A fox tests a chicken run the way water tests a boat: patiently, at the weakest seam, and usually at night. Foxes dig quickly and quietly, climb wire surprisingly well, and remember a run that fed them once. Proofing therefore has to answer both routes at once, digging and climbing, because a run secured against only one is simply a puzzle with the second half left unsolved.
Stop the Digging at the Base
Digging is defeated at ground level. Fit a mesh skirt or apron around the base of the run: wire mesh laid flat on the ground outward from the fence line and pinned or buried shallowly, so a fox digging at the fence, which is where foxes start, strikes wire under her paws and abandons the attempt. Digging straight down beside the mesh rarely occurs to them; they dig where they stand, and where they stand is on your apron. Use sturdy welded mesh rather than light hexagonal wire for the skirt and the lower walls, because a determined fox can tear thin wire that looks perfectly adequate in daylight.
Check the apron as a system, not a gesture. It must run the full perimeter without a break at the gate, overlap at corners instead of merely meeting, and stay pinned as frost, rain and scratching hens shift the soil above it. The gate itself needs the same treatment, with no inviting trench line underneath where paws have already loosened the earth. Inspect the whole perimeter after heavy rain and after any digging by your own dogs, because softened, shifted soil is exactly where a skirt lifts and a determined fox finds the one unguarded stretch of fence.
Roof the Run and Lock It Properly
Climbing is defeated overhead. A covered top, solid roofing on at least part of the run and strong mesh over the rest, removes the over-the-top route entirely and brings the bonus of dry ground and shade beneath it. Open-topped runs rely on height alone, and height is a challenge a climbing fox accepts. Where a full cover is impossible, an outward-angled overhang of mesh at the top makes the final move far harder, but treat that as a compromise, not an equal.
Latches deserve the same skepticism. A simple sliding bolt or a hook that lifts with a nose is an invitation; fit strong latches that need two distinct movements, or a latch backed by a carabiner clip, on every door including the small pop door into the coop. Walk the run at hen height and at fox height alike, looking for daylight where there should be none. Then close the gaps foxes actually use: the seams where wire panels meet their posts, the junction between run and coop, and any hole wider than a hen's head is wide, because a fox needs surprisingly little room once her head is through. Bring the hens in well before dusk each night, since the hour the run should stand empty of birds is the hour the fox arrives.
If a Bird Gets Out Anyway
Proofing is the prevention, and prevention can still fail in the chaos of an attack: a hen who bursts out through a door opened in panic, or one missed at shut-in, is outside the wire while the fox is still near. That is when the alert layer matters, as a fallback running in parallel rather than a substitute for wire. For a hen who slips away anyway, the Lost Pet Alert System entry on KinsLe carries her description to homes close by while letting whoever spots her message her owner directly.