Simple OPA mutation policy by Mr. Torin Sandall

main.rego

package system

main = {
    "apiVersion": "admission.k8s.io/v1beta1",
    "kind": "AdmissionReview",
    "response": {
        "allowed": true,
        "patchType": "JSONPatch",
        "patch": patch_bytes,
    }
} {
    is_create_or_update

    input.request.object.metadata.annotations["test-mutation"]

    patch = [
        {"op": "add", "path": "/metadata/annotations/foo", "value": "bar"},
    ]
    patch_json = json.marshal(patch)
    patch_bytes = base64url.encode(patch_json)
}

is_create_or_update { is_create }
is_create_or_update { is_update }
is_create { input.request.operation == "CREATE" }
is_update { input.request.operation == "UPDATE" }