diff --git a/object/service.proto b/object/service.proto
index 4e51f6d..c7a292b 100644
--- a/object/service.proto
+++ b/object/service.proto
@@ -527,6 +527,8 @@ message SearchRequest {
     //   object_id of parent
     // * $Object:split.splitID \
     //   16 byte UUIDv4 used to identify the split object hierarchy parts
+    // * $Object:split.first \
+    //   object_id of the first part in split chain; non-acceptable for deprecated V1 split scheme
     //
     // There are some well-known filter aliases to match objects by certain
     // properties:
diff --git a/proto-docs/object.md b/proto-docs/object.md
index 074ae9b..a20791b 100644
--- a/proto-docs/object.md
+++ b/proto-docs/object.md
@@ -800,6 +800,8 @@ prefix to the name. Here is the list of fields available via this prefix:
   object_id of parent
 * $Object:split.splitID \
   16 byte UUIDv4 used to identify the split object hierarchy parts
+* $Object:split.first \
+  object_id of the first part in split chain; non-acceptable for deprecated V1 split scheme
 
 There are some well-known filter aliases to match objects by certain
 properties: