diff --git a/lib/services/route53-dns-delegation.ts b/lib/services/route53-dns-delegation.ts
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+++ b/lib/services/route53-dns-delegation.ts
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+/**
+ * D-12 DNS-delegation health check helpers.
+ *
+ * Split into a pure half (normalization + set comparison, fully unit-testable)
+ * and an I/O half (live public DNS lookup via a dedicated `dns.Resolver()`
+ * instance). The pure half never touches the network; the I/O half never
+ * touches the process-global resolver — see the CRITICAL comment on
+ * `resolveLiveNs` below (24-RESEARCH.md Pitfall 5 / threat T-24-13).
+ */
+
+import { Resolver } from 'dns';
+
+// ============================================================================
+// Pure half — NS normalization + comparison (unit-testable, no I/O)
+// ============================================================================
+
+/**
+ * Normalize an unknown input (expected to be a JSONB-sourced string array,
+ * e.g. route53_zones.authoritative_name_servers or a raw DNS answer) into a
+ * lowercase, trailing-dot-stripped, de-duplicated, sorted list of hostnames.
+ *
+ * Returns [] for null, undefined, or any non-array input rather than
+ * throwing — callers treat an empty list as "unjudgeable", not an error.
+ */
+export function normalizeNsList(input: unknown): string[] {
+  if (!Array.isArray(input)) return [];
+  const set = new Set<string>();
+  for (const entry of input) {
+    const normalized = String(entry).trim().toLowerCase().replace(/\.$/, '');
+    if (normalized.length > 0) set.add(normalized);
+  }
+  return Array.from(set).sort();
+}
+
+export interface NsDelegationComparison {
+  mismatch: boolean;
+  authoritative: string[];
+  live: string[];
+  missingFromLive: string[];
+  extraInLive: string[];
+}
+
+/**
+ * Compare Route 53's authoritative NS list against a live public DNS answer.
+ *
+ * - An empty (normalized) authoritative list is unjudgeable — a zone Pulse
+ *   has no recorded NS data for cannot be flagged as drifted. Returns
+ *   `mismatch: false`.
+ * - A non-empty authoritative list with an empty live answer IS a delegation
+ *   problem (the domain resolved to nothing) — returns `mismatch: true`.
+ * - Otherwise mismatch is true iff either side has an entry the other lacks.
+ */
+export function compareNsDelegation(authoritative: unknown, live: unknown): NsDelegationComparison {
+  const authNorm = normalizeNsList(authoritative);
+  const liveNorm = normalizeNsList(live);
+
+  if (authNorm.length === 0) {
+    return { mismatch: false, authoritative: authNorm, live: liveNorm, missingFromLive: [], extraInLive: [] };
+  }
+  if (liveNorm.length === 0) {
+    return {
+      mismatch: true,
+      authoritative: authNorm,
+      live: liveNorm,
+      missingFromLive: [...authNorm],
+      extraInLive: [],
+    };
+  }
+
+  const authSet = new Set(authNorm);
+  const liveSet = new Set(liveNorm);
+  const missingFromLive = authNorm.filter((ns) => !liveSet.has(ns));
+  const extraInLive = liveNorm.filter((ns) => !authSet.has(ns));
+
+  return {
+    mismatch: missingFromLive.length > 0 || extraInLive.length > 0,
+    authoritative: authNorm,
+    live: liveNorm,
+    missingFromLive,
+    extraInLive,
+  };
+}
+
+// ============================================================================
+// I/O half — live public DNS lookup (dedicated resolver instance)
+// ============================================================================
+
+export type ResolveLiveNsResult =
+  | { ok: true; nameServers: string[] }
+  | { ok: false; error: string };
+
+/**
+ * Resolve a domain's live NS records against a dedicated public resolver
+ * (Cloudflare 1.1.1.1 + Google 8.8.8.8), NOT the container's default
+ * resolver (24-RESEARCH.md Pitfall 5).
+ *
+ * CRITICAL: this constructs its own `new Resolver()` instance and calls
+ * `setServers()` on THAT instance only. The process-global `dns.setServers()`
+ * is never called anywhere in this file — doing so would repoint DNS
+ * resolution for the entire Node process, including Postgres/Redis/AWS
+ * hostname resolution (T-24-13).
+ */
+export function resolveLiveNs(domain: string, timeoutMs = 5000): Promise<ResolveLiveNsResult> {
+  const target = domain.trim().replace(/\.$/, '');
+  const resolver = new Resolver();
+  resolver