diff --git a/control/Director.php b/control/Director.php
index d6ddc97ca..3f62ecc39 100644
--- a/control/Director.php
+++ b/control/Director.php
@@ -918,6 +918,9 @@ class Director implements TemplateGlobalProvider {
 	 * by checking for a custom header set by jQuery or
 	 * wether a manually set request-parameter 'ajax' is present.
 	 *
+	 * Note that if you plan to use this to alter your HTTP response on a cached page,
+	 * you should add X-Requested-With to the Vary header.
+	 *
 	 * @return boolean
 	 */
 	public static function is_ajax() {
diff --git a/docs/en/02_Developer_Guides/08_Performance/02_HTTP_Cache_Headers.md b/docs/en/02_Developer_Guides/08_Performance/02_HTTP_Cache_Headers.md
index c47c1fcd6..e4df33def 100644
--- a/docs/en/02_Developer_Guides/08_Performance/02_HTTP_Cache_Headers.md
+++ b/docs/en/02_Developer_Guides/08_Performance/02_HTTP_Cache_Headers.md
@@ -209,7 +209,7 @@ when calculating a cache key, usually in addition to the full URL path.
 By default, SilverStripe will output a `Vary` header with the following content: 
 
 ```
-Vary: X-Requested-With, X-Forwarded-Protocol
+Vary: X-Forwarded-Protocol
 ```
 
 To change the value of the `Vary` header, you can change this value by specifying the header in configuration.
@@ -218,3 +218,6 @@ To change the value of the `Vary` header, you can change this value by specifyin
 HTTP:
   vary: ""
 ```
+
+Note that if you use `Director::is_ajax()` on cached pages then you should add `X-Requested-With` to the vary
+header.