It took me some time to work out how to get my ssh agent set up in Niri so I though I would share what I did. I’m using NixOS and Home Manager. I put this in my Home Manager config:

services.gnome-keyring = {
  enable = true;
  components = [ "pkcs11" "secrets" "ssh" ];
};
home.sessionVariables.SSH_AUTH_SOCK = "$XDG_RUNTIME_DIR/keyring/ssh";

I’m using GDM according to NixOS’ default configuration which I think runs gnome-keyring (I thought I saw it in the process list before I set up the user unit), and I think that configuration is automatically unlocking gnome-keyring when I log in via PAM integration. But apparently I need to run gnome-keyring again in my window manager session. Home Manager’s services.gnome-keyring adds a systemd user unit that does that.