Please don't break these lines up so much. Just put it on one line, possibly two.

Ig, I can use 80 character limit to determine whether to distribute a line over several ones or not. Wanna keep completions readable and not to make myself scroll horizontally.

I will apply the same logic for all other my PRs. 😄

These are well below 80 characters, which is a limit that we do not actually have in our fish script style.

If you must split them up, here's how I would do that:

complete -c $command \
    -s o \
    -d 'The output file of a migrated schema' \
    -n "__fish_seen_subcommand_from validate migrate" \
    -F -r

becomes

complete -c $command -s o -F -r \
    -n "__fish_seen_subcommand_from validate migrate" \
    -d 'The output file of a migrated schema'

Basically put the big condition and description on their own lines, but don't give each little option its own line. That just wastes a ton of vertical space and gives you very empty lines (-s o is basically nothing).

I like your suggestions, I also put -a on a separate line, to keep the following structure of the completion:

• option's values
• options with additional flags like -F
• description
• condition

That's fine.

These should be local-scoped, or you'll overwrite someone's variable named "command".

Tho tbh just remove this entirely and use command -c ajv.

It's not like you're gonna copy these into other files anyway.

These definitely need to be set -l.

We don't really do the [h] mnemonic, please remove it.

Should all other mnemonics be removed as well?

I've removed all mnemonics in square brackets.

Missed these.