Wrap the haskell-language-server-wrapper in the way of bash tee stderr. Record HLS stderr output in the file stderr.log.
hls-wrapper = pkgs.writeShellApplication {
name = "haskell-language-server-wrapper";
text = ''
export PATH=$DIRENV_DIR/dist-newstyle/build/x86_64-linux/ghc-9.6.3/haskell-language-server-2.8.0.0/x/haskell-language-server/build/haskell-language-server:$PATH
./dist-newstyle/build/x86_64-linux/ghc-9.6.3/haskell-language-server-2.8.0.0/x/haskell-language-server-wrapper/build/haskell-language-server-wrapper/haskell-language-server-wrapper "$@" 2> >(tee -a stderr.log >&2)
'';
};
Follow the stderr.log.
tail -f stderr.log | bat -l log -pp