.../tests/src/Functional/BasicAuthTest.php         |  4 +-
 .../tests/src/Functional/DbLogResourceTest.php     |  2 +-
 .../EventSubscriber/DynamicPageCacheSubscriber.php | 10 +++-
 .../Functional/DynamicPageCacheIntegrationTest.php | 18 +++----
 .../file/tests/src/Functional/DownloadTest.php     |  2 +-
 .../jsonapi/tests/src/Functional/NodeTest.php      |  4 +-
 .../tests/src/Functional/ResourceTestBase.php      | 60 +++++++++++-----------
 .../tests/src/Functional/LayoutSectionTest.php     | 11 ++--
 .../src/Functional/NodeBlockFunctionalTest.php     |  2 +-
 .../page_cache/src/StackMiddleware/PageCache.php   | 12 ++++-
 .../tests/src/Functional/PageCacheTest.php         | 30 ++++++++---
 .../EntityResource/EntityResourceTestBase.php      | 14 ++---
 .../rest/tests/src/Functional/ResourceTestBase.php | 14 ++++-
 .../tests/src/Functional/Session/SessionTest.php   |  2 +-
 .../src/Functional/System/ErrorHandlerTest.php     |  2 +-
 .../tests/src/Functional/UserPasswordResetTest.php |  4 +-
 .../standard/tests/src/Functional/StandardTest.php |  2 +-
 17 files changed, 118 insertions(+), 75 deletions(-)

diff --git a/core/modules/basic_auth/tests/src/Functional/BasicAuthTest.php b/core/modules/basic_auth/tests/src/Functional/BasicAuthTest.php
index fc69ee5912..20f7a3bc59 100644
--- a/core/modules/basic_auth/tests/src/Functional/BasicAuthTest.php
+++ b/core/modules/basic_auth/tests/src/Functional/BasicAuthTest.php
@@ -41,7 +41,7 @@ public function testBasicAuth() {
     $this->assertText($account->getAccountName(), 'Account name is displayed.');
     $this->assertResponse('200', 'HTTP response is OK');
     $this->mink->resetSessions();
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'));
+    $this->assertSame('UNCACHEABLE (request policy)', $this->drupalGetHeader('X-Drupal-Cache'));
     $this->assertIdentical(strpos($this->drupalGetHeader('Cache-Control'), 'public'), FALSE, 'Cache-Control is not set to public');
 
     $this->basicAuthGet($url, $account->getAccountName(), $this->randomMachineName());
@@ -69,7 +69,7 @@ public function testBasicAuth() {
     $this->drupalGet($url);
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS');
     $this->basicAuthGet($url, $account->getAccountName(), $account->pass_raw);
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'));
+    $this->assertSame('UNCACHEABLE (request policy)', $this->drupalGetHeader('X-Drupal-Cache'));
     $this->assertIdentical(strpos($this->drupalGetHeader('Cache-Control'), 'public'), FALSE, 'No page cache response when requesting a cached page with basic auth credentials.');
   }
 
diff --git a/core/modules/dblog/tests/src/Functional/DbLogResourceTest.php b/core/modules/dblog/tests/src/Functional/DbLogResourceTest.php
index fc22d49182..348ceff847 100644
--- a/core/modules/dblog/tests/src/Functional/DbLogResourceTest.php
+++ b/core/modules/dblog/tests/src/Functional/DbLogResourceTest.php
@@ -67,7 +67,7 @@ public function testWatchdog() {
     $request_options = $this->getAuthenticationRequestOptions('GET');
 
     $response = $this->request('GET', $url, $request_options);
-    $this->assertResourceErrorResponse(403, "The 'restful get dblog' permission is required.", $response, ['4xx-response', 'http_response'], ['user.permissions'], FALSE, FALSE);
+    $this->assertResourceErrorResponse(403, "The 'restful get dblog' permission is required.", $response, ['4xx-response', 'http_response'], ['user.permissions'], 'UNCACHEABLE (request policy)', FALSE);
 
     // Create a user account that has the required permissions to read
     // the watchdog resource via the REST API.
diff --git a/core/modules/dynamic_page_cache/src/EventSubscriber/DynamicPageCacheSubscriber.php b/core/modules/dynamic_page_cache/src/EventSubscriber/DynamicPageCacheSubscriber.php
index e99caa1b89..08945f7714 100644
--- a/core/modules/dynamic_page_cache/src/EventSubscriber/DynamicPageCacheSubscriber.php
+++ b/core/modules/dynamic_page_cache/src/EventSubscriber/DynamicPageCacheSubscriber.php
@@ -156,6 +156,7 @@ public function onResponse(FilterResponseEvent $event) {
     // access and modify the cacheability metadata associated with the
     // response.)
     if (!$response instanceof CacheableResponseInterface) {
+      $response->headers->set(self::HEADER, 'UNCACHEABLE (no cacheability)');
       return;
     }
 
@@ -167,7 +168,7 @@ public function onResponse(FilterResponseEvent $event) {
     // There's no work left to be done if this is an uncacheable response.
     if (!$this->shouldCacheResponse($response)) {
       // The response is uncacheable, mark it as such.
-      $response->headers->set(self::HEADER, 'UNCACHEABLE');
+      $response->headers->set(self::HEADER, 'UNCACHEABLE (poor cacheability)');
       return;
     }
 
@@ -190,7 +191,12 @@ public function onResponse(FilterResponseEvent $event) {
     // Don't cache the response if the Dynamic Page Cache request & response
     // policies are not met.
     // @see onRequest()
-    if ($this->requestPolicyResults[$request] === RequestPolicyInterface::DENY || $this->responsePolicy->check($response, $request) === ResponsePolicyInterface::DENY) {
+    if ($this->requestPolicyResults[$request] === RequestPolicyInterface::DENY) {
+      $response->headers->set(self::HEADER, 'UNCACHEABLE (request policy)');
+      return;
+    }
+    if ($this->responsePolicy->check($response, $request) === ResponsePolicyInterface::DENY) {
+      $response->headers->set(self::HEADER, 'UNCACHEABLE (response policy)');
       return;
     }
 
diff --git a/core/modules/dynamic_page_cache/tests/src/Functional/DynamicPageCacheIntegrationTest.php b/core/modules/dynamic_page_cache/tests/src/Functional/DynamicPageCacheIntegrationTest.php
index ffa8f4375f..232b5b5fd3 100644
--- a/core/modules/dynamic_page_cache/tests/src/Functional/DynamicPageCacheIntegrationTest.php
+++ b/core/modules/dynamic_page_cache/tests/src/Functional/DynamicPageCacheIntegrationTest.php
@@ -51,7 +51,7 @@ public function testDynamicPageCache() {
     // Cache.
     $url = Url::fromUri('route:dynamic_page_cache_test.response');
     $this->drupalGet($url);
-    $this->assertFalse($this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Response object returned: Dynamic Page Cache is ignoring.');
+    $this->assertSame('UNCACHEABLE (no cacheability)', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Response object returned: Dynamic Page Cache is ignoring.');
 
