By running

OUTPUT=$(niri msg --json focused-output); niri msg output $(echo $OUTPUT | jq -r .name) scale $([ $(echo $OUTPUT | jq .logical.scale) = 1.0 ] && echo 2.0 || echo 1.0)

the current output's scale is toggled between 1.0 and 2.0.

Can be bound to a keystroke like this:

e.g. Mod+Shift+U:

    Mod+Shift+U repeat=false {
        spawn-sh "OUTPUT=$(niri msg --json focused-output); niri msg output $(echo $OUTPUT | jq -r .name) scale $([ $(echo $OUTPUT | jq .logical.scale) = 1.0 ] && echo 2.0 || echo 1.0)"
    }