diff --git a/src/InvalidDataThrowable.php b/src/InvalidDataThrowable.php
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--- /dev/null
+++ b/src/InvalidDataThrowable.php
@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+/**
+ * Copyright 2026 The Horde Project (http://www.horde.org/)
+ *
+ * See the enclosed file LICENSE for license information (LGPL). If you
+ * did not receive this file, see http://www.horde.org/licenses/lgpl21.
+ *
+ * @author   Torben Dannhauer <torben@dannhauer.de>
+ * @category Horde
+ * @license  http://www.horde.org/licenses/lgpl21 LGPL-2.1
+ * @package  Exception
+ */
+
+namespace Horde\Exception;
+
+/**
+ * Marker interface for exceptions caused by invalid caller-supplied data.
+ *
+ * Thrown by an application when a value it was asked to store or process
+ * violates that application's own constraints (field length, format,
+ * required field) - as opposed to an infrastructure failure. Consumers
+ * such as sync protocol handlers use it to tell "the item is unacceptable,
+ * do not retry it" from "the server failed, try again later" without
+ * depending on the throwing application's exception classes.
+ *
+ * Intentionally a noop / marker interface, like PermissionDeniedThrowable.
+ *
+ * @author    Torben Dannhauer <torben@dannhauer.de>
+ * @category  Horde
+ * @copyright 2026 The Horde Project
+ * @license   http://www.horde.org/licenses/lgpl21 LGPL-2.1
+ * @package   Exception
+ */
+interface InvalidDataThrowable extends HordeThrowable
+{
+}