Defense in Depth Needs Visibility

Defense-in-depth only works if every fallback layer is visible and monitored. Unmonitored fallbacks mask root causes, allowing decay to accumulate silently until catastrophic failure. The pattern: a cheap primary mechanism fails silently, an expensive fallback kicks in unnoticed, works fine on small inputs, and nobody investigates. Then larger inputs arrive and the fallback blows up.

If a deeper defense activates, treat it as an incident signal. align-alerts-to-sev-criteria when fallbacks fire. The whole point of defense-in-depth is redundancy, not invisibility. If you don’t know your primary defense failed, your “resilience” is just hiding decay.