diff --git a/databuild/databuild.proto b/databuild/databuild.proto
index 3231f77..a9a44ea 100644
--- a/databuild/databuild.proto
+++ b/databuild/databuild.proto
@@ -74,6 +74,9 @@ message PartitionManifest {
 
   // The configuration used to run the job
   Task task = 5;
+
+  // Arbitrary metadata about the produced partitions, keyed by partition ref
+  map<string, string> metadata = 6;
 }
 
 message JobExecuteRequest { repeated PartitionRef outputs = 1; }