tests/src/Functional/FileUploadTest.php   | 2 ++
 tests/src/Functional/ResourceTestBase.php | 3 +++
 tests/src/Functional/UserTest.php         | 2 ++
 3 files changed, 7 insertions(+)

diff --git a/tests/src/Functional/FileUploadTest.php b/tests/src/Functional/FileUploadTest.php
index 0c2a336..afbe9e2 100644
--- a/tests/src/Functional/FileUploadTest.php
+++ b/tests/src/Functional/FileUploadTest.php
@@ -207,6 +207,7 @@ class FileUploadTest extends ResourceTestBase {
     // DX: 405 when read-only mode is enabled.
     $response = $this->fileRequest($uri, $this->testFileData);
     $this->assertResourceErrorResponse(405, sprintf("JSON:API's read-only mode is enabled. Site administrators can enable writes at %s.", Url::fromUri('base:/admin/config/services/jsonapi')->setAbsolute()->toString(TRUE)->getGeneratedUrl()), $uri, $response);
+    $this->assertSame(['GET'], $response->getHeader('Allow'));
 
     $this->config('jsonapi.settings')->set('read_only', FALSE)->save(TRUE);
 
@@ -284,6 +285,7 @@ class FileUploadTest extends ResourceTestBase {
     // DX: 405 when read-only mode is enabled.
     $response = $this->fileRequest($uri, $this->testFileData);
     $this->assertResourceErrorResponse(405, sprintf("JSON:API's read-only mode is enabled. Site administrators can enable writes at %s.", Url::fromUri('base:/admin/config/services/jsonapi')->setAbsolute()->toString(TRUE)->getGeneratedUrl()), $uri, $response);
+    $this->assertSame(['GET'], $response->getHeader('Allow'));
 
     $this->config('jsonapi.settings')->set('read_only', FALSE)->save(TRUE);
 
diff --git a/tests/src/Functional/ResourceTestBase.php b/tests/src/Functional/ResourceTestBase.php
index c06b67f..3670bef 100644
--- a/tests/src/Functional/ResourceTestBase.php
+++ b/tests/src/Functional/ResourceTestBase.php
@@ -1902,6 +1902,7 @@ abstract class ResourceTestBase extends BrowserTestBase {
     // DX: 405 when read-only mode is enabled.
     $response = $this->request('POST', $url, $request_options);
     $this->assertResourceErrorResponse(405, sprintf("JSON:API's read-only mode is enabled. Site administrators can enable writes at %s.", Url::fromUri('base:/admin/config/services/jsonapi')->setAbsolute()->toString(TRUE)->getGeneratedUrl()), $url, $response);
+    $this->assertSame(['GET'], $response->getHeader('Allow'));
 
     $this->config('jsonapi.settings')->set('read_only', FALSE)->save(TRUE);
 
@@ -2119,6 +2120,7 @@ abstract class ResourceTestBase extends BrowserTestBase {
     // DX: 405 when read-only mode is enabled.
     $response = $this->request('PATCH', $url, $request_options);
     $this->assertResourceErrorResponse(405, sprintf("JSON:API's read-only mode is enabled. Site administrators can enable writes at %s.", Url::fromUri('base:/admin/config/services/jsonapi')->setAbsolute()->toString(TRUE)->getGeneratedUrl()), $url, $response);
+    $this->assertSame(['GET'], $response->getHeader('Allow'));
 
     $this->config('jsonapi.settings')->set('read_only', FALSE)->save(TRUE);
 
@@ -2412,6 +2414,7 @@ abstract class ResourceTestBase extends BrowserTestBase {
     // DX: 405 when read-only mode is enabled.
     $response = $this->request('DELETE', $url, $request_options);
     $this->assertResourceErrorResponse(405, sprintf("JSON:API's read-only mode is enabled. Site administrators can enable writes at %s.", Url::fromUri('base:/admin/config/services/jsonapi')->setAbsolute()->toString(TRUE)->getGeneratedUrl()), $url, $response);
+    $this->assertSame(['GET'], $response->getHeader('Allow'));
 
     $this->config('jsonapi.settings')->set('read_only', FALSE)->save(TRUE);
 
diff --git a/tests/src/Functional/UserTest.php b/tests/src/Functional/UserTest.php
index 7b96783..94d3d5a 100644
--- a/tests/src/Functional/UserTest.php
+++ b/tests/src/Functional/UserTest.php
@@ -225,6 +225,7 @@ class UserTest extends ResourceTestBase {
     // DX: 405 when read-only mode is enabled.
     $response = $this->request('PATCH', $url, $request_options);
     $this->assertResourceErrorResponse(405, sprintf("JSON:API's read-only mode is enabled. Site administrators can enable writes at %s.", Url::fromUri('base:/admin/config/services/jsonapi')->setAbsolute()->toString(TRUE)->getGeneratedUrl()), $url, $response);
+    $this->assertSame(['GET'], $response->getHeader('Allow'));
 
     $this->config('jsonapi.settings')->set('read_only', FALSE)->save(TRUE);
 
@@ -340,6 +341,7 @@ class UserTest extends ResourceTestBase {
     // DX: 405 when read-only mode is enabled.
     $response = $this->request('PATCH', $url, $request_options);
     $this->assertResourceErrorResponse(405, sprintf("JSON:API's read-only mode is enabled. Site administrators can enable writes at %s.", Url::fromUri('base:/admin/config/services/jsonapi')->setAbsolute()->toString(TRUE)->getGeneratedUrl()), $url, $response);
+    $this->assertSame(['GET'], $response->getHeader('Allow'));
 
     $this->config('jsonapi.settings')->set('read_only', FALSE)->save(TRUE);