diff --git a/includes/class-activitypub.php b/includes/class-activitypub.php
index 4730515..16a1f22 100644
--- a/includes/class-activitypub.php
+++ b/includes/class-activitypub.php
@@ -39,6 +39,9 @@ class Activitypub {
 			return $template;
 		}
 
+		// Ensure that edge caches know that this page can deliver both HTML and JSON.
+		header( 'Vary: Accept' );
+
 		// check if user can publish posts
 		if ( \is_author() && ! user_can( \get_the_author_meta( 'ID' ), 'publish_posts' ) ) {
 			return $template;