     // Controllers returning CacheableResponseInterface (cacheable response)
     // objects are handled by Dynamic Page Cache.
@@ -95,27 +95,27 @@ public function testDynamicPageCache() {
     // response, are ignored by Dynamic Page Cache (but only because those
     // wrapper formats' responses do not implement CacheableResponseInterface).
     $this->drupalGet('dynamic-page-cache-test/html', ['query' => [MainContentViewSubscriber::WRAPPER_FORMAT => 'drupal_ajax']]);
-    $this->assertFalse($this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Render array returned, rendered as AJAX response: Dynamic Page Cache is ignoring.');
+    $this->assertSame('UNCACHEABLE (no cacheability)', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Render array returned, rendered as AJAX response: Dynamic Page Cache is ignoring.');
     $this->drupalGet('dynamic-page-cache-test/html', ['query' => [MainContentViewSubscriber::WRAPPER_FORMAT => 'drupal_dialog']]);
-    $this->assertFalse($this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Render array returned, rendered as dialog response: Dynamic Page Cache is ignoring.');
+    $this->assertSame('UNCACHEABLE (no cacheability)', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Render array returned, rendered as dialog response: Dynamic Page Cache is ignoring.');
     $this->drupalGet('dynamic-page-cache-test/html', ['query' => [MainContentViewSubscriber::WRAPPER_FORMAT => 'drupal_modal']]);
-    $this->assertFalse($this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Render array returned, rendered as modal response: Dynamic Page Cache is ignoring.');
+    $this->assertSame('UNCACHEABLE (no cacheability)', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Render array returned, rendered as modal response: Dynamic Page Cache is ignoring.');
 
     // Admin routes are ignored by Dynamic Page Cache.
     $this->drupalGet('dynamic-page-cache-test/html/admin');
-    $this->assertFalse($this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Response returned, rendered as HTML response, admin route: Dynamic Page Cache is ignoring');
+    $this->assertSame('UNCACHEABLE (response policy)', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Response returned, rendered as HTML response, admin route: Dynamic Page Cache is ignoring');
     $this->drupalGet('dynamic-page-cache-test/response/admin');
-    $this->assertFalse($this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Response returned, plain response, admin route: Dynamic Page Cache is ignoring');
+    $this->assertSame('UNCACHEABLE (no cacheability)', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Response returned, plain response, admin route: Dynamic Page Cache is ignoring');
     $this->drupalGet('dynamic-page-cache-test/cacheable-response/admin');
-    $this->assertFalse($this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Response returned, cacheable response, admin route: Dynamic Page Cache is ignoring');
+    $this->assertSame('UNCACHEABLE (response policy)', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Response returned, cacheable response, admin route: Dynamic Page Cache is ignoring');
 
     // Max-age = 0 responses are ignored by Dynamic Page Cache.
     $this->drupalGet('dynamic-page-cache-test/html/uncacheable/max-age');
-    $this->assertEqual('UNCACHEABLE', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Render array returned, rendered as HTML response, but uncacheable: Dynamic Page Cache is running, but not caching.');
+    $this->assertSame('UNCACHEABLE (poor cacheability)', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Render array returned, rendered as HTML response, but uncacheable: Dynamic Page Cache is running, but not caching.');
 
     // 'user' cache context responses are ignored by Dynamic Page Cache.
     $this->drupalGet('dynamic-page-cache-test/html/uncacheable/contexts');
-    $this->assertEqual('UNCACHEABLE', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Render array returned, rendered as HTML response, but uncacheable: Dynamic Page Cache is running, but not caching.');
+    $this->assertSame('UNCACHEABLE (poor cacheability)', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Render array returned, rendered as HTML response, but uncacheable: Dynamic Page Cache is running, but not caching.');
 
     // 'current-temperature' cache tag responses are ignored by Dynamic Page
     // Cache.
diff --git a/core/modules/file/tests/src/Functional/DownloadTest.php b/core/modules/file/tests/src/Functional/DownloadTest.php
index bf25a44cad..e4b8a6f4fb 100644
--- a/core/modules/file/tests/src/Functional/DownloadTest.php
+++ b/core/modules/file/tests/src/Functional/DownloadTest.php
@@ -70,7 +70,7 @@ protected function doPrivateFileTransferTest() {
     file_test_set_return('download', ['x-foo' => 'Bar']);
     $this->drupalGet($url);
     $this->assertEqual($this->drupalGetHeader('x-foo'), 'Bar', 'Found header set by file_test module on private download.');
-    $this->assertFalse($this->drupalGetHeader('x-drupal-cache'), 'Page cache is disabled on private file download.');
+    $this->assertIdentical('UNCACHEABLE (no cacheability)', $this->drupalGetHeader('X-Drupal-Cache'), 'Page cache is disabled on private file download.');
     $this->assertResponse(200, 'Correctly allowed access to a file when file_test provides headers.');
 
