libssh2_agent_get_identity(3) on Linux
The text acdccde633dd, as Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie), Debian 7 (Wheezy), Debian 6.0 (Squeeze) shipped it.
get a public key off the collection of public keys managed by ssh-agent