Build fails because vendorSha256 is missing. modSha256 will be removed with the next release. See: #89454

I see. I tested the build only with : nix-env -f $NIXPKGS -i kpt
From a stable channel.

Is it possible to test the build fast without using `nixos-rebuild build``on my git branch of nixpkgs ?

Is it possible to test the build fast without using `nixos-rebuild build``on my git branch of nixpkgs ?

If you are in the top-level nixpkgs directory, you can run

nix-build -A kpt

This will build the derivation and create a result symlink to the output. You can then peek around in result/ and e.g. run result/bin/kpt. Although I rarely use it, you can also use nix-shell. E.g.:

nix-shell -I nixpkgs=. --pure -p kpt

Which will put you in a development shell as if kpt was a dependency (so you can also run it).

Finally, there is nixpkgs-review, which also builds any package dependent on the package that you changed (though of course, that is less important for a new package).