diff --git a/payload_consumer/payload_constants.cc b/payload_consumer/payload_constants.cc
index 72abf8c..e47b4f8 100644
--- a/payload_consumer/payload_constants.cc
+++ b/payload_consumer/payload_constants.cc
@@ -52,6 +52,8 @@
       return "DISCARD";
     case InstallOperation::REPLACE_XZ:
       return "REPLACE_XZ";
+    case InstallOperation::IMGDIFF:
+      return "IMGDIFF";
   }
   return "<unknown_op>";
 }
diff --git a/update_metadata.proto b/update_metadata.proto
index c856405..222542f 100644
--- a/update_metadata.proto
+++ b/update_metadata.proto
@@ -162,6 +162,9 @@
     ZERO = 6;  // Write zeros in the destination.
     DISCARD = 7;  // Discard the destination blocks, reading as undefined.
     REPLACE_XZ = 8; // Replace destination extents w/ attached xz data.
+
+    // On minor version 4 or newer, these operations are supported:
+    IMGDIFF = 9; // The data is in imgdiff format.
   }
   required Type type = 1;
   // The offset into the delta file (after the protobuf)