Another attempt at thoroughly fixing tmux ssh agent.

home/default.nix

@@ -29,7 +29,9 @@
 
     file.".ssh/rc".text = ''
       #!/bin/sh
-      ln -sf "$SSH_AUTH_SOCK" "$HOME/.ssh/ssh_auth_sock"
+      if test "$SSH_AUTH_SOCK"; then
+        ln -sf "$SSH_AUTH_SOCK" "$HOME/.ssh/ssh_auth_sock"
+      fi
     '';
     file.".ssh/rc".executable = true;
   };

home/programs/server.nix

@@ -398,7 +398,11 @@
         setw -g automatic-rename on
         set -g set-titles on
 
-        set-environment -g 'SSH_AUTH_SOCK' $HOME/.ssh/ssh_auth_sock
+        # first, unset update-environment[SSH_AUTH_SOCK] (idx 3), to prevent
+        # the client overriding the global value
+        set-option -g -u update-environment[3]
+        # And set the global value to our static symlink'd path:
+        set-environment -g SSH_AUTH_SOCK $HOME/.ssh/ssh_auth_sock
       '';
     };
   };