     // Test that the file transferred correctly.
diff --git a/core/modules/jsonapi/tests/src/Functional/NodeTest.php b/core/modules/jsonapi/tests/src/Functional/NodeTest.php
index 81c8b54489..037494bbe2 100644
--- a/core/modules/jsonapi/tests/src/Functional/NodeTest.php
+++ b/core/modules/jsonapi/tests/src/Functional/NodeTest.php
@@ -335,7 +335,7 @@ public function testGetIndividual() {
       $response,
       ['4xx-response', 'http_response', 'node:1'],
       ['url.query_args:resourceVersion', 'url.site', 'user.permissions'],
-      FALSE,
+      'UNCACHEABLE (request policy)',
       'MISS'
     );
     /* $this->assertResourceErrorResponse(403, 'The current user is not allowed to GET the selected resource.', $response, '/data'); */
@@ -347,7 +347,7 @@ public function testGetIndividual() {
     // context to be optimized away.
     $expected_cache_contexts = Cache::mergeContexts($this->getExpectedCacheContexts(), ['user']);
     $expected_cache_contexts = array_diff($expected_cache_contexts, ['user.permissions']);
-    $this->assertResourceResponse(200, FALSE, $response, $this->getExpectedCacheTags(), $expected_cache_contexts, FALSE, 'UNCACHEABLE');
+    $this->assertResourceResponse(200, FALSE, $response, $this->getExpectedCacheTags(), $expected_cache_contexts, 'UNCACHEABLE (request policy)', 'UNCACHEABLE (poor cacheability)');
   }
 
   /**
diff --git a/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php b/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php
index f50c15804a..c9eec4e625 100644
--- a/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php
+++ b/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php
@@ -920,7 +920,7 @@ public function testGetIndividual() {
       $expected_403_cacheability = $this->getExpectedUnauthorizedAccessCacheability();
       $reason = $this->getExpectedUnauthorizedAccessMessage('GET');
       $message = trim("The current user is not allowed to GET the selected resource. $reason");
-      $this->assertResourceErrorResponse(403, $message, $url, $response, '/data', $expected_403_cacheability->getCacheTags(), $expected_403_cacheability->getCacheContexts(), FALSE, 'MISS');
+      $this->assertResourceErrorResponse(403, $message, $url, $response, '/data', $expected_403_cacheability->getCacheTags(), $expected_403_cacheability->getCacheContexts(), 'UNCACHEABLE (request policy)', 'MISS');
       $this->assertArrayNotHasKey('Link', $response->getHeaders());
     }
     else {
@@ -929,8 +929,8 @@ public function testGetIndividual() {
       $expected_document['data']['attributes'] = array_intersect_key($expected_document['data']['attributes'], [$label_field_name => TRUE]);
       unset($expected_document['data']['relationships']);
       // MISS or UNCACHEABLE depends on data. It must not be HIT.
-      $dynamic_cache_label_only = !empty(array_intersect(['user', 'session'], $this->getExpectedCacheContexts([$label_field_name]))) ? 'UNCACHEABLE' : 'MISS';
-      $this->assertResourceResponse(200, $expected_document, $response, $this->getExpectedCacheTags(), $this->getExpectedCacheContexts([$label_field_name]), FALSE, $dynamic_cache_label_only);
+      $dynamic_cache_label_only = !empty(array_intersect(['user', 'session'], $this->getExpectedCacheContexts([$label_field_name]))) ? 'UNCACHEABLE (poor cacheability)' : 'MISS';
+      $this->assertResourceResponse(200, $expected_document, $response, $this->getExpectedCacheTags(), $this->getExpectedCacheContexts([$label_field_name]), 'UNCACHEABLE (request policy)', $dynamic_cache_label_only);
     }
 
     $this->setUpAuthorization('GET');
@@ -956,20 +956,20 @@ public function testGetIndividual() {
         ],
       ],
     ];
-    $this->assertResourceResponse(400, $expected_document, $response, ['4xx-response', 'http_response'], ['url.query_args', 'url.site'], FALSE, 'MISS');
+    $this->assertResourceResponse(400, $expected_document, $response, ['4xx-response', 'http_response'], ['url.query_args', 'url.site'], 'UNCACHEABLE (request policy)', 'MISS');
 
     // 200 for well-formed HEAD request.
     $response = $this->request('HEAD', $url, $request_options);
     // MISS or UNCACHEABLE depends on data. It must not be HIT.
-    $dynamic_cache = !empty(array_intersect(['user', 'session'], $this->getExpectedCacheContexts())) ? 'UNCACHEABLE' : 'MISS';
-    $this->assertResourceResponse(200, NULL, $response, $this->getExpectedCacheTags(), $this->getExpectedCacheContexts(), FALSE, $dynamic_cache);
+    $dynamic_cache = !empty(array_intersect(['user', 'session'], $this->getExpectedCacheContexts())) ? 'UNCACHEABLE (poor cacheability)' : 'MISS';
+    $this->assertResourceResponse(200, NULL, $response, $this->getExpectedCacheTags(), $this->getExpectedCacheContexts(), 'UNCACHEABLE (request policy)', $dynamic_cache);
     $head_headers = $response->getHeaders();
 
     // 200 for well-formed GET request. Page Cache hit because of HEAD request.
     // Same for Dynamic Page Cache hit.
     $response = $this->request('GET', $url, $request_options);
 
-    $this->assertResourceResponse(200, $this->getExpectedDocument(), $response, $this->getExpectedCacheTags(), $this->getExpectedCacheContexts(), FALSE, $dynamic_cache === 'MISS' ? 'HIT' : 'UNCACHEABLE');
+    $this->assertResourceResponse(200, $this->getExpectedDocument(), $response, $this->getExpectedCacheTags(), $this->getExpectedCacheContexts(), 'UNCACHEABLE (request policy)', $dynamic_cache === 'MISS' ? 'HIT' : 'UNCACHEABLE (poor cacheability)');
     // Assert that Dynamic Page Cache did not store a ResourceResponse object,
     // which needs serialization after every cache hit. Instead, it should
     // contain a flattened response. Otherwise performance suffers.
@@ -1050,12 +1050,12 @@ public function testGetIndividual() {
     $message_url = clone $url;
     $path = str_replace($random_uuid, '{entity}', $message_url->setAbsolute()->setOptions(['base_url' => '', 'query' => []])->toString());
     $message = 'The "entity" parameter was not converted for the path "' . $path . '" (route name: "jsonapi.' . static::$resourceTypeName . '.individual")';
-    $this->assertResourceErrorResponse(404, $message, $url, $response, FALSE, ['4xx-response', 'http_response'], ['url.site'], FALSE, 'UNCACHEABLE');
+    $this->assertResourceErrorResponse(404, $message, $url, $response, FALSE, ['4xx-response', 'http_response'], ['url.site'], 'UNCACHEABLE (request policy)', 'UNCACHEABLE (poor cacheability)');
 
