The pkgs parameter looks unused. Note also that lib can be (and is typically) accessed via stdenv.lib.

makeWrapper can be treated as a nativeBuildInputs

Please do not explicitly list phases, instead disable those that are causing problems. While it may not matter to this package in particular, it is a common source of bugs.

Please simplify Foo is a ... to A ... and drop trailing punctuation.

pkgs seems to be unused

fixed

Not important, but I wonder if creating the dir/symlink is necessary; what if you just pass -jar $jar?
In that case, the whole expression reduces to, basically

writeScriptBin "metabase" ''
  #! /bin/sh
  exec ${jre}/bin/java -jar ${jar} "$@"
''

Just something to consider.

It is probably not necessary, I am only not very Nix experienced yet.
How does writeScriptBin work?
I tried

  installPhase = writeScriptBin "metabase" ''
    #!/bin/sh
    exec ${jre}/bin/java -jar ${jar} "$@"
  '';

But it fails with /nix/store/…/*-metabase: Is a directory. I grepped for usage examples but could only find packages where writeScriptBin is used in the let scope for dependencies.

writeScriptBin is a function that given a name and text returns a derivation that produces an output comprising an executable script of that name containing that text under a bin/ directory.

In your case, installPhase is supposed to be a Nix string (which is then exported to the builder); you're instead setting it to be a Nix derivation (evaluating to a store path).

What I was suggesting was that the whole package expression would reduce, essentially, to writeScriptBin. That is, no installPhase or anything. I'm not suggesting you do literally that :)

In the context of your original expression, something like (untested)

installPhase = ''
  mkdir -p $out/bin
  cat >$out/bin/metabase <<EOF
  #! /bin/sh
  exec ${jre}/bin/java -jar ${jar} "$@"
  EOF
''

might work.

Anyway, this is not at all a blocker. Please just remove the redundant pkgs parameter and I'd be happy to merge as-is.

Thanks a lot for your explanation! ❤️ I decided to stick with the verbose approach because it is more common.

Not important, but I wonder if creating the dir/symlink is necessary; what if you just pass -jar $jar?

Fixed.