Whoops! Guess I was wrong. After some experimenting it looks like the flake system parses, but does not evaluate flake.nix to read inputs. I also experimented with string concatenation, and that failed with the same error:

nixpkgs.url = "github:nixos/nixpkgs" ++ "/nixos-25.05"; # error: expected a string or path, but got a thunk

A "thunk" is an expression whose evaluation has been delayed. It's a key piece of lazy evaluation. Remember that every expression in Nix is lazily evaluated.

It looks only literal attribute set, string, and path expressions will work in inputs. I think that means it is not possible to split inputs over multiple files.