     $request_options[RequestOptions::BODY] = Json::encode($request_document);
     $response = $this->request('POST', Url::fromUri('base:/ca/jsonapi/node/article/'), $request_options);
     $this->assertSame(403, $response->getStatusCode());
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertSame('The current user is not allowed to POST the selected field (langcode).', $document['errors'][0]['detail']);
 
     // Omitting a langcode results in an entity in 'en': the default language of
@@ -271,8 +271,8 @@ class JsonApiFunctionalMultilingualTest extends JsonApiFunctionalTestBase {
     unset($request_document['data']['attributes']['langcode']);
     $request_options[RequestOptions::BODY] = Json::encode($request_document);
     $response = $this->request('POST', Url::fromUri('base:/ca/jsonapi/node/article/'), $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(201, $response->getStatusCode());
-    $document = Json::decode((string) $response->getBody());
     $this->assertSame($title, $document['data']['attributes']['title']);
     $this->assertSame('en', $document['data']['attributes']['langcode']);
     $this->assertSame(['en'], array_keys(Node::load($document['data']['attributes']['drupal_internal__nid'])->getTranslationLanguages()));
@@ -285,16 +285,16 @@ class JsonApiFunctionalMultilingualTest extends JsonApiFunctionalTestBase {
     $request_document['data']['attributes']['langcode'] = 'ca';
     $request_options[RequestOptions::BODY] = Json::encode($request_document);
     $response = $this->request('POST', Url::fromUri('base:/ca/jsonapi/node/article/'), $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(201, $response->getStatusCode());
-    $document = Json::decode((string) $response->getBody());
     $this->assertSame($title, $document['data']['attributes']['title']);
     $this->assertSame('ca', $document['data']['attributes']['langcode']);
     $this->assertSame(['ca'], array_keys(Node::load($document['data']['attributes']['drupal_internal__nid'])->getTranslationLanguages()));
 
     // Same request, but sent to the URL without the language prefix.
     $response = $this->request('POST', Url::fromUri('base:/jsonapi/node/article/'), $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(201, $response->getStatusCode());
-    $document = Json::decode((string) $response->getBody());
     $this->assertSame($title, $document['data']['attributes']['title']);
     $this->assertSame('ca', $document['data']['attributes']['langcode']);
     $this->assertSame(['ca'], array_keys(Node::load($document['data']['attributes']['drupal_internal__nid'])->getTranslationLanguages()));
@@ -311,12 +311,12 @@ class JsonApiFunctionalMultilingualTest extends JsonApiFunctionalTestBase {
 
     $response = $this->request('DELETE', Url::fromUri('base:/ca/jsonapi/node/article/' . $this->nodes[0]->uuid()), []);
     $this->assertSame(405, $response->getStatusCode());
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertSame('Deleting a resource object translation is not yet supported. See https://www.drupal.org/docs/8/modules/jsonapi/translations.', $document['errors'][0]['detail']);
 
     $response = $this->request('DELETE', Url::fromUri('base:/ca-fr/jsonapi/node/article/' . $this->nodes[0]->uuid()), []);
     $this->assertSame(405, $response->getStatusCode());
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertSame('Deleting a resource object translation is not yet supported. See https://www.drupal.org/docs/8/modules/jsonapi/translations.', $document['errors'][0]['detail']);
 
     $response = $this->request('DELETE', Url::fromUri('base:/jsonapi/node/article/' . $this->nodes[0]->uuid()), []);
diff --git a/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalTest.php b/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalTest.php
index 71daad24ffc..211eae28377 100644
--- a/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalTest.php
@@ -282,7 +282,7 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
         $this->userCanViewProfiles->pass_raw,
       ],
     ]);
-    $single_output = Json::decode($response->getBody()->__toString());
+    $single_output = $this->getDocumentFromResponse($response);
     $this->assertEquals(200, $response->getStatusCode());
     $this->assertEquals('user--user', $single_output['data']['type']);
     $this->assertEquals($this->user->get('name')->value, $single_output['data']['attributes']['name']);
@@ -608,22 +608,22 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
       'auth' => [$this->user->getAccountName(), $this->user->pass_raw],
       'headers' => ['Content-Type' => 'application/vnd.api+json'],
     ]);
-    $created_response = Json::decode($response->getBody()->__toString());
+    $document = $this->getDocumentFromResponse($response);
     $this->assertEquals(201, $response->getStatusCode());
-    $this->assertArrayNotHasKey('uuid', $created_response['data']['attributes']);
-    $uuid = $created_response['data']['id'];
-    $this->assertCount(2, $created_response['data']['relationships']['field_tags']['data']);
-    $this->assertEquals($created_response['data']['links']['self']['href'], $response->getHeader('Location')[0]);
+    $this->assertArrayNotHasKey('uuid', $document['data']['attributes']);
+    $uuid = $document['data']['id'];
+    $this->assertCount(2, $document['data']['relationships']['field_tags']['data']);
+    $this->assertEquals($document['data']['links']['self']['href'], $response->getHeader('Location')[0]);
 
     // 2. Authorization error.
     $response = $this->request('POST', $collection_url, [
       'body' => Json::encode($body),
       'headers' => ['Content-Type' => 'application/vnd.api+json'],
     ]);
-    $created_response = Json::decode($response->getBody()->__toString());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertEquals(401, $response->getStatusCode());
-    $this->assertNotEmpty($created_response['errors']);
-    $this->assertEquals('Unauthorized', $created_response['errors'][0]['title']);
+    $this->assertNotEmpty($document['errors']);
+    $this->assertEquals('Unauthorized', $document['errors'][0]['title']);
 
