> A much simpler way to do it is to have a server switch to enable
> new features.
> It is less flexible, of course, because you can't have old and new
> clients, but do you really want to have old and new clients?


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.

-- 
Thank you and best regards,
Eugene Leonovich