diff --git a/tests/php-unit-tests/unitary-tests/application/applicationextension/ApplicationExtensionTest.php b/tests/php-unit-tests/unitary-tests/application/applicationextension/ApplicationExtensionTest.php
new file mode 100644
index 0000000000..997a52258c
--- /dev/null
+++ b/tests/php-unit-tests/unitary-tests/application/applicationextension/ApplicationExtensionTest.php
@@ -0,0 +1,124 @@
+<?php
+/*
+ * @copyright   Copyright (C) 2010-2023 Combodo SARL
+ * @license     http://opensource.org/licenses/AGPL-3.0
+ */
+
+namespace Combodo\iTop\Test\UnitTest\Application;
+
+use Combodo\iTop\Test\UnitTest\ItopCustomDatamodelTestCase;
+use MetaModel;
+use utils;
+
+/**
+ * @covers \iBackofficeLinkedScriptsExtension
+ */
+class ApplicationExtensionTest extends ItopCustomDatamodelTestCase
+{
+	protected const ENUM_API_CALL_METHOD_ENUMPLUGINS = 'MetaModel::EnumPlugins';
+	protected const ENUM_API_CALL_METHOD_GETCLASSESFORINTERFACE = 'utils::GetClassesForInterface';
+
+	/**
+	 * @inheritDoc
+	 */
+	public function GetDatamodelDeltaAbsPath(): string
+	{
+		return __DIR__ . '/Delta/application-extension-usages-in-snippets.xml';
+	}
+
+	/**
+	 * This test ensures that APIs are discovered / registered / called.
+	 *
+	 * It was introduced after {@since N°6436} when some APIs registration in \MetaModel::InitClasses() were lost during a merge {@link https://github.com/Combodo/iTop/commit/6432678de9f635990e22a6512e5b30713c22204a#diff-c94890a26989b5a5ce638f82e8cc7d4c7aa24e6fbb9c2ca89850e8fa4e0e9adaL3004} preventing them from being called when requested. This was hard to detect as it needed an extension to use the lost API to witness that it was no longer called.
+	 *
+	 * For each new API, a new test case should be added here to ensure that we don't lose it later.
+	 * To do so:
+	 * - Add the API to the provider
+	 * - Add a class extending / implementing the API in ./Delta/application-extension-usages-in-snippets.xml
+	 *
+	 * @param string $sAPIFQCN
+	 * @param string $sCallMethod
+	 *
+	 * @return void
+	 * @dataProvider ExtensionAPIRegisteredAndCalledProvider
+	 */
+	public function testExtensionAPIRegisteredAndCalled(string $sAPIFQCN, string $sCallMethod)
+	{
+		if ($sCallMethod === static::ENUM_API_CALL_METHOD_ENUMPLUGINS) {
+			$iExtendingClassesCount = count(MetaModel::EnumPlugins($sAPIFQCN));
+		} else {
+			$iExtendingClassesCount = count(utils::GetClassesForInterface($sAPIFQCN, '', ['[\\\\/]lib[\\\\/]', '[\\\\/]node_modules[\\\\/]', '[\\\\/]test[\\\\/]', '[\\\\/]tests[\\\\/]']));
+		}
+		$this->assertGreaterThan(0, $iExtendingClassesCount, "Found no class extending the $sAPIFQCN API");
+	}
+
+	public function ExtensionAPIRegisteredAndCalledProvider(): array
+	{
+		// APIs not concerned by this test:
+		// * \iRestServiceProvider as it is discovered by iterating over declared classes directly
+		// * \iLoginUIExtension as it is not iterated directly, only its derived interfaces
+
+		return [
+			\iLoginFSMExtension::class => [
+				\iLoginFSMExtension::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iLogoutExtension::class => [
+				\iLogoutExtension::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iLoginUIExtension::class => [
+				\iLoginUIExtension::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iPreferencesExtension::class => [
+				\iPreferencesExtension::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iApplicationUIExtension::class => [
+				\iApplicationUIExtension::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iApplicationObjectExtension::class => [
+				\iApplicationObjectExtension::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iPopupMenuExtension::class => [
+				\iPopupMenuExtension::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iPageUIExtension::class => [
+				\iPageUIExtension::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iPortalUIExtension::class => [
+				\iPortalUIExtension::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iQueryModifier::class => [
+				\iQueryModifier::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iOnClassInitialization::class => [
+				\iOnClassInitialization::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iModuleExtension::class => [
+				\iModuleExtension::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iKPILoggerExtension::class => [
+				\iKPILoggerExtension::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\ModuleHandlerApiInterface::class => [
+				\ModuleHandlerApiInterface::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+			\iNewsroomProvider::class => [
+				\iNewsroomProvider::class,
+				static::ENUM_API_CALL_METHOD_ENUMPLUGINS,
+			],
+		];
+	}
+}
diff --git a/tests/php-unit-tests/unitary-tests/application/applicationextension/Delta/application-extension-usages-in-snippets.xml b/tests/php-unit-tests/unitary-tests/application/applicationextension/Delta/application-extension-usages-in-snippets.xml
new file mode 100644
index 0000000000..e404ac49d2
--- /dev/null
+++ b/tests/php-unit-tests/unitary-tests/application/applicationextension/Delta/application-extension-usages-in-snippets.xml
@@ -0,0 +1,233 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<itop_design xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="1.7">
+  <snippets>
+    <!-- These snippets just implements application/applicationextension.inc.php APIs for the ApplicationExtensionTest unit test -->
+    <snippet id="ExampleFor_iLoginFSMExtension" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iLoginFSMExtension extends \AbstractLoginFSMExtension
+{
+	public function ListSupportedLoginModes()
+	{
+		// Do nothing, we just need the class to exists for the unit test