     // DX: when Accept request header is missing, still 404, same response.
     unset($request_options[RequestOptions::HEADERS]['Accept']);
     $response = $this->request('GET', $url, $request_options);
-    $this->assertResourceErrorResponse(404, $message, $url, $response, FALSE, ['4xx-response', 'http_response'], ['url.site'], FALSE, 'UNCACHEABLE');
+    $this->assertResourceErrorResponse(404, $message, $url, $response, FALSE, ['4xx-response', 'http_response'], ['url.site'], 'UNCACHEABLE (request policy)', 'UNCACHEABLE (poor cacheability)');
   }
 
   /**
@@ -1077,7 +1077,7 @@ public function testCollection() {
     $response = $this->request('HEAD', $collection_url, $request_options);
     // MISS or UNCACHEABLE depends on the collection data. It must not be HIT.
     $dynamic_cache = $expected_cacheability->getCacheMaxAge() === 0 ? 'UNCACHEABLE' : 'MISS';
-    $this->assertResourceResponse(200, NULL, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), FALSE, $dynamic_cache);
+    $this->assertResourceResponse(200, NULL, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), 'UNCACHEABLE (request policy)', $dynamic_cache);
 
     // Different databases have different sort orders, so a sort is required so
     // test expectations do not need to vary per database.
@@ -1091,7 +1091,7 @@ public function testCollection() {
     $expected_cacheability = $expected_response->getCacheableMetadata();
     $expected_document = $expected_response->getResponseData();
     $response = $this->request('GET', $collection_url, $request_options);
-    $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), FALSE, $dynamic_cache);
+    $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), 'UNCACHEABLE (request policy)', $dynamic_cache);
 
     $this->setUpAuthorization('GET');
 
@@ -1099,7 +1099,7 @@ public function testCollection() {
     $expected_response = $this->getExpectedCollectionResponse($entity_collection, $collection_url->toString(), $request_options);
     $expected_cacheability = $expected_response->getCacheableMetadata();
     $response = $this->request('HEAD', $collection_url, $request_options);
-    $this->assertResourceResponse(200, NULL, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), FALSE, $dynamic_cache);
+    $this->assertResourceResponse(200, NULL, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), 'UNCACHEABLE (request policy)', $dynamic_cache);
 
     // 200 for well-formed GET request.
     $expected_response = $this->getExpectedCollectionResponse($entity_collection, $collection_url->toString(), $request_options);
@@ -1107,8 +1107,8 @@ public function testCollection() {
     $expected_document = $expected_response->getResponseData();
     $response = $this->request('GET', $collection_url, $request_options);
     // Dynamic Page Cache HIT unless the HEAD request was UNCACHEABLE.
-    $dynamic_cache = $dynamic_cache === 'UNCACHEABLE' ? 'UNCACHEABLE' : 'HIT';
-    $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), FALSE, $dynamic_cache);
+    $dynamic_cache = $expected_cacheability->getCacheMaxAge() === 0 || !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE (poor cacheability)' : 'MISS';
+    $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), 'UNCACHEABLE (request policy)', $dynamic_cache);
 
     if ($this->entity instanceof FieldableEntityInterface) {
       // 403 for filtering on an unauthorized field on the base resource type.
@@ -1131,14 +1131,14 @@ public function testCollection() {
         'url.site',
         'user.permissions',
       ];
-      $this->assertResourceErrorResponse(403, $expected_error_message, $unauthorized_filter_url, $response, FALSE, $expected_cache_tags, $expected_cache_contexts, FALSE, 'MISS');
+      $this->assertResourceErrorResponse(403, $expected_error_message, $unauthorized_filter_url, $response, FALSE, $expected_cache_tags, $expected_cache_contexts, 'UNCACHEABLE (request policy)', 'MISS');
 
       $this->grantPermissionsToTestedRole(['field_jsonapi_test_entity_ref view access']);
 
       // 403 for filtering on an unauthorized field on a related resource type.
       $response = $this->request('GET', $unauthorized_filter_url, $request_options);
       $expected_error_message = "The current user is not authorized to filter by the `status` field, given in the path `field_jsonapi_test_entity_ref.entity:user.status`.";
-      $this->assertResourceErrorResponse(403, $expected_error_message, $unauthorized_filter_url, $response, FALSE, $expected_cache_tags, $expected_cache_contexts, FALSE, 'MISS');
+      $this->assertResourceErrorResponse(403, $expected_error_message, $unauthorized_filter_url, $response, FALSE, $expected_cache_tags, $expected_cache_contexts, 'UNCACHEABLE (request policy)', 'MISS');
     }
 
     // Remove an entity from the collection, then filter it out.
@@ -1162,8 +1162,8 @@ public function testCollection() {
     $expected_document = $expected_response->getResponseData();
     $response = $this->request('GET', $filtered_collection_url, $request_options);
     // MISS or UNCACHEABLE depends on the collection data. It must not be HIT.
-    $dynamic_cache = $expected_cacheability->getCacheMaxAge() === 0 || !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE' : 'MISS';
-    $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), FALSE, $dynamic_cache);
+    $dynamic_cache = $expected_cacheability->getCacheMaxAge() === 0 || !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE (poor cacheability)' : 'MISS';
+    $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), 'UNCACHEABLE (request policy)', $dynamic_cache);
 
     // Filtered collection with includes.
     $relationship_field_names = array_reduce($filtered_entity_collection, function ($relationship_field_names, $entity) {
@@ -1178,8 +1178,8 @@ public function testCollection() {
     $expected_document = $expected_response->getResponseData();
     $response = $this->request('GET', $filtered_collection_include_url, $request_options);
     // MISS or UNCACHEABLE depends on the included data. It must not be HIT.
-    $dynamic_cache = $expected_cacheability->getCacheMaxAge() === 0 || !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE' : 'MISS';
-    $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), FALSE, $dynamic_cache);
+    $dynamic_cache = $expected_cacheability->getCacheMaxAge() === 0 || !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE (poor cacheability)' : 'MISS';
+    $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), 'UNCACHEABLE (request policy)', $dynamic_cache);
 
