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Thanks for the review. The empty string is preserved through the operator pipeline because SetDefault only replaces nil, not empty string:

// pkg/apis/kai/v1/common/set_default.go
func SetDefault[T any](target *T, value *T) *T {
    if target == nil {
        return value
    }
    return target  // *string("") is not nil, so it's kept
}

Full flow when user sets admission.gpuPodRuntimeClassName: "":

1. Helm (after PR fix: allow setting empty gpuPodRuntimeClassName during helm install #972) renders gpuPodRuntimeClassName: "" in ConfigMap
2. Operator deserializes → *string pointing to "" (not nil)
3. SetDefaultsWhereNeeded at admission.go:70 → SetDefault keeps "" because target is not nil
4. buildArgsList at resources.go:362 → config.GPUPodRuntimeClassName is non-nil, so it passes --gpu-pod-runtime-class-name ""
5. This fix → Mutate() sees empty string, returns early, no runtimeClassName injection

Without this fix, step 5 would still proceed to evaluate the pod and call setRuntimeClass(pod, ""), which sets runtimeClassName to an empty string pointer rather than leaving it unset.

The cmd/admission/app/options.go:70 default (constants.DefaultRuntimeClassName) only applies when the flag is not provided at all — since the operator explicitly passes the flag via buildArgsList, the pflag default is overridden.

Merging this branch will increase overall coverage

Backport failed for v0.9, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin v0.9
git worktree add -d .worktree/backport-1035-to-v0.9 origin/v0.9
cd .worktree/backport-1035-to-v0.9
git switch --create backport-1035-to-v0.9
git cherry-pick -x 9675f37d32d202b7eeb96485c85fdd399d37b12b

Backport failed for v0.10, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin v0.10
git worktree add -d .worktree/backport-1035-to-v0.10 origin/v0.10
cd .worktree/backport-1035-to-v0.10
git switch --create backport-1035-to-v0.10
git cherry-pick -x 9675f37d32d202b7eeb96485c85fdd399d37b12b

Backport failed for v0.12, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin v0.12
git worktree add -d .worktree/backport-1035-to-v0.12 origin/v0.12
cd .worktree/backport-1035-to-v0.12
git switch --create backport-1035-to-v0.12
git cherry-pick -x 9675f37d32d202b7eeb96485c85fdd399d37b12b