I am not an expert, but do have experience. First, I always have capacity to contain birds when needed. Then have a range of approaches that can be used in many combinations. Most of time my foxes are not coming first for chickens. They are coming in for whatever they find as part of patrolling their territory. Currently, fox is coming in only at night based on game cameras and when losses have occurred. I have had broody hens roosting on ground because sometimes they want to which I must work against. Therefore all birds, excepting a few in the barn, are penned about dark. All the birds are currently imprinted on roosting sites I can protect. Fox seldom visits during day in part because birds (songbirds included) alert dogs fox is about so dogs check out the disturbance. When chickens are roosting on ground or in pens fox might try to beat, then I have an electrified perimeter going. It is easy to see the fox respects fencing using game camera. My fencing has gaps made for dogs that most critters do not figure out. When a fox figures out the gap, then we setup to eliminate the fox using foot-hold trap. I have a bait station out all the time with a depression made for the trap. If fox to be trapped, the station baited for a couple nights or more to get fox coming to it regularly for a snack. Then I set out trap and catch that fox. We may then go for a night or two watching camera to make certain that fox is the only one coming in. When cost appears clear, then chickens are released again.
I use dogs, but they must cover several acres which can be tough when your predator is the Red Fox. My property has about 3 acres of mowed ground and an additional 3 acres chickens use that are early successional with lots of American Persimmon trees, Red Oaks, Smooth Sumac and brambles. The area is exceptional productive with respect to forages for chickens. That comes at the cost of ground that can appear unkempt and supports a lot of wildlife.