diff --git a/api/client-server/sync.yaml b/api/client-server/sync.yaml
index 4b399d07..7ce04c3f 100644
--- a/api/client-server/sync.yaml
+++ b/api/client-server/sync.yaml
@@ -32,7 +32,9 @@ paths:
           name: filter
           type: string
           description: |-
-            The ID of a filter created using the filter API.
+            The ID of a filter created using the filter API or a filter JSON
+            object. The server will detect whether it is an ID or a JSON object
+            by whether the first character is a ``"{"`` open brace.
           x-example: "66696p746572"
         - in: query
           name: since