diff --git a/docs/ZapHandler.md b/docs/ZapHandler.md
new file mode 100644
index 0000000..b6c6e9a
--- /dev/null
+++ b/docs/ZapHandler.md
@@ -0,0 +1,16 @@
+# ZAPHandler Interface
+
+| File | Change |
+|------|--------|
+| **`lib/ZAPHandler.interface.php`** | **Created** — defines `ZAPHandler` interface with `public function getContent()` |
+| **`lib/ZAPHome.class.php`** | Added `implements ZAPHandler` |
+| **`lib/ZAPDisplay.class.php`** | Added `implements ZAPHandler` |
+| **`lib/ZAPZap.class.php`** | Added `implements ZAPHandler` |
+| **`lib/ZAPController.class.php`** | Added `instanceof ZAPHandler` guard inside `fetch()` before calling `getContent()` |
+
+### How it works
+
+- Any class named `ZAP{Mode}` in `lib/` is autoloaded and instantiated by `ZAPController`
+- The controller now checks `$this->action instanceof ZAPHandler` **before** calling `getContent()`
+- If a handler forgets to implement the interface, you get a clear `die()` message instead of a cryptic runtime error
+- The interface file uses the `.interface.php` naming convention, which the existing autoloader already handles (it checks `.interface.php` first, then `.class.php`)
\ No newline at end of file
diff --git a/lib/ZAPController.class.php b/lib/ZAPController.class.php
index 27214f1..115b416 100755
--- a/lib/ZAPController.class.php
+++ b/lib/ZAPController.class.php
@@ -35,6 +35,9 @@ class ZAPController {
 	 * @return string content
 	 */
 	public function fetch() {
+		if (!($this->action instanceof ZAPHandler)) {
+			die('Handler does not implement ZAPHandler interface');
+		}
 		return $this->action->getContent();
 	}
 }
diff --git a/lib/ZAPDisplay.class.php b/lib/ZAPDisplay.class.php
index a1398ab..2d1ef71 100755
--- a/lib/ZAPDisplay.class.php
+++ b/lib/ZAPDisplay.class.php
@@ -2,7 +2,7 @@
 /**
  * Class is responsible for starting up ZAP machine
  */
-class ZAPDisplay {
+class ZAPDisplay implements ZAPHandler {
 	/**
 	 * Template file
 	 */
diff --git a/lib/ZAPHandler.interface.php b/lib/ZAPHandler.interface.php
new file mode 100644
index 0000000..4f38e1a
--- /dev/null
+++ b/lib/ZAPHandler.interface.php
@@ -0,0 +1,12 @@
+<?php
+/**
+ * Interface for ZAP mode handlers.
+ * All ZAP handler classes must implement this interface.
+ */
+interface ZAPHandler {
+	/**
+	 * Renders and returns the content for this mode.
+	 * @return string
+	 */
+	public function getContent();
+}
diff --git a/lib/ZAPHome.class.php b/lib/ZAPHome.class.php
index 38764b5..21de450 100755
--- a/lib/ZAPHome.class.php
+++ b/lib/ZAPHome.class.php
@@ -2,7 +2,7 @@
 /**
  * Index class
  */
-class ZAPHome {
+class ZAPHome implements ZAPHandler {
 	/**
 	 * Template file prefix
 	 */
diff --git a/lib/ZAPZap.class.php b/lib/ZAPZap.class.php
index 175f10a..143fef6 100755
--- a/lib/ZAPZap.class.php
+++ b/lib/ZAPZap.class.php
@@ -2,7 +2,7 @@
 /**
  * ZAP factory
  */
-class ZAPZap {
+class ZAPZap implements ZAPHandler {
 	private $tpl = '';
 	private $folder;
 	private $height;