When canonical (gRPC) error codes are mapped to HTTP status codes, in some cases the mapping is lossy. Multiple canonical error codes map to one HTTP status code. For example ABORTED and ALREADY_EXISTS both map to 409. By using only the HTTP status code (409) a client would not be able to differentiate how to retry these errors.

The error response returned from Google's JSON HTTP APIs includes the gRPC error code in error.status, so this behavior can be addressed by parsing error.status and only falling back to the http status code if the grpc code is not provided.