How do I copy a derivation and all its build dependencies recursively to a binary cache?

I tried copying nixpkgs#hello.drvPath (well actually my nixos toplevel.drvPath but hello is easier to focus on) but that just got me this weird error Error message "string '...' has a context which refers to a complete source and binary closure" makes no sense. · Issue #10651 · NixOS/nix · GitHub

Other than a few implications that drvPath might help, I can’t find any info on how to copy all recursive build deps.