diff --git a/protos/backend_service.proto b/protos/backend_service.proto
index 970dd9f..7fbc497 100644
--- a/protos/backend_service.proto
+++ b/protos/backend_service.proto
@@ -187,6 +187,10 @@ message CompleteOrchestrationWorkItemRequest {
 
     // True if this is a partial (chunked) completion. The backend must keep the work item open until the final chunk (isPartial=false).
     bool isPartial = 11;
+    
+    // Zero-based position of the current chunk within a chunked completion sequence.
+    // This field is omitted for non-chunked completions.
+    google.protobuf.Int32Value chunkIndex = 12;
 }
 
 // Response payload for completing an orchestration work item.
diff --git a/protos/orchestrator_service.proto b/protos/orchestrator_service.proto
index 1a86c0a..f58cf37 100644
--- a/protos/orchestrator_service.proto
+++ b/protos/orchestrator_service.proto
@@ -356,6 +356,13 @@ message OrchestratorResponse {
 
     // Whether or not a history is required to complete the original OrchestratorRequest and none was provided.
     bool requiresHistory = 7;
+
+    // True if this is a partial (chunked) completion. The backend must keep the work item open until the final chunk (isPartial=false).
+    bool isPartial = 8;
+
+    // Zero-based position of the current chunk within a chunked completion sequence.
+    // This field is omitted for non-chunked completions.
+    google.protobuf.Int32Value chunkIndex = 9;
 }
 
 message CreateInstanceRequest {