diff --git a/interop/test.proto b/interop/test.proto
index 8380ebb3..996f11aa 100644
--- a/interop/test.proto
+++ b/interop/test.proto
@@ -14,7 +14,7 @@ service TestService {
   rpc EmptyCall(grpc.testing.Empty) returns (grpc.testing.Empty);
 
   // One request followed by one response.
-  // The server returns the client payload as-is.
+  // TODO(Issue 527): Describe required server behavior.
   rpc UnaryCall(SimpleRequest) returns (SimpleResponse);
 
   // One request followed by a sequence of responses (streamed download).