     // 2.1 Authorization error with a user without create permissions.
     $response = $this->request('POST', $collection_url, [
@@ -631,10 +631,10 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
       'auth' => [$this->userCanViewProfiles->getAccountName(), $this->userCanViewProfiles->pass_raw],
       'headers' => ['Content-Type' => 'application/vnd.api+json'],
     ]);
-    $created_response = Json::decode($response->getBody()->__toString());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertEquals(403, $response->getStatusCode());
-    $this->assertNotEmpty($created_response['errors']);
-    $this->assertEquals('Forbidden', $created_response['errors'][0]['title']);
+    $this->assertNotEmpty($document['errors']);
+    $this->assertEquals('Forbidden', $document['errors'][0]['title']);
 
     // 3. Missing Content-Type error.
     $response = $this->request('POST', $collection_url, [
@@ -655,10 +655,10 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
         'Content-Type' => 'application/vnd.api+json',
       ],
     ]);
-    $created_response = Json::decode($response->getBody()->__toString());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertEquals(409, $response->getStatusCode());
-    $this->assertNotEmpty($created_response['errors']);
-    $this->assertEquals('Conflict', $created_response['errors'][0]['title']);
+    $this->assertNotEmpty($document['errors']);
+    $this->assertEquals('Conflict', $document['errors'][0]['title']);
     // 5. Article with wrong reference UUIDs for tags.
     $body_invalid_tags = $body;
     $body_invalid_tags['data']['relationships']['field_tags']['data'][0]['id'] = 'lorem';
@@ -678,10 +678,10 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
         'Accept' => 'application/vnd.api+json',
       ],
     ]);
-    $created_response = Json::decode($response->getBody()->__toString());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertEquals(400, $response->getStatusCode());
-    $this->assertNotEmpty($created_response['errors']);
-    $this->assertEquals('Bad Request', $created_response['errors'][0]['title']);
+    $this->assertNotEmpty($document['errors']);
+    $this->assertEquals('Bad Request', $document['errors'][0]['title']);
     // 6.1 Denormalizing error.
     $response = $this->request('POST', $collection_url, [
       'body' => '{"data":{"type":"something"},"valid yet nonsensical json":[]}',
@@ -691,10 +691,10 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
         'Accept' => 'application/vnd.api+json',
       ],
     ]);
-    $created_response = Json::decode($response->getBody()->__toString());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertEquals(422, $response->getStatusCode());
-    $this->assertNotEmpty($created_response['errors']);
-    $this->assertStringStartsWith('Unprocessable', $created_response['errors'][0]['title']);
+    $this->assertNotEmpty($document['errors']);
+    $this->assertStringStartsWith('Unprocessable', $document['errors'][0]['title']);
     // 6.2 Relationships are not included in "data".
     $malformed_body = $body;
     unset($malformed_body['data']['relationships']);
@@ -707,11 +707,11 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
         'Content-Type' => 'application/vnd.api+json',
       ],
     ]);
-    $created_response = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertSame(400, $response->getStatusCode());
-    $this->assertNotEmpty($created_response['errors']);
-    $this->assertSame("Bad Request", $created_response['errors'][0]['title']);
-    $this->assertSame("Found \"relationships\" within the document's top level. The \"relationships\" key must be within resource object.", $created_response['errors'][0]['detail']);
+    $this->assertNotEmpty($document['errors']);
+    $this->assertSame("Bad Request", $document['errors'][0]['title']);
+    $this->assertSame("Found \"relationships\" within the document's top level. The \"relationships\" key must be within resource object.", $document['errors'][0]['detail']);
     // 6.2 "type" not included in "data".
     $missing_type = $body;
     unset($missing_type['data']['type']);
@@ -723,11 +723,11 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
         'Content-Type' => 'application/vnd.api+json',
       ],
     ]);
-    $created_response = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response, FALSE);
     $this->assertSame(400, $response->getStatusCode());
-    $this->assertNotEmpty($created_response['errors']);
-    $this->assertSame("Bad Request", $created_response['errors'][0]['title']);
-    $this->assertSame("Resource object must include a \"type\".", $created_response['errors'][0]['detail']);
+    $this->assertNotEmpty($document['errors']);
+    $this->assertSame("Bad Request", $document['errors'][0]['title']);
+    $this->assertSame("Resource object must include a \"type\".", $document['errors'][0]['detail']);
     // 7. Successful PATCH.
     $body = [
       'data' => [
@@ -744,7 +744,7 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
       'auth' => [$this->user->getAccountName(), $this->user->pass_raw],
       'headers' => ['Content-Type' => 'application/vnd.api+json'],
     ]);
-    $updated_response = Json::decode($response->getBody()->__toString());
+    $updated_response = $this->getDocumentFromResponse($response);
     $this->assertEquals(200, $response->getStatusCode());
     $this->assertEquals('My updated title', $updated_response['data']['attributes']['title']);
 
