yubikey-agent is a seamless ssh-agent for YubiKeys.

  (*) Easy to use. A one-command setup, one environment variable,
and it just runs in the background.
  (*) Indestructible. Tolerates unplugging, sleep, and suspend.
Never needs restarting.
  (*) Compatible. Provides a public key that works with all services
and servers.
  (*) Secure. The key is generated on the YubiKey and can't be
extracted. Every session requires the PIN, every login requires a
touch. Setup takes care of PUK and management key.

WWW: https://filippo.io/yubikey-agent