cliff complete command only supports bash, but fish offers much more user friendly completions.
cliff uses a pluggable system, but this where new shell completions can be registered, but the plugin (CompleteFish) can not access the command manager to get more details about options, since fish can also show help of suggestion, we want to access the help of each option to show this during suggestions. Also we want to make some more clever suggestions (for example slapos node start complete with services identifiers). For these reasons we need to register a command to replace the default complete command and can not achieve this with simply a cliff.formatter.completion entry point.
This can be installed with:
slapos complete > ~/.bash_completion.d/slapos
slapos complete --shell fish > ~/.config/fish/completions/slapos.fish