diff --git a/api/client-server/to_device.yaml b/api/client-server/to_device.yaml
index 739d9a4f..a4d2f6dd 100644
--- a/api/client-server/to_device.yaml
+++ b/api/client-server/to_device.yaml
@@ -79,6 +79,7 @@ paths:
                       }
                     }
                   }
+            required: ["messages"]
       responses:
         200:
           description:
diff --git a/changelogs/client_server/newsfragments/2928.clarification b/changelogs/client_server/newsfragments/2928.clarification
new file mode 100644
index 00000000..60653eae
--- /dev/null
+++ b/changelogs/client_server/newsfragments/2928.clarification
@@ -0,0 +1 @@
+Mark `messages` as a required JSON body field in `PUT /_matrix/client/r0/sendToDevice/{eventType}/{txnId}` calls.