What's the best way of checking whether a task runs in a TTY?

[vctls]

I want to check whether a task is running in a TTY in order to set relevant flags to a command (docker exec -i -t, for example).
But the only thing that seems to work for substitution is tty -s.
The other results are a little weird. test doesn't work in a sub-command.
And inside dynamic variables, nothing seems to work. Example task:

  tty:
    cmd: |-
      echo -n test stdin: ; test -t 1 && echo X \
      ; echo -n test stderr: ; test -t 2 && echo X \
      ; echo -n sub test stdin: ; echo $(test -t 1 && echo X) \
      ; echo -n sub test stderr: ; echo $(test -t 2 && echo X) \
      ; echo -n tty: ; tty -s && echo X \
      ; echo -n sub tty: ; echo $(tty -s && echo X) \
      ; echo "{{.TTY}}"
    interactive: true
    vars:
      TTY:
        sh: |-
          echo -n var test stdin: ; if [ -t 1 ]; then echo -n X; fi; echo '';
          echo -n var test stderr: ; if [ -t 2 ]; then echo -n X; fi; echo '';
          echo -n var tty: ; if [ $(tty -s) ]; then echo -n X; fi; echo '';

❯ task -s tty
test stdin:X
test stderr:X
sub test stdin:
sub test stderr:X
tty:X
sub tty:X
var test stdin:
var test stderr:X
var tty:

Not sure why we still get stderr without stdin, I'm not a terminal specialist.
The interactive option doesn't change anything.
So the best I could find is $(tty -s && echo flags). Is there any better way?
Is it possible to put that in a variable?