SSH agent has been implemented on first version of the protocol definition (00) but at least a blocking change exists according last version (11)
Constraint extension identifier is set to 3 but should be 255 here: https://github.com/golang/crypto/blob/master/ssh/agent/client.go#L146
Openssh implementation is ok: https://github.com/openssh/openssh-portable/blob/master/authfd.h#L109