Not the author. But thank you, I didn’t know that.

What you’re doing is equivalent to

nix-shell -p "grim slurp"

Which won’t work because nix-shell expects

nix-shell -p "grim" "slurp"

Which then becomes

{...}@args: with import <nixpkgs> args; (pkgs.runCommandCC or pkgs.runCommand) "shell" { buildInputs = [ (grim) (slurp) ]; } ""

According to the manual

nix-shell --packages interprets each command line arguments as attribute names inside the Nix packages collection.

The error message is because you are giving multiple package names as a single argument.