@@ -782,7 +782,7 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
       'auth' => [$this->user->getAccountName(), $this->user->pass_raw],
       'headers' => ['Content-Type' => 'application/vnd.api+json'],
     ]);
-    $updated_response = Json::decode($response->getBody()->__toString());
+    $updated_response = $this->getDocumentFromResponse($response, FALSE);
     $this->assertEquals(403, $response->getStatusCode());
     $this->assertEquals("The current user is not allowed to PATCH the selected field (status). The 'administer nodes' permission is required.",
       $updated_response['errors'][0]['detail']);
@@ -806,7 +806,7 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
       'auth' => [$this->user->getAccountName(), $this->user->pass_raw],
       'headers' => ['Content-Type' => 'application/vnd.api+json'],
     ]);
-    $updated_response = Json::decode($response->getBody()->__toString());
+    $updated_response = $this->getDocumentFromResponse($response);
     $this->assertEquals(200, $response->getStatusCode());
     $this->assertCount(3, $updated_response['data']);
     $this->assertEquals('taxonomy_term--tags', $updated_response['data'][2]['type']);
@@ -836,7 +836,7 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
         'Accept' => 'application/vnd.api+json',
       ],
     ]);
-    $updated_response = Json::decode($response->getBody()->__toString());
+    $updated_response = $this->getDocumentFromResponse($response, FALSE);
     $this->assertEquals(
       'You need to provide a body for DELETE operations on a relationship (field_tags).',
       $updated_response['errors'][0]['detail']
@@ -879,7 +879,7 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
         'Accept' => 'application/vnd.api+json',
       ],
     ]);
-    $updated_response = Json::decode($response->getBody()->__toString());
+    $updated_response = $this->getDocumentFromResponse($response, FALSE);
     $this->assertEquals(422, $response->getStatusCode());
     $this->assertCount(2, $updated_response['errors']);
     for ($i = 0; $i < 2; $i++) {
@@ -908,7 +908,7 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
         'Accept' => 'application/vnd.api+json',
       ],
     ]);
-    $updated_response = Json::decode($response->getBody()->__toString());
+    $updated_response = $this->getDocumentFromResponse($response, FALSE);
     $this->assertEquals(422, $response->getStatusCode());
     $this->assertEquals("The attribute field_that_does_not_exist does not exist on the node--article resource type.",
       $updated_response['errors']['0']['detail']);
diff --git a/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalTestBase.php b/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalTestBase.php
index 239e8b9014d..1ed47faa06a 100644
--- a/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalTestBase.php
+++ b/core/modules/jsonapi/tests/src/Functional/JsonApiFunctionalTestBase.php
@@ -14,6 +14,7 @@ use Drupal\taxonomy\Entity\Vocabulary;
 use Drupal\Tests\BrowserTestBase;
 use Drupal\Tests\field\Traits\EntityReferenceFieldCreationTrait;
 use Drupal\Tests\image\Kernel\ImageFieldCreationTrait;
+use Drupal\Tests\jsonapi\Traits\GetDocumentFromResponseTrait;
 use Drupal\user\Entity\Role;
 use Drupal\user\RoleInterface;
 use GuzzleHttp\Exception\ClientException;
@@ -27,6 +28,7 @@ use GuzzleHttp\Exception\ServerException;
 abstract class JsonApiFunctionalTestBase extends BrowserTestBase {
 
   use EntityReferenceFieldCreationTrait;
+  use GetDocumentFromResponseTrait;
   use ImageFieldCreationTrait;
 
   const IS_MULTILINGUAL = TRUE;
diff --git a/core/modules/jsonapi/tests/src/Functional/JsonApiPatchRegressionTest.php b/core/modules/jsonapi/tests/src/Functional/JsonApiPatchRegressionTest.php
index 8b3aa4a3b53..c2cd0d9e6cf 100644
--- a/core/modules/jsonapi/tests/src/Functional/JsonApiPatchRegressionTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/JsonApiPatchRegressionTest.php
@@ -7,7 +7,6 @@ namespace Drupal\Tests\jsonapi\Functional;
 use Drupal\comment\Entity\Comment;
 use Drupal\comment\Plugin\Field\FieldType\CommentItemInterface;
 use Drupal\comment\Tests\CommentTestTrait;
-use Drupal\Component\Serialization\Json;
 use Drupal\Core\Entity\TranslatableInterface;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\Core\Language\LanguageInterface;
@@ -271,18 +270,18 @@ class JsonApiPatchRegressionTest extends JsonApiFunctionalTestBase {
     ];
     $node_url = Url::fromUri('internal:/jsonapi/node/page/' . $page->uuid());
     $response = $this->request('GET', $node_url, $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $doc = Json::decode((string) $response->getBody());
-    $this->assertSame('2018-12-19', $doc['data']['attributes']['when']);
-    $this->assertSame('2018-12-20T04:00:00+11:00', $doc['data']['attributes']['when_exactly']);
-    $doc['data']['attributes']['when'] = '2018-12-20';
-    $doc['data']['attributes']['when_exactly'] = '2018-12-19T19:00:00+01:00';
-    $request_options = $request_options + [RequestOptions::JSON => $doc];
+    $this->assertSame('2018-12-19', $document['data']['attributes']['when']);
+    $this->assertSame('2018-12-20T04:00:00+11:00', $document['data']['attributes']['when_exactly']);
+    $document['data']['attributes']['when'] = '2018-12-20';
+    $document['data']['attributes']['when_exactly'] = '2018-12-19T19:00:00+01:00';
+    $request_options = $request_options + [RequestOptions::JSON => $document];
     $response = $this->request('PATCH', $node_url, $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $doc = Json::decode((string) $response->getBody());
-    $this->assertSame('2018-12-20', $doc['data']['attributes']['when']);
-    $this->assertSame('2018-12-20T05:00:00+11:00', $doc['data']['attributes']['when_exactly']);
+    $this->assertSame('2018-12-20', $document['data']['attributes']['when']);
+    $this->assertSame('2018-12-20T05:00:00+11:00', $document['data']['attributes']['when_exactly']);
   }
 
