Good to know. Note regarding your final sentence, however: this design does not necessarily prevent the SSH agent from having a bug which can cause key disclos

Good to know. Note regarding your final sentence, however: this design does not necessarily prevent the SSH agent from having a bug which can cause key disclosure via the agent protocol. It returns data buffers, so if it could be tricked into leaking key material into those data buffers, it may end up getting sent back to the server just the same (e.g. as a signature value). This would, however, be a bug in the agent and not the client.