fish is a bit insonsistent on it. For instance, `git add <tab>` will only autocomplete for modified files. It will also fill in wildcards, e.g. `cat *.txt <tab>` will expand to show all .txt files. On the failure side, `rm foo <tab>` will still show `foo` as an option.

IME, zsh has better autocompletion (which, at the time at least, was a separate install).

There are a couple differen things going on

- completions being aware of the subcommand

- dynamic look ups for specific values

- completions being aware of previous options, flags, and values

A lot of completions have the first. Some have the second. The last is rare. The completer needs knowledge of when flags, options, and value can be repeated and change which future options and values are suggested.