   /**
@@ -325,10 +324,10 @@ class JsonApiPatchRegressionTest extends JsonApiFunctionalTestBase {
       ],
     ];
     $response = $this->request('PATCH', $url, $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $doc = Json::decode((string) $response->getBody());
-    $this->assertArrayHasKey('included', $doc);
-    $this->assertSame($user->label(), $doc['included'][0]['attributes']['name']);
+    $this->assertArrayHasKey('included', $document);
+    $this->assertSame($user->label(), $document['included'][0]['attributes']['name']);
   }
 
   /**
@@ -369,8 +368,8 @@ class JsonApiPatchRegressionTest extends JsonApiFunctionalTestBase {
     // GET the test entity via JSON:API.
     $entity_url = Url::fromUri('internal:/jsonapi/entity_test/entity_test/' . $entity->uuid());
     $response = $this->request('GET', $entity_url, $request_options);
+    $response_document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $response_document = Json::decode($response->getBody());
     // Ensure that the entity's langcode attribute is 'und'.
     $this->assertSame(LanguageInterface::LANGCODE_NOT_SPECIFIED, $response_document['data']['attributes']['langcode']);
     // Prepare to PATCH the entity via JSON:API.
@@ -387,8 +386,8 @@ class JsonApiPatchRegressionTest extends JsonApiFunctionalTestBase {
     // Issue the PATCH request and verify that the test entity was successfully
     // updated.
     $response = $this->request('PATCH', $entity_url, $request_options);
+    $response_document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode(), (string) $response->getBody());
-    $response_document = Json::decode($response->getBody());
     // Ensure that the entity's langcode attribute is still 'und' and the name
     // was successfully updated.
     $this->assertSame(LanguageInterface::LANGCODE_NOT_SPECIFIED, $response_document['data']['attributes']['langcode']);
@@ -450,7 +449,7 @@ class JsonApiPatchRegressionTest extends JsonApiFunctionalTestBase {
     $response = $this->request('PATCH', $url, $request_options);
 
     // Assert a helpful error response is present.
-    $data = Json::decode((string) $response->getBody());
+    $data = $this->getDocumentFromResponse($response, FALSE);
     $this->assertSame(422, $response->getStatusCode());
     $this->assertNotNull($data);
     // cSpell:disable-next-line
@@ -483,7 +482,7 @@ class JsonApiPatchRegressionTest extends JsonApiFunctionalTestBase {
     $response = $this->request('PATCH', $url, $request_options);
 
     // Assert a helpful error response is present.
-    $data = Json::decode((string) $response->getBody());
+    $data = $this->getDocumentFromResponse($response, FALSE);
     $this->assertSame(422, $response->getStatusCode());
     $this->assertNotNull($data);
     // cSpell:disable-next-line
diff --git a/core/modules/jsonapi/tests/src/Functional/JsonApiRegressionTest.php b/core/modules/jsonapi/tests/src/Functional/JsonApiRegressionTest.php
index c6b4259ea65..d40774144c3 100644
--- a/core/modules/jsonapi/tests/src/Functional/JsonApiRegressionTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/JsonApiRegressionTest.php
@@ -260,6 +260,7 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
     ];
     $issue_node->delete();
     $response = $this->request('GET', $url, $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
 
     // Entity reference field allowing a single bundle: dangling reference's
@@ -279,7 +280,7 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
           ],
         ],
       ],
-    ], Json::decode((string) $response->getBody())['data']['relationships']['field_issue']['data']);
+    ], $document['data']['relationships']['field_issue']['data']);
 
     // Entity reference field allowing multiple bundles: dangling reference's
     // resource type is NOT deduced.
@@ -305,7 +306,7 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
           'drupal_internal__target_id' => (int) $conference_node->id(),
         ],
       ],
-    ], Json::decode((string) $response->getBody())['data']['relationships']['field_mentioned_in']['data']);
+    ], $document['data']['relationships']['field_mentioned_in']['data']);
   }
 
   /**
@@ -513,8 +514,8 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
         $user->pass_raw,
       ],
     ]);
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $doc = Json::decode((string) $response->getBody());
     $this->assertSame('2018-09-16T22:00:00+10:00', $doc['data']['attributes']['when']);
   }
 
@@ -549,8 +550,8 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
       ],
     ];
     $response = $this->request('POST', $url, $request_options);
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertSame(201, $response->getStatusCode());
-    $doc = Json::decode((string) $response->getBody());
     $this->assertArrayHasKey('included', $doc);
     $this->assertSame($user->label(), $doc['included'][0]['attributes']['name']);
   }
@@ -586,8 +587,8 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
       RequestOptions::AUTH => [$user->getAccountName(), $user->pass_raw],
     ];
     $response = $this->request('GET', $url, $request_options);
+    $data = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $data = Json::decode((string) $response->getBody());
     $this->assertSame([
       'foo' => 'bar',
       'baz' => 'qux',
@@ -597,8 +598,8 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
       'foo' => 'bar',
     ])->save();
     $response = $this->request('GET', $url, $request_options);
+    $data = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $data = Json::decode((string) $response->getBody());
     $this->assertSame(['foo' => 'bar'], $data['data'][0]['attributes']['data']);
   }
 
@@ -633,7 +634,7 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
     $response = $this->request('POST', $url, $request_options);
 
     // Assert that the response has a body.
-    $data = Json::decode((string) $response->getBody());
+    $data = $this->getDocumentFromResponse($response, FALSE);
     $this->assertSame(422, $response->getStatusCode());
     $this->assertNotNull($data);
     $this->assertSame(sprintf('title: This value should not be null.'), $data['errors'][0]['detail']);
@@ -644,7 +645,7 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
     $response = $this->request('POST', $url, $request_options);
 
