In reply to @adis:blad.is
You have a uvicorn derivation created by poetry2nix & a fastapi-cli derivation from nixpkgs. Derivations generated by poetry2nix doesn't have passthru.optional-dependencies because poetry2nix predates it.

    overrides = poetry2nix.overrides.withDefaults (self: super: {
      fastapi-cli = python312.pkgs.fastapi-cli;
      bcrypt = python312.pkgs.bcrypt;
    });

Seems to work for me

I'm not sure how the "right" script is chosen in the first place.

I have this now:

[tool.poetry.scripts]
remarks = 'remarks.main:main'
remarks-server = "server:main"

And if I build with

poetryConfig = {
          projectDir = ./.;
          python = pkgs.python310;
          preferWheels = true;
          overrides = defaultPoetryOverrides.extend (final: prev: {
            click = prev.click.overridePythonAttrs (old: {
              buildInputs = (old.buildInputs or [ ]) ++ [ prev.flit-scm ];
            });
          });
          # Optional overrides if needed:
          # overrides = poetry2nix.overrides.withDefaults (final: prev: { });
        };

        remarksBin = mkPoetryApplication poetryConfig;
        remarksServer = mkPoetryApplication (poetryConfig // {
          extras = [ "server" ];
        });