diff --git a/CHANGELOG.md b/CHANGELOG.md
index f7e4137..b114a52 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- Dev: added a test for `ItkDevOpenIdConnectBundle::getContainerExtension()`
+  asserting the custom extension is created and memoized (same instance on
+  repeated calls), prompted by mutation testing findings. No effect on the
+  published package.
 - Dev: strengthened DependencyInjection tests based on mutation testing
   findings — the extension's container wiring (cache pool reference,
   provider options mapping, CLI login route arguments) is now asserted
diff --git a/tests/ItkDevOpenIdConnectBundleTest.php b/tests/ItkDevOpenIdConnectBundleTest.php
index 6948230..1593cd3 100644
--- a/tests/ItkDevOpenIdConnectBundleTest.php
+++ b/tests/ItkDevOpenIdConnectBundleTest.php
@@ -4,6 +4,8 @@
 
 use ItkDev\OpenIdConnectBundle\Command\UserLoginCommand;
 use ItkDev\OpenIdConnectBundle\Controller\LoginController;
+use ItkDev\OpenIdConnectBundle\DependencyInjection\ItkDevOpenIdConnectExtension;
+use ItkDev\OpenIdConnectBundle\ItkDevOpenIdConnectBundle;
 use ItkDev\OpenIdConnectBundle\Security\CliLoginTokenAuthenticator;
 use ItkDev\OpenIdConnectBundle\Security\OpenIdConfigurationProviderManager;
 use ItkDev\OpenIdConnectBundle\Security\OpenIdLoginAuthenticator;
@@ -57,4 +59,18 @@ public function testServiceWiring(): void
         $authenticator = $container->get(CliLoginTokenAuthenticator::class);
         $this->assertInstanceOf(CliLoginTokenAuthenticator::class, $authenticator);
     }
+
+    /**
+     * Test that the custom container extension is created and memoized.
+     */
+    public function testGetContainerExtension(): void
+    {
+        $bundle = new ItkDevOpenIdConnectBundle();
+
+        $extension = $bundle->getContainerExtension();
+        $this->assertInstanceOf(ItkDevOpenIdConnectExtension::class, $extension);
+
+        // Repeated calls must return the same instance, not recreate it.
+        $this->assertSame($extension, $bundle->getContainerExtension());
+    }
 }