     // Assert that the response has a body.
-    $data = Json::decode((string) $response->getBody());
+    $data = $this->getDocumentFromResponse($response, FALSE);
     $this->assertSame(422, $response->getStatusCode());
     $this->assertNotNull($data);
     $this->assertSame(sprintf('title: This value should not be null.'), $data['errors'][0]['detail']);
@@ -707,8 +708,8 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
     ];
     // Retrieve the current representation of the entity.
     $response = $this->request('GET', $url, $request_options);
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $doc = Json::decode((string) $response->getBody());
     // Modify the title. The @FieldType=map normalization is not changed. (The
     // name of this field is confusingly also 'data'.)
     $doc['data']['attributes']['name'] = 'bar';
@@ -718,8 +719,9 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
     ];
     $request_options[RequestOptions::BODY] = Json::encode($doc);
     $response = $this->request('PATCH', $url, $request_options);
+    $patched_document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $this->assertSame($doc['data']['attributes']['data'], Json::decode((string) $response->getBody())['data']['attributes']['data']);
+    $this->assertSame($doc['data']['attributes']['data'], $patched_document['data']['attributes']['data']);
   }
 
   /**
@@ -759,12 +761,12 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
       'query' => ['include' => 'field_tags'],
     ]);
     $response = $this->request('GET', $url, $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
 
-    $response = Json::decode((string) $response->getBody());
-    $this->assertArrayNotHasKey('included', $response, 'JSON API response does not contain "included" taxonomy term as the latter is not published, i.e not accessible.');
+    $this->assertArrayNotHasKey('included', $document, 'JSON API response does not contain "included" taxonomy term as the latter is not published, i.e not accessible.');
 
-    $omitted = $response['meta']['omitted']['links'];
+    $omitted = $document['meta']['omitted']['links'];
     unset($omitted['help']);
     $omitted = reset($omitted);
     $expected_url = Url::fromUri('internal:/jsonapi/' . $term->getEntityTypeId() . '/' . $term->bundle() . '/' . $term->uuid());
@@ -774,8 +776,9 @@ class JsonApiRegressionTest extends JsonApiFunctionalTestBase {
     $term->setPublished();
     $term->save();
     $response = $this->request('GET', $url, $request_options);
+    $document = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $this->assertEquals($term->uuid(), Json::decode((string) $response->getBody())['included'][0]['id'], 'JSON API response contains "included" taxonomy term as it became published, i.e accessible.');
+    $this->assertEquals($term->uuid(), $document['included'][0]['id'], 'JSON API response contains "included" taxonomy term as it became published, i.e accessible.');
   }
 
   /**
diff --git a/core/modules/jsonapi/tests/src/Functional/MenuLinkContentTest.php b/core/modules/jsonapi/tests/src/Functional/MenuLinkContentTest.php
index 779a4ca7865..b1130951263 100644
--- a/core/modules/jsonapi/tests/src/Functional/MenuLinkContentTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/MenuLinkContentTest.php
@@ -223,7 +223,7 @@ class MenuLinkContentTest extends ResourceTestBase {
     unset($document['data']['attributes']['link']['options']);
     $request_options[RequestOptions::BODY] = Json::encode($document);
     $response = $this->request('POST', $url, $request_options);
-    $document = Json::decode((string) $response->getBody());
+    $document = $this->getDocumentFromResponse($response);
     $internal_id = $document['data']['attributes']['drupal_internal__id'];
 
     // Load the created menu item and add link options to it.
diff --git a/core/modules/jsonapi/tests/src/Functional/NodeTest.php b/core/modules/jsonapi/tests/src/Functional/NodeTest.php
index 7d5ec952390..3e4a4d7dacd 100644
--- a/core/modules/jsonapi/tests/src/Functional/NodeTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/NodeTest.php
@@ -292,7 +292,7 @@ class NodeTest extends ResourceTestBase {
 
     // GET node's current normalization.
     $response = $this->request('GET', $url, $this->getAuthenticationRequestOptions());
-    $normalization = Json::decode((string) $response->getBody());
+    $normalization = $this->getDocumentFromResponse($response);
 
     // Change node's path alias.
     $normalization['data']['attributes']['path']['alias'] .= 's-rule-the-world';
@@ -311,8 +311,8 @@ class NodeTest extends ResourceTestBase {
 
     // Repeat PATCH request: 200.
     $response = $this->request('PATCH', $url, $request_options);
+    $updated_normalization = $this->getDocumentFromResponse($response);
     $this->assertResourceResponse(200, FALSE, $response);
-    $updated_normalization = Json::decode((string) $response->getBody());
     $this->assertSame($normalization['data']['attributes']['path']['alias'], $updated_normalization['data']['attributes']['path']['alias']);
   }
 
@@ -501,21 +501,21 @@ class NodeTest extends ResourceTestBase {
 
     // 0 results because the node is unpublished.
     $response = $this->request('GET', $collection_filter_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(0, $doc['data']);
 
     $this->grantPermissionsToTestedRole(['view own unpublished content']);
 
     // 1 result because the current user is the owner of the unpublished node.
     $response = $this->request('GET', $collection_filter_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(1, $doc['data']);
 
     $this->entity->setOwnerId(0)->save();
 
     // 0 results because the current user is no longer the owner.
     $response = $this->request('GET', $collection_filter_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(0, $doc['data']);
 