     // If the response should vary by a user's authorizations, grant permissions
     // for the included resources and execute another request.
@@ -1198,8 +1198,8 @@ public function testCollection() {
       $response = $this->request('GET', $filtered_collection_include_url, $request_options);
       $requires_include_only_permissions = !empty($flattened_permissions);
       $uncacheable = $expected_cacheability->getCacheMaxAge() === 0 || !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts()));
-      $dynamic_cache = !$uncacheable ? $requires_include_only_permissions ? 'MISS' : 'HIT' : 'UNCACHEABLE';
-      $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), FALSE, $dynamic_cache);
+      $dynamic_cache = !$uncacheable ? $requires_include_only_permissions ? 'MISS' : 'HIT' : 'UNCACHEABLE (poor cacheability)';
+      $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), 'UNCACHEABLE (request policy)', $dynamic_cache);
     }
 
     // Sorted collection with includes.
@@ -1216,8 +1216,8 @@ public function testCollection() {
     $expected_document = $expected_response->getResponseData();
     $response = $this->request('GET', $sorted_collection_include_url, $request_options);
     // MISS or UNCACHEABLE depends on the included data. It must not be HIT.
-    $dynamic_cache = $expected_cacheability->getCacheMaxAge() === 0 ? 'UNCACHEABLE' : 'MISS';
-    $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), FALSE, $dynamic_cache);
+    $dynamic_cache = $expected_cacheability->getCacheMaxAge() === 0 || !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE (poor cacheability)' : 'MISS';
+    $this->assertResourceResponse(200, $expected_document, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), 'UNCACHEABLE (request policy)', $dynamic_cache);
   }
 
   /**
@@ -1408,7 +1408,7 @@ protected function doTestRelationshipGet(array $request_options) {
         $actual_response,
         $expected_cacheability->getCacheTags(),
         $expected_cacheability->getCacheContexts(),
-        FALSE,
+        'UNCACHEABLE (request policy)',
         $expected_resource_response->isSuccessful() ? 'MISS' : FALSE
       );
     }
@@ -2592,21 +2592,21 @@ protected function doTestSparseFieldSets(Url $url, array $request_options) {
       $response = $this->request('GET', $url, $request_options);
       // Dynamic Page Cache MISS because cache should vary based on the 'field'
       // query param. (Or uncacheable if expensive cache context.)
-      $dynamic_cache = !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE' : 'MISS';
+      $dynamic_cache = !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE (poor cacheability)' : 'MISS';
       $this->assertResourceResponse(
         200,
         $expected_document,
         $response,
         $expected_cacheability->getCacheTags(),
         $expected_cacheability->getCacheContexts(),
-        FALSE,
+        'UNCACHEABLE (request policy)',
         $dynamic_cache
       );
     }
     // Test Dynamic Page Cache HIT for a query with the same field set (unless
     // expensive cache context is present).
     $response = $this->request('GET', $url, $request_options);
-    $this->assertResourceResponse(200, FALSE, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), FALSE, $dynamic_cache === 'MISS' ? 'HIT' : 'UNCACHEABLE');
+    $this->assertResourceResponse(200, FALSE, $response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), 'UNCACHEABLE (request policy)', $dynamic_cache === 'MISS' ? 'HIT' : 'UNCACHEABLE (poor cacheability)');
   }
 
   /**
@@ -2658,7 +2658,7 @@ protected function doTestIncluded(Url $url, array $request_options) {
         $actual_response,
         $expected_cacheability->getCacheTags(),
         $expected_cacheability->getCacheContexts(),
-        FALSE,
+        'UNCACHEABLE (request policy)',
         $dynamic_cache
       );
     }
@@ -3098,7 +3098,7 @@ public function testRevisions() {
       $expected_cacheability = $expected_response->getCacheableMetadata();
       $expected_document['links']['self']['href'] = $related_url->toString();
       // MISS or UNCACHEABLE depends on data. It must not be HIT.
-      $dynamic_cache = !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE' : 'MISS';
+      $dynamic_cache = !empty(array_intersect(['user', 'session'], $expected_cacheability->getCacheContexts())) ? 'UNCACHEABLE (poor cacheability)' : 'MISS';
       $this->assertResourceResponse(200, $expected_document, $actual_response, $expected_cacheability->getCacheTags(), $expected_cacheability->getCacheContexts(), FALSE, $dynamic_cache);
     }
 
diff --git a/core/modules/layout_builder/tests/src/Functional/LayoutSectionTest.php b/core/modules/layout_builder/tests/src/Functional/LayoutSectionTest.php
index 86ae521b10..dee89dab5b 100644
--- a/core/modules/layout_builder/tests/src/Functional/LayoutSectionTest.php
+++ b/core/modules/layout_builder/tests/src/Functional/LayoutSectionTest.php
@@ -73,7 +73,7 @@ public function providerTestLayoutSectionFormatter() {
       ],
       'user',
       'user:2',
-      'UNCACHEABLE',
+      'UNCACHEABLE (poor cacheability)',
     ];
     $data['block_with_entity_context'] = [
       [
@@ -167,7 +167,7 @@ public function testLayoutSectionFormatter($layout_data, $expected_selector, $ex
     $this->assertLayoutSection($expected_selector, $expected_content, $expected_cache_contexts, $expected_cache_tags, $expected_dynamic_cache);
 
     $this->drupalGet($canonical_url->toString() . '/layout');
-    $this->assertLayoutSection($expected_selector, $expected_content, $expected_cache_contexts, $expected_cache_tags, 'UNCACHEABLE');
+    $this->assertLayoutSection($expected_selector, $expected_content, $expected_cache_contexts, $expected_cache_tags, 'UNCACHEABLE (poor cacheability)');
   }
 
   /**
@@ -188,14 +188,14 @@ public function testLayoutSectionFormatterAccess() {
     $this->container->get('state')->set('test_block_access', FALSE);
 
     $this->drupalGet($node->toUrl('canonical'));
-    $this->assertLayoutSection('.layout--onecol', NULL, '', '', 'UNCACHEABLE');
+    $this->assertLayoutSection('.layout--onecol', NULL, '', '', 'UNCACHEABLE (poor cacheability)');
     // Ensure the block was not rendered.
     $this->assertSession()->pageTextNotContains('Hello test world');
 
     // Grant access to the block, and ensure it was rendered.
     $this->container->get('state')->set('test_block_access', TRUE);
     $this->drupalGet($node->toUrl('canonical'));
-    $this->assertLayoutSection('.layout--onecol', 'Hello test world', '', '', 'UNCACHEABLE');
+    $this->assertLayoutSection('.layout--onecol', 'Hello test world', '', '', 'UNCACHEABLE (poor cacheability)');
   }
 
