ssh install no longer runs a command on the host. It reads .ssh/authorized_keys over sftp, takes the empty reading only from sftp's own message about that path, appends the derived key locally when it is not already present, uploads the result beside the file with mode 0600 and renames it over the original. Any other failure prints what sftp said, writes nothing and exits 1. sftp batch mode disables password prompts, so a key or agent is required; a directory the owner cannot enter reads as a host with no file, which README.md states.
Model: opus-5 (implementation); fable-5-1 (landing)