     // Assert bubbling of cacheability from query alter hook.
diff --git a/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php b/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php
index ff14e5a3722..9586bc1accc 100644
--- a/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php
+++ b/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php
@@ -43,6 +43,7 @@ use Drupal\jsonapi\ResourceResponse;
 use Drupal\path\Plugin\Field\FieldType\PathItem;
 use Drupal\Tests\BrowserTestBase;
 use Drupal\Tests\content_moderation\Traits\ContentModerationTestTrait;
+use Drupal\Tests\jsonapi\Traits\GetDocumentFromResponseTrait;
 use Drupal\user\Entity\Role;
 use Drupal\user\EntityOwnerInterface;
 use Drupal\user\RoleInterface;
@@ -58,6 +59,7 @@ abstract class ResourceTestBase extends BrowserTestBase {
 
   use ResourceResponseTestTrait;
   use ContentModerationTestTrait;
+  use GetDocumentFromResponseTrait;
   use JsonApiRequestTestTrait;
 
   /**
@@ -728,7 +730,7 @@ abstract class ResourceTestBase extends BrowserTestBase {
     else {
       $this->assertSame(['application/vnd.api+json'], $response->getHeader('Content-Type'));
       if ($expected_document !== FALSE) {
-        $response_document = Json::decode((string) $response->getBody());
+        $response_document = $this->getDocumentFromResponse($response, FALSE);
         if ($expected_document === NULL) {
           $this->assertNull($response_document);
         }
@@ -1627,7 +1629,8 @@ abstract class ResourceTestBase extends BrowserTestBase {
       $resource->set($relationship_field_name, [$target_resource]);
       $expected_document = $this->getExpectedGetRelationshipDocument($relationship_field_name, $resource);
       $response = $this->request('GET', $url, $request_options);
-      $this->assertSameDocument($expected_document, Json::decode((string) $response->getBody()));
+      $document = $this->getDocumentFromResponse($response);
+      $this->assertSameDocument($expected_document, $document);
 
       // Test DELETE: one existing relationship, removed.
       $request_options[RequestOptions::BODY] = Json::encode(['data' => [$target_identifier]]);
@@ -1636,7 +1639,8 @@ abstract class ResourceTestBase extends BrowserTestBase {
       $this->assertResourceResponse(204, NULL, $response);
       $expected_document = $this->getExpectedGetRelationshipDocument($relationship_field_name, $resource);
       $response = $this->request('GET', $url, $request_options);
-      $this->assertSameDocument($expected_document, Json::decode((string) $response->getBody()));
+      $document = $this->getDocumentFromResponse($response);
+      $this->assertSameDocument($expected_document, $document);
 
       // Test DELETE: no existing relationships, no op, success.
       $request_options[RequestOptions::BODY] = Json::encode(['data' => [$target_identifier]]);
@@ -1644,7 +1648,8 @@ abstract class ResourceTestBase extends BrowserTestBase {
       $this->assertResourceResponse(204, NULL, $response);
       $expected_document = $this->getExpectedGetRelationshipDocument($relationship_field_name, $resource);
       $response = $this->request('GET', $url, $request_options);
-      $this->assertSameDocument($expected_document, Json::decode((string) $response->getBody()));
+      $document = $this->getDocumentFromResponse($response);
+      $this->assertSameDocument($expected_document, $document);
 
       // Test PATCH: success, new value is different than existing value.
       $request_options[RequestOptions::BODY] = Json::encode([
@@ -1672,7 +1677,8 @@ abstract class ResourceTestBase extends BrowserTestBase {
       $resource->set($relationship_field_name, []);
       $expected_document = $this->getExpectedGetRelationshipDocument($relationship_field_name, $resource);
       $response = $this->request('GET', $url, $request_options);
-      $this->assertSameDocument($expected_document, Json::decode((string) $response->getBody()));
+      $document = $this->getDocumentFromResponse($response);
+      $this->assertSameDocument($expected_document, $document);
     }
     else {
       $request_options[RequestOptions::BODY] = Json::encode(['data' => [$target_identifier]]);
@@ -2106,7 +2112,7 @@ abstract class ResourceTestBase extends BrowserTestBase {
       // Assert that the entity was indeed created, and that the response body
       // contains the serialized created entity.
       $created_entity_document = $this->normalize($created_entity, $url);
-      $decoded_response_body = Json::decode((string) $response->getBody());
+      $decoded_response_body = $this->getDocumentFromResponse($response);
       $this->assertEquals($created_entity_document, $decoded_response_body);
       // Assert that the entity was indeed created using the POSTed values.
       foreach ($this->getPostDocument()['data']['attributes'] as $field_name => $field_normalization) {
@@ -2370,7 +2376,8 @@ abstract class ResourceTestBase extends BrowserTestBase {
       }
     }
     $updated_entity_document = $this->normalize($updated_entity, $url);
-    $this->assertSame($updated_entity_document, Json::decode((string) $response->getBody()));
+    $document = $this->getDocumentFromResponse($response);
+    $this->assertSame($updated_entity_document, $document);
     $prior_revision_id = (int) $updated_entity->getRevisionId();
     // Assert that the entity was indeed created using the PATCHed values.
     foreach ($this->getPatchDocument()['data']['attributes'] as $field_name => $field_normalization) {
diff --git a/core/modules/jsonapi/tests/src/Functional/ShortcutTest.php b/core/modules/jsonapi/tests/src/Functional/ShortcutTest.php
index fdb64885ee6..20dd4b5f4ed 100644
--- a/core/modules/jsonapi/tests/src/Functional/ShortcutTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/ShortcutTest.php
@@ -4,7 +4,6 @@ declare(strict_types=1);
 
 namespace Drupal\Tests\jsonapi\Functional;
 
-use Drupal\Component\Serialization\Json;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\Url;
@@ -183,7 +182,7 @@ class ShortcutTest extends ResourceTestBase {
     // No results because the current user does not have access to shortcuts
     // not in the user's assigned set or the default set.
     $response = $this->request('GET', $collection_filter_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(0, $doc['data']);
 