   /**
@@ -281,7 +281,8 @@ public function testLayoutDeletingBundle() {
    * @param string $expected_cache_tags
    *   A string of cache tags to be found in the header.
    * @param string $expected_dynamic_cache
-   *   The expected dynamic cache header. Either 'HIT', 'MISS' or 'UNCACHEABLE'.
+   *   The expected dynamic cache header. Either 'HIT', 'MISS' or
+   *   'UNCACHEABLE (poor cacheability)'.
    */
   protected function assertLayoutSection($expected_selector, $expected_content, $expected_cache_contexts = '', $expected_cache_tags = '', $expected_dynamic_cache = 'MISS') {
     $assert_session = $this->assertSession();
diff --git a/core/modules/node/tests/src/Functional/NodeBlockFunctionalTest.php b/core/modules/node/tests/src/Functional/NodeBlockFunctionalTest.php
index cc8e5703bb..1660bf9e23 100644
--- a/core/modules/node/tests/src/Functional/NodeBlockFunctionalTest.php
+++ b/core/modules/node/tests/src/Functional/NodeBlockFunctionalTest.php
@@ -154,7 +154,7 @@ public function testRecentNodeBlock() {
     $this->assertCacheContexts(['languages:language_content', 'languages:language_interface', 'session', 'theme', 'url.path', 'url.query_args', 'user', 'route']);
 
     // The node/add/article page is an admin path and currently uncacheable.
-    $this->assertSame('UNCACHEABLE', $this->getSession()->getResponseHeader('X-Drupal-Dynamic-Cache'));
+    $this->assertSame('UNCACHEABLE (poor cacheability)', $this->getSession()->getResponseHeader('X-Drupal-Dynamic-Cache'));
 
     $this->drupalGet('node/' . $node1->id());
     $this->assertText($label, 'Block was displayed on the node/N when node is of type article.');
diff --git a/core/modules/page_cache/src/StackMiddleware/PageCache.php b/core/modules/page_cache/src/StackMiddleware/PageCache.php
index ba07414d63..1b2176ba02 100644
--- a/core/modules/page_cache/src/StackMiddleware/PageCache.php
+++ b/core/modules/page_cache/src/StackMiddleware/PageCache.php
@@ -19,6 +19,11 @@
  */
 class PageCache implements HttpKernelInterface {
 
+  /**
+   * Name of Page Cache's response header.
+   */
+  const HEADER = 'X-Drupal-Cache';
+
   /**
    * The wrapped HTTP kernel.
    *
@@ -83,6 +88,7 @@ public function handle(Request $request, $type = self::MASTER_REQUEST, $catch =
     }
     else {
       $response = $this->pass($request, $type, $catch);
+      $response->headers->set(self::HEADER, 'UNCACHEABLE (request policy)');
     }
 
     return $response;
@@ -122,7 +128,7 @@ protected function pass(Request $request, $type = self::MASTER_REQUEST, $catch =
    */
   protected function lookup(Request $request, $type = self::MASTER_REQUEST, $catch = TRUE) {
     if ($response = $this->get($request)) {
-      $response->headers->set('X-Drupal-Cache', 'HIT');
+      $response->headers->set(static::HEADER, 'HIT');
     }
     else {
       $response = $this->fetch($request, $type, $catch);
@@ -193,7 +199,7 @@ protected function fetch(Request $request, $type = self::MASTER_REQUEST, $catch
     // Only set the 'X-Drupal-Cache' header if caching is allowed for this
     // response.
     if ($this->storeResponse($request, $response)) {
-      $response->headers->set('X-Drupal-Cache', 'MISS');
+      $response->headers->set(static::HEADER, 'MISS');
     }
 
     return $response;
@@ -232,6 +238,7 @@ protected function storeResponse(Request $request, Response $response) {
     //   so by replacing/extending this middleware service or adding another
     //   one.
     if (!$response instanceof CacheableResponseInterface) {
+      $response->headers->set(self::HEADER, 'UNCACHEABLE (no cacheability)');
       return FALSE;
     }
 
@@ -245,6 +252,7 @@ protected function storeResponse(Request $request, Response $response) {
 
     // Allow policy rules to further restrict which responses to cache.
     if ($this->responsePolicy->check($response, $request) === ResponsePolicyInterface::DENY) {
+      $response->headers->set(self::HEADER, 'UNCACHEABLE (response policy)');
       return FALSE;
     }
 
diff --git a/core/modules/page_cache/tests/src/Functional/PageCacheTest.php b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
index 7b0dade84c..84c72a86dd 100644
--- a/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
+++ b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
@@ -228,7 +228,7 @@ public function testConditionalRequests() {
     $this->drupalLogin($user);
     $this->drupalGet('', [], ['If-Modified-Since' => $last_modified, 'If-None-Match' => $etag]);
     $this->assertResponse(200, 'Conditional request returned 200 OK for authenticated user.');
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'), 'Absence of Page was not cached.');
+    $this->assertSame('UNCACHEABLE (request policy)', $this->drupalGetHeader('X-Drupal-Cache'), 'Absence of Page was not cached.');
   }
 
   /**
@@ -268,7 +268,7 @@ public function testPageCache() {
     $user = $this->drupalCreateUser();
     $this->drupalLogin($user);
     $this->drupalGet('system-test/set-header', ['query' => ['name' => 'Foo', 'value' => 'bar']]);
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'), 'Caching was bypassed.');
+    $this->assertSame('UNCACHEABLE (request policy)', $this->drupalGetHeader('X-Drupal-Cache'), 'Caching was bypassed.');
     $this->assertTrue(strpos(strtolower($this->drupalGetHeader('Vary')), 'cookie') === FALSE, 'Vary: Cookie header was not sent.');
     $this->assertEqual($this->drupalGetHeader('Cache-Control'), 'must-revalidate, no-cache, private', 'Cache-Control header was sent.');
     $this->assertEqual($this->drupalGetHeader('Expires'), 'Sun, 19 Nov 1978 05:00:00 GMT', 'Expires header was sent.');
@@ -488,22 +488,22 @@ public function testCacheableResponseResponses() {
 
