See my other comment and let me know if that script contains ssh-agent $SHELL ... or exec $SHELL ... or eval "$SHELL ..." or similar. :) If it ends

See my other comment and let me know if that script contains ssh-agent $SHELL ... or exec $SHELL ... or eval "$SHELL ..." or similar. :) If it ends up passing a script file or arguments to $SHELL and you wanna fix it without rewriting it, just add some conditionals where $SHELL is used and use the equivalent flags or syntax when $SHELL indicates fish. Alternatively, if you just wanna work around it, just launch that script like env SHELL=(which zsh) whatever-script.sh