     // Assign the alternate shortcut set to the current user.
@@ -192,7 +191,7 @@ class ShortcutTest extends ResourceTestBase {
     // 1 result because the alternate shortcut set is now assigned to the
     // current user.
     $response = $this->request('GET', $collection_filter_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(1, $doc['data']);
   }
 
diff --git a/core/modules/jsonapi/tests/src/Functional/TermTest.php b/core/modules/jsonapi/tests/src/Functional/TermTest.php
index 649237d997a..05d9212fdef 100644
--- a/core/modules/jsonapi/tests/src/Functional/TermTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/TermTest.php
@@ -416,7 +416,7 @@ class TermTest extends ResourceTestBase {
 
     // GET term's current normalization.
     $response = $this->request('GET', $url, $request_options);
-    $normalization = Json::decode((string) $response->getBody());
+    $normalization = $this->getDocumentFromResponse($response);
 
     // Change term's path alias.
     $normalization['data']['attributes']['path']['alias'] .= 's-rule-the-world';
@@ -426,8 +426,8 @@ class TermTest extends ResourceTestBase {
 
     // PATCH request: 200.
     $response = $this->request('PATCH', $url, $request_options);
+    $updated_normalization = $this->getDocumentFromResponse($response);
     $this->assertResourceResponse(200, FALSE, $response);
-    $updated_normalization = Json::decode((string) $response->getBody());
     $this->assertSame($normalization['data']['attributes']['path']['alias'], $updated_normalization['data']['attributes']['path']['alias']);
   }
 
diff --git a/core/modules/jsonapi/tests/src/Functional/UserTest.php b/core/modules/jsonapi/tests/src/Functional/UserTest.php
index 49eca5f48b4..a281a743c7f 100644
--- a/core/modules/jsonapi/tests/src/Functional/UserTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/UserTest.php
@@ -230,7 +230,7 @@ class UserTest extends ResourceTestBase {
     $request_options = NestedArray::mergeDeep($request_options, $this->getAuthenticationRequestOptions());
 
     $response = $this->request('GET', $url, $request_options);
-    $original_normalization = Json::decode((string) $response->getBody());
+    $original_normalization = $this->getDocumentFromResponse($response);
 
     // Test case 1: changing email.
     $normalization = $original_normalization;
@@ -263,7 +263,7 @@ class UserTest extends ResourceTestBase {
     $this->assertResourceResponse(200, FALSE, $response);
 
     // Test case 2: changing password.
-    $normalization = Json::decode((string) $response->getBody());
+    $normalization = $this->getDocumentFromResponse($response);
     $normalization['data']['attributes']['mail'] = 'new-email@example.com';
     $new_password = $this->randomString();
     $normalization['data']['attributes']['pass']['value'] = $new_password;
@@ -291,7 +291,7 @@ class UserTest extends ResourceTestBase {
     $request_options = NestedArray::mergeDeep($request_options, $this->getAuthenticationRequestOptions());
 
     // Test case 3: changing name.
-    $normalization = Json::decode((string) $response->getBody());
+    $normalization = $this->getDocumentFromResponse($response);
     $normalization['data']['attributes']['mail'] = 'new-email@example.com';
     $normalization['data']['attributes']['pass']['existing'] = $new_password;
     $normalization['data']['attributes']['name'] = 'Cooler Llama';
@@ -405,11 +405,11 @@ class UserTest extends ResourceTestBase {
 
     // Viewing user A as user A: "mail" field is accessible.
     $response = $this->request('GET', $user_a_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertArrayHasKey('mail', $doc['data']['attributes']);
     // Also when looking at the collection.
     $response = $this->request('GET', $collection_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertSame($user_a->uuid(), $doc['data']['2']['id']);
     $this->assertArrayHasKey('mail', $doc['data'][2]['attributes'], "Own user--user resource's 'mail' field is visible.");
     $this->assertSame($user_b->uuid(), $doc['data'][count($doc['data']) - 1]['id']);
@@ -420,11 +420,11 @@ class UserTest extends ResourceTestBase {
     $request_options = NestedArray::mergeDeep($request_options, $this->getAuthenticationRequestOptions());
     // Viewing user A as user B: "mail" field should be inaccessible.
     $response = $this->request('GET', $user_a_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertArrayNotHasKey('mail', $doc['data']['attributes']);
     // Also when looking at the collection.
     $response = $this->request('GET', $collection_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertSame($user_a->uuid(), $doc['data']['2']['id']);
     $this->assertArrayNotHasKey('mail', $doc['data'][2]['attributes']);
     $this->assertSame($user_b->uuid(), $doc['data'][count($doc['data']) - 1]['id']);
@@ -434,11 +434,11 @@ class UserTest extends ResourceTestBase {
     $this->grantPermissionsToTestedRole(['view user email addresses']);
     // Viewing user A as user B: "mail" field should be accessible.
     $response = $this->request('GET', $user_a_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertArrayHasKey('mail', $doc['data']['attributes']);
     // Also when looking at the collection.
     $response = $this->request('GET', $collection_url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertSame($user_a->uuid(), $doc['data']['2']['id']);
     $this->assertArrayHasKey('mail', $doc['data'][2]['attributes']);
   }
@@ -472,7 +472,7 @@ class UserTest extends ResourceTestBase {
     $request_options = NestedArray::mergeDeep($request_options, $this->getAuthenticationRequestOptions());
 
     $response = $this->request('GET', $url, $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
 