     // GET a URL, which would be marked as a cache miss if it were cacheable.
     $this->drupalGet('/system-test/respond-reponse');
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'), 'Drupal page cache header not found.');
+    $this->assertSame('UNCACHEABLE (no cacheability)', $this->drupalGetHeader('X-Drupal-Cache'));
     $this->assertEqual($this->drupalGetHeader('Cache-Control'), 'must-revalidate, no-cache, private', 'Cache-Control header was sent');
 
     // GET it again, verify it's still not cached.
     $this->drupalGet('/system-test/respond-reponse');
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'), 'Drupal page cache header not found.');
+    $this->assertSame('UNCACHEABLE (no cacheability)', $this->drupalGetHeader('X-Drupal-Cache'));
     $this->assertEqual($this->drupalGetHeader('Cache-Control'), 'must-revalidate, no-cache, private', 'Cache-Control header was sent');
 
     // GET a URL, which would be marked as a cache miss if it were cacheable.
     $this->drupalGet('/system-test/respond-public-response');
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'), 'Drupal page cache header not found.');
+    $this->assertSame('UNCACHEABLE (no cacheability)', $this->drupalGetHeader('X-Drupal-Cache'));
     $this->assertEqual($this->drupalGetHeader('Cache-Control'), 'max-age=60, public', 'Cache-Control header was sent');
 
     // GET it again, verify it's still not cached.
     $this->drupalGet('/system-test/respond-public-response');
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'), 'Drupal page cache header not found.');
+    $this->assertSame('UNCACHEABLE (no cacheability)', $this->drupalGetHeader('X-Drupal-Cache'));
     $this->assertEqual($this->drupalGetHeader('Cache-Control'), 'max-age=60, public', 'Cache-Control header was sent');
 
     // GET a URL, which should be marked as a cache miss.
@@ -526,6 +526,24 @@ public function testCacheableResponseResponses() {
     $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'), 'Drupal page cache header not found.');
   }
 
+  /**
+   * Tests uncacheable responses' X-Drupal-Cache response headers.
+   */
+  public function testUncacheableResponseHeader() {
+    // A controller that responds with a cacheable response, but the policy
+    // \Drupal\Core\PageCache\ResponsePolicy\DenyNoCacheRoutes prevents caching.
+    $this->drupalGet(Url::fromRoute('system_test.lock_acquire'));
+    $this->assertSame('UNCACHEABLE (response policy)', $this->drupalGetHeader('X-Drupal-Cache'));
+
+    // A controller that responds with an uncacheable response.
+    $this->drupalGet(Url::fromRoute('system_test.date'));
+    $this->assertSame('UNCACHEABLE (no cacheability)', $this->drupalGetHeader('X-Drupal-Cache'));
+
+    $this->drupalLogin($this->drupalCreateUser());
+    $this->drupalGet(Url::fromRoute('system_test.date'));
+    $this->assertSame('UNCACHEABLE (request policy)', $this->drupalGetHeader('X-Drupal-Cache'));
+  }
+
   /**
    * Tests that HEAD requests are treated the same as GET requests.
    */
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index 1bc41be84e..2753d21660 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -504,7 +504,7 @@ public function testGet() {
     else {
       $this->assertResourceErrorResponse(403, FALSE, $response, $expected_403_cacheability->getCacheTags(), $expected_403_cacheability->getCacheContexts(), static::$auth ? FALSE : 'MISS', FALSE);
     }
-    $this->assertSame(static::$auth ? [] : ['MISS'], $response->getHeader('X-Drupal-Cache'));
+    $this->assertSame(static::$auth ? ['UNCACHEABLE (request policy)'] : ['MISS'], $response->getHeader('X-Drupal-Cache'));
     // DX: 403 because unauthorized.
     $url->setOption('query', ['_format' => static::$format]);
     $response = $this->request('GET', $url, $request_options);
@@ -521,10 +521,10 @@ public function testGet() {
     }
     else {
       $this->assertSame(406, $response->getStatusCode());
-      $this->assertSame(['UNCACHEABLE'], $response->getHeader('X-Drupal-Dynamic-Cache'));
+      $this->assertSame(['UNCACHEABLE (poor cacheability)'], $response->getHeader('X-Drupal-Dynamic-Cache'));
     }
     $this->assertSame(['text/html; charset=UTF-8'], $response->getHeader('Content-Type'));
-    $this->assertSame(static::$auth ? [] : ['MISS'], $response->getHeader('X-Drupal-Cache'));
+    $this->assertSame(static::$auth ? ['UNCACHEABLE (request policy)'] : ['MISS'], $response->getHeader('X-Drupal-Cache'));
     // DX: 403 because unauthorized.
     $url->setOption('query', ['_format' => static::$format]);
     $response = $this->request('GET', $url, $request_options);
@@ -945,7 +945,7 @@ public function testPost() {
     else {
       $this->assertSame([], $response->getHeader('Location'));
     }
-    $this->assertFalse($response->hasHeader('X-Drupal-Cache'));
+    $this->assertSame(['UNCACHEABLE (request policy)'], $response->getHeader('X-Drupal-Cache'));
     // If the entity is stored, perform extra checks.
     if (get_class($this->entityStorage) !== ContentEntityNullStorage::class) {
       // Assert that the entity was indeed created, and that the response body
@@ -982,7 +982,7 @@ public function testPost() {
     else {
       $this->assertSame([], $response->getHeader('Location'));
     }
-    $this->assertFalse($response->hasHeader('X-Drupal-Cache'));
+    $this->assertSame(['UNCACHEABLE (request policy)'], $response->getHeader('X-Drupal-Cache'));
 
     if ($this->entity->getEntityType()->getStorageClass() !== ContentEntityNullStorage::class && $this->entity->getEntityType()->hasKey('uuid')) {
       // 500 when creating an entity with a duplicate UUID.
@@ -1231,7 +1231,7 @@ public function testPatch() {
     // 200 for well-formed request.
     $response = $this->request('PATCH', $url, $request_options);
     $this->assertResourceResponse(200, FALSE, $response);
-    $this->assertFalse($response->hasHeader('X-Drupal-Cache'));
+    $this->assertSame(['UNCACHEABLE (request policy)'], $response->getHeader('X-Drupal-Cache'));
     // Assert that the entity was indeed updated, and that the response body
     // contains the serialized updated entity.
     $updated_entity = $this->entityStorage->loadUnchanged($this->entity->id());
@@ -1275,7 +1275,7 @@ public function testPatch() {
     // 200 for well-formed request.
     $response = $this->request('PATCH', $url, $request_options);
     $this->assertResourceResponse(200, FALSE, $response);
-    $this->assertFalse($response->hasHeader('X-Drupal-Cache'));
+    $this->assertSame(['UNCACHEABLE (request policy)'], $response->getHeader('X-Drupal-Cache'));
   }
 
