diff --git a/src/main/protobuf/build.proto b/src/main/protobuf/build.proto
index 0f2fdb0..469df9e 100644
--- a/src/main/protobuf/build.proto
+++ b/src/main/protobuf/build.proto
@@ -311,8 +311,7 @@
   // The rule's class's public by default value.
   optional bool public_by_default = 9;
 
-  // If this rule is of a skylark-defined RuleClass.
-  optional bool is_skylark = 10;
+  optional bool DEPRECATED_is_skylark = 10;
 
   // Hash encapsulating the behavior of this Skylark rule. Any change to this
   // rule's definition that could change its behavior will be reflected here.