     $this->assertCount(4, $doc['data']);
     $this->assertSame(User::load(0)->uuid(), $doc['data'][0]['id']);
@@ -530,58 +530,58 @@ class UserTest extends ResourceTestBase {
     // ?filter[uid.id]=OWN_UUID requires no permissions: 1 result.
     $response = $this->request('GET', $collection_url->setOption('query', ['filter[uid.id]' => $this->account->uuid()]), $request_options);
     $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Contexts', 'user.permissions');
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(1, $doc['data']);
     $this->assertSame($node_auth_1->uuid(), $doc['data'][0]['id']);
     // ?filter[uid.id]=ANONYMOUS_UUID: 0 results.
     $response = $this->request('GET', $collection_url->setOption('query', ['filter[uid.id]' => User::load(0)->uuid()]), $request_options);
     $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Contexts', 'user.permissions');
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(0, $doc['data']);
     // ?filter[uid.name]=A: 0 results.
     $response = $this->request('GET', $collection_url->setOption('query', ['filter[uid.name]' => 'A']), $request_options);
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(0, $doc['data']);
     // /jsonapi/user/user?filter[field_favorite_animal]: 0 results.
     $response = $this->request('GET', $favorite_animal_test_url, $request_options);
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $doc = Json::decode((string) $response->getBody());
     $this->assertCount(0, $doc['data']);
     // Grant "view" permission.
     $this->grantPermissionsToTestedRole(['access user profiles']);
     // ?filter[uid.id]=ANONYMOUS_UUID: 0 results.
     $response = $this->request('GET', $collection_url->setOption('query', ['filter[uid.id]' => User::load(0)->uuid()]), $request_options);
     $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Contexts', 'user.permissions');
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(0, $doc['data']);
     // ?filter[uid.name]=A: 1 result since user A is active.
     $response = $this->request('GET', $collection_url->setOption('query', ['filter[uid.name]' => 'A']), $request_options);
     $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Contexts', 'user.permissions');
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(1, $doc['data']);
     $this->assertSame($node_a->uuid(), $doc['data'][0]['id']);
     // ?filter[uid.name]=B: 0 results since user B is blocked.
     $response = $this->request('GET', $collection_url->setOption('query', ['filter[uid.name]' => 'B']), $request_options);
     $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Contexts', 'user.permissions');
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(0, $doc['data']);
     // /jsonapi/user/user?filter[field_favorite_animal]: 0 results.
     $response = $this->request('GET', $favorite_animal_test_url, $request_options);
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $doc = Json::decode((string) $response->getBody());
     $this->assertCount(0, $doc['data']);
     // Grant "admin" permission.
     $this->grantPermissionsToTestedRole(['administer users']);
     // ?filter[uid.name]=B: 1 result.
     $response = $this->request('GET', $collection_url->setOption('query', ['filter[uid.name]' => 'B']), $request_options);
     $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Contexts', 'user.permissions');
-    $doc = Json::decode((string) $response->getBody());
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertCount(1, $doc['data']);
     $this->assertSame($node_b->uuid(), $doc['data'][0]['id']);
     // /jsonapi/user/user?filter[field_favorite_animal]: 1 result.
     $response = $this->request('GET', $favorite_animal_test_url, $request_options);
+    $doc = $this->getDocumentFromResponse($response);
     $this->assertSame(200, $response->getStatusCode());
-    $doc = Json::decode((string) $response->getBody());
     $this->assertCount(1, $doc['data']);
     $this->assertSame($user_b->uuid(), $doc['data'][0]['id']);
   }
diff --git a/core/modules/jsonapi/tests/src/Traits/GetDocumentFromResponseTrait.php b/core/modules/jsonapi/tests/src/Traits/GetDocumentFromResponseTrait.php
new file mode 100644
index 00000000000..955d24c922f
--- /dev/null
+++ b/core/modules/jsonapi/tests/src/Traits/GetDocumentFromResponseTrait.php
@@ -0,0 +1,52 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\jsonapi\Traits;
+
+use Drupal\Component\Serialization\Json;
+use PHPUnit\Framework\TestCase;
+use Psr\Http\Message\ResponseInterface;
+
+/**
+ * Test trait for retrieving the JSON:API document from a response.
+ */
+trait GetDocumentFromResponseTrait {
+
+  /**
+   * Retrieve document from response, with basic validation.
+   *
+   * @param \Psr\Http\Message\ResponseInterface $response
+   *   Response to extract JSON:API document from.
+   * @param bool $validate
+   *   Determines whether the data is validated or not. Defaults to TRUE.
+   *
+   * @return ?array
+   *   JSON:API document extracted from the response, or NULL.
+   *
+   * @throws \PHPUnit\Framework\AssertionFailedError
+   *   Thrown when the document does not pass basic validation against the spec.
+   */
+  protected function getDocumentFromResponse(ResponseInterface $response, bool $validate = TRUE): ?array {
+    assert($this instanceof TestCase);
+
+    $document = Json::decode((string) $response->getBody());
+
+    if (isset($document['data']) && isset($document['errors'])) {
+      $this->fail('Document contains both data and errors members; only one is allowed.');
+    }
+
+    if ($validate === TRUE && !isset($document['data'])) {
+      if (isset($document['errors'])) {
+        $errors = [];
+        foreach ($document['errors'] as $error) {
+          $errors[] = $error['title'] . ' (' . $error['status'] . '): ' . $error['detail'];
+        }
+        $this->fail('Missing expected data member in document. Error(s): ' . PHP_EOL . '  ' . implode('  ' . PHP_EOL, $errors));
+      }
+      $this->fail('Missing both data and errors members in document; either is required. Response body: ' . PHP_EOL . '  ' . $response->getBody());
+    }
+    return $document;
+  }
+
+}