   /**
diff --git a/core/modules/rest/tests/src/Functional/ResourceTestBase.php b/core/modules/rest/tests/src/Functional/ResourceTestBase.php
index d7ca1e34f3..0d994d0cb7 100644
--- a/core/modules/rest/tests/src/Functional/ResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/ResourceTestBase.php
@@ -412,7 +412,8 @@ protected function assertResourceResponse($expected_status_code, $expected_body,
       $this->assertSame($expected_page_cache_header_value, $response->getHeader('X-Drupal-Cache')[0]);
     }
     else {
-      $this->assertFalse($response->hasHeader('X-Drupal-Cache'));
+      $this->assertTrue($response->hasHeader('X-Drupal-Cache'));
+      $this->stringStartsWith('UNCACHEABLE (', $response->getHeader('X-Drupal-Cache')[0]);
     }
 
     // Expected Dynamic Page Cache header value: X-Drupal-Dynamic-Cache header.
@@ -421,7 +422,16 @@ protected function assertResourceResponse($expected_status_code, $expected_body,
       $this->assertSame($expected_dynamic_page_cache_header_value, $response->getHeader('X-Drupal-Dynamic-Cache')[0]);
     }
     else {
-      $this->assertFalse($response->hasHeader('X-Drupal-Dynamic-Cache'));
+      if ($expected_status_code === 403) {
+        if ($response->hasHeader('X-Drupal-Dynamic-Cache')) {
+          $this->assertTrue($response->hasHeader('X-Drupal-Dynamic-Cache'));
+          $this->stringStartsWith('UNCACHEABLE (', $response->getHeader('X-Drupal-Dynamic-Cache')[0]);
+        }
+      }
+      else {
+        $this->assertTrue($response->hasHeader('X-Drupal-Dynamic-Cache'));
+        $this->stringStartsWith('UNCACHEABLE (', $response->getHeader('X-Drupal-Dynamic-Cache')[0]);
+      }
     }
   }
 
diff --git a/core/modules/system/tests/src/Functional/Session/SessionTest.php b/core/modules/system/tests/src/Functional/Session/SessionTest.php
index 0cee31f68c..e6844b6996 100644
--- a/core/modules/system/tests/src/Functional/Session/SessionTest.php
+++ b/core/modules/system/tests/src/Functional/Session/SessionTest.php
@@ -192,7 +192,7 @@ public function testEmptyAnonymousSession() {
     $this->drupalGet('');
     $this->assertSessionCookie(FALSE);
     $this->assertSessionEmpty(FALSE);
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'), 'Caching was bypassed.');
+    $this->assertIdentical('UNCACHEABLE (request policy)', $this->drupalGetHeader('X-Drupal-Cache'), 'Caching was bypassed.');
     $this->assertText(t('This is a dummy message.'), 'Message was displayed.');
     $this->assertTrue(preg_match('/SESS\w+=deleted/', $this->drupalGetHeader('Set-Cookie')), 'Session cookie was deleted.');
 
diff --git a/core/modules/system/tests/src/Functional/System/ErrorHandlerTest.php b/core/modules/system/tests/src/Functional/System/ErrorHandlerTest.php
index 3ab502e864..7da191ac9b 100644
--- a/core/modules/system/tests/src/Functional/System/ErrorHandlerTest.php
+++ b/core/modules/system/tests/src/Functional/System/ErrorHandlerTest.php
@@ -136,7 +136,7 @@ public function testExceptionHandler() {
       ->save();
 
     $this->drupalGet('error-test/trigger-exception');
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'));
+    $this->assertIdentical('UNCACHEABLE (no cacheability)', $this->drupalGetHeader('X-Drupal-Cache'));
     $this->assertIdentical(strpos($this->drupalGetHeader('Cache-Control'), 'public'), FALSE, 'Received expected HTTP status line.');
     $this->assertSession()->statusCodeEquals(500);
     $this->assertNoErrorMessage($error_exception);
diff --git a/core/modules/user/tests/src/Functional/UserPasswordResetTest.php b/core/modules/user/tests/src/Functional/UserPasswordResetTest.php
index 2f73e55506..717f8f0574 100644
--- a/core/modules/user/tests/src/Functional/UserPasswordResetTest.php
+++ b/core/modules/user/tests/src/Functional/UserPasswordResetTest.php
@@ -93,11 +93,11 @@ public function testUserPasswordReset() {
     // Ensure that the current url does not contain the hash and timestamp.
     $this->assertUrl(Url::fromRoute('user.reset.form', ['uid' => $this->account->id()]));
 
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'));
+    $this->assertIdentical('UNCACHEABLE (request policy)', $this->drupalGetHeader('X-Drupal-Cache'));
 
     // Ensure the password reset URL is not cached.
     $this->drupalGet($resetURL);
-    $this->assertFalse($this->drupalGetHeader('X-Drupal-Cache'));
+    $this->assertIdentical('UNCACHEABLE (request policy)', $this->drupalGetHeader('X-Drupal-Cache'));
 
     // Check the one-time login page.
     $this->assertText($this->account->getAccountName(), 'One-time login page contains the correct username.');
diff --git a/core/profiles/standard/tests/src/Functional/StandardTest.php b/core/profiles/standard/tests/src/Functional/StandardTest.php
index 79584b4420..79c5d1a857 100644
--- a/core/profiles/standard/tests/src/Functional/StandardTest.php
+++ b/core/profiles/standard/tests/src/Functional/StandardTest.php
@@ -183,7 +183,7 @@ public function testStandard() {
     $this->drupalLogin($this->adminUser);
     $url = Url::fromRoute('contact.site_page');
     $this->drupalGet($url);
-    $this->assertEqual('UNCACHEABLE', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Site-wide contact page cannot be cached by Dynamic Page Cache.');
+    $this->assertEqual('UNCACHEABLE (poor cacheability)', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER), 'Site-wide contact page cannot be cached by Dynamic Page Cache.');
 
     $url = Url::fromRoute('<front>');
     $this->drupalGet($url);