diff -u b/core/modules/page_cache/src/StackMiddleware/PageCache.php b/core/modules/page_cache/src/StackMiddleware/PageCache.php
--- b/core/modules/page_cache/src/StackMiddleware/PageCache.php
+++ b/core/modules/page_cache/src/StackMiddleware/PageCache.php
@@ -81,7 +81,6 @@
     }
     else {
       $response = $this->pass($request, $type, $catch);
-      $response->headers->set(self::HEADER, 'UNCACHEABLE (request policy)');
     }
 
     return $response;
@@ -89,6 +88,7 @@
     }
     else {
       $response = $this->pass($request, $type, $catch);
+      $response->headers->set(self::HEADER, 'UNCACHEABLE (request policy)');
     }
 
     return $response;
@@ -121,14 +121,14 @@
    */
   protected function lookup(Request $request, $type = self::MASTER_REQUEST, $catch = TRUE) {
     if ($response = $this->get($request)) {
-      $response->headers->set(static::HEADER, 'HIT');
+      $response->headers->set('X-Drupal-Cache', 'HIT');
     }
     else {
       $response = $this->fetch($request, $type, $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, $catch);
@@ -192,14 +192,14 @@
     // Only set the 'X-Drupal-Cache' header if caching is allowed for this
     // response.
     if ($this->storeResponse($request, $response)) {
-      $response->headers->set(static::HEADER, 'MISS');
+      $response->headers->set('X-Drupal-Cache', 'MISS');
     }
 
     return $response;
     // Only set the 'X-Drupal-Cache' header if caching is allowed for this
     // response.
     if ($this->storeResponse($request, $response)) {
-      $response->headers->set('X-Drupal-Cache', 'MISS');
+      $response->headers->set(static::HEADER, 'MISS');
     }
 
     return $response;
@@ -231,7 +231,6 @@
     //   so by replacing/extending this middleware service or adding another
     //   one.
     if (!$response instanceof CacheableResponseInterface) {
-      $response->headers->set(self::HEADER, 'UNCACHEABLE (no cacheability)');
       return FALSE;
     }
 
@@ -239,13 +238,13 @@
     //   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;
     }
 
 
     // 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;
     }
 
@@ -253,6 +252,7 @@
 
     // 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 -u b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
--- b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -485,7 +485,7 @@
     else {
       $this->assertResourceErrorResponse(403, FALSE, $response, $expected_403_cacheability->getCacheTags(), $expected_403_cacheability->getCacheContexts(), static::$auth ? FALSE : 'MISS', 'MISS');
     }
-    $this->assertSame(static::$auth ? ['UNCACHEABLE (request policy)'] : ['MISS'], $response->getHeader('X-Drupal-Cache'));
+    $this->assertSame(static::$auth ? [] : ['MISS'], $response->getHeader('X-Drupal-Cache'));
     // DX: 403 because unauthorized.
     $url->setOption('query', ['_format' => static::$format]);
     $response = $this->request('GET', $url, $request_options);
@@ -504,7 +504,7 @@
     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);
reverted:
--- b/core/modules/system/src/Tests/System/ErrorHandlerTest.php
+++ a/core/modules/system/src/Tests/System/ErrorHandlerTest.php
@@ -170,7 +170,7 @@
       ->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.');
     $this->assertTrue(strpos($this->drupalGetHeader(':status'), '500 Service unavailable (with message)'), 'Received expected HTTP status line.');
     $this->assertNoErrorMessage($error_exception);
only in patch2:
unchanged:
--- 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);