I do agree with Kostya, I think it's a good compromise. In my humble opinion, the extended error feature in its current state is not worth the complexity and overhead it adds. Instead, why not introduce a Tarantool setting to choose whether you want to deal with a legacy or extended error response type (and it's very unlikely that someone will need to have 2 types at the same time). By default this setting will be set to use the legacy mode, then after N minor 2.x releases, it can be changed to use the new error type, the setting itself will be marked as deprecated and removed in the next major release. From a connector's point of view, it will also be easy to check if there are any additional fields in the response body, which would mean that the connector has received a "new" error.