diff --git a/packcheck.sh b/packcheck.sh
index 439c7cd..9fb527b 100755
--- a/packcheck.sh
+++ b/packcheck.sh
@@ -2540,14 +2540,20 @@ setup_environment() {
   # XXX add paths only if not already on PATH, but sometimes we have to add path
   # at head for precedence even if it is already on PATH.
   #echo "Original PATH is [$PATH]..."
-  PATH_PREFIX=$LOCAL_BIN:$GHCUP_BIN
+
+  # XXX We should not add anything to PATH, respect the user's PATH. For example,
+  # we may be running in a nix shell which has overridden ghc, and adding
+  # ghcup/bin to the front of PATH will change the ghc and all the packages
+  # that were available.
+  #
+  PATH_SUFFIX=$LOCAL_BIN:$GHCUP_BIN
   if test "$BUILD" = "cabal-v2"
   then
-      PATH_PREFIX=$PATH_PREFIX:$OS_APP_HOME/$OS_CABAL_DIR/bin
+      PATH_SUFFIX=$PATH_PREFIX:$OS_APP_HOME/$OS_CABAL_DIR/bin
   fi
 
-  echo "Prefixing [$PATH_PREFIX] to PATH"
-  export PATH=$PATH_PREFIX:$PATH
+  echo "Suffixing [$PATH_SUFFIX] to PATH"
+  export PATH=$PATH:$PATH_SUFFIX
   echo
 
   # XXX we should be able to set multiple paths in it
@@ -2560,10 +2566,6 @@ setup_environment() {
   #  TOOLS_DIR="$GHCUP_PATH"
   #fi
 
-  # if we are running from a stack environment remove GHC_PACKAGE_PATH so that
-  # cabal does not complain
-  # XXX this should be done from outside via env
-  unset GHC_PACKAGE_PATH
   # stack does not work well with empty STACK_YAML env var
   test -n "$STACK_YAML" || unset STACK_YAML
 
@@ -2612,6 +2614,14 @@ case $BUILD in
   *) ;;
 esac
 
+# if we are running from a stack environment remove GHC_PACKAGE_PATH so that
+# cabal does not complain.
+# XXX this should be done from outside via env
+# NOTE: do this before eval_env, so that it can later be set by eval_env if
+# passed on command line.
+echo "WARNING! unsetting GHC_PACKAGE_PATH if you really want to use it pass it on the command line"
+unset GHC_PACKAGE_PATH
+
 # This allows only SYSTEM_ENVVARS to be set on the command line
 eval_env "$@"
 # After eval_env the PATH changes