+	}
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iLogoutExtension" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iLogoutExtension implements \iLogoutExtension
+{
+	public function LogoutAction()
+	{
+		// Do nothing, we just need the class to exists for the unit test
+	}
+
+	public function ListSupportedLoginModes()
+	{
+		// Do nothing, we just need the class to exists for the unit test
+	}
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iLoginUIExtension" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iLoginUIExtension implements \iLoginUIExtension
+{
+	public function GetTwigContext()
+	{
+		// Do nothing, we just need the class to exists for the unit test
+	}
+
+	public function ListSupportedLoginModes()
+	{
+		// Do nothing, we just need the class to exists for the unit test
+	}
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iPreferencesExtension" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iPreferencesExtension extends \AbstractPreferencesExtension
+{
+	// Do nothing, we just need the class to exists for the unit test
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iApplicationUIExtension" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iApplicationUIExtension extends \AbstractApplicationUIExtension
+{
+	// Do nothing, we just need the class to exists for the unit test
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iApplicationObjectExtension" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iApplicationObjectExtension extends \AbstractApplicationObjectExtension
+{
+	// Do nothing, we just need the class to exists for the unit test
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iPopupMenuExtension" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iPopupMenuExtension implements \iPopupMenuExtension
+{
+  public static function EnumItems($iMenuId, $param)
+  {
+	  // Do nothing, we just need the class to exists for the unit test
+  }
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_ApplicationPopupMenuItem" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_ApplicationPopupMenuItem extends \ApplicationPopupMenuItem
+{
+  public function GetMenuItem()
+  {
+	  // Do nothing, we just need the class to exists for the unit test
+  }
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iPageUIExtension" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iPageUIExtension extends \AbstractPageUIExtension
+{
+	// Do nothing, we just need the class to exists for the unit test
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iPortalUIExtension" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iPortalUIExtension extends \AbstractPortalUIExtension
+{
+	// Do nothing, we just need the class to exists for the unit test
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iQueryModifier" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iQueryModifier implements \iQueryModifier
+{
+	public function __construct()
+	{
+	  // Do nothing, we just need the class to exists for the unit test
+	}
+
+	public function GetFieldExpression(QueryBuilderContext &$oBuild, $sClass, $sAttCode, $sColId, Expression $oFieldSQLExp, SQLQuery &$oSelect)
+	{
+	  // Do nothing, we just need the class to exists for the unit test
+	}
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iOnClassInitialization" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iOnClassInitialization implements \iOnClassInitialization
+{
+	public function OnAfterClassInitialization($sClass)
+	{
+	  // Do nothing, we just need the class to exists for the unit test
+	}
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iModuleExtension" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iModuleExtension implements \iModuleExtension
+{
+	public function __construct()
+	{
+	  // Do nothing, we just need the class to exists for the unit test
+	}
+}
+      ]]></content>
+    </snippet>
+    <snippet id="ExampleFor_iKPILoggerExtension" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iKPILoggerExtension implements \iKPILoggerExtension
+{
+	public function InitStats()
+	{
+		// Do nothing, we just need the class to exists for the unit test
+	}
+
+	public function LogOperation($oKpiLogData)
+	{
+	  // Do nothing, we just need the class to exists for the unit test
+	}
+}
+      ]]></content>
+    </snippet>
+    <!-- These snippets just implements core/modulehandler.class.inc.php APIs for the ApplicationExtensionTest unit test -->
+    <snippet id="ExampleFor_ModuleHandlerApiInterface" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_ModuleHandlerApiInterface extends \ModuleHandlerAPI
+{
+	// Do nothing, we just need the class to exists for the unit test
+}
+      ]]></content>
+    </snippet>
+    <!-- These snippets just implements application/newsroomprovider.class.inc.php APIs for the ApplicationExtensionTest unit test -->
+    <snippet id="ExampleFor_iNewsroomProvider" _delta="define">
+      <placement>core</placement>
+      <rank>0</rank>
+      <content><![CDATA[
+class ExampleFor_iNewsroomProvider extends \NewsroomProviderBase
+{
+	public function GetLabel()
+	{
+	  // Do nothing, we just need the class to exists for the unit test
+  }
+
+	public function GetFetchURL()
+	{
+	  // Do nothing, we just need the class to exists for the unit test
+  }
+
+	public function GetMarkAllAsReadURL()
+	{
+	  // Do nothing, we just need the class to exists for the unit test
+  }
+
+	public function GetViewAllURL()
+	{
+	  // Do nothing, we just need the class to exists for the unit test
+  }
+}
+      ]]></content>
+    </snippet>
+  </snippets>
+</itop_design>
\ No newline at end of file