If a proto field is unknown to ESP, the transcoded response data maybe truncated. This could happen when proto files used by ESP (the proto descriptor file used in "gcloud endpoints service deploy") and gRPC server are not in sync.

Example 1. a new field is recently added, but gRPC server is using updated proto, but ESP is using the old one.
Example 2: a removed field, gRPC server is using old proto with that field, but ESP is using new proto.

This is because ESP did not implement Skip() function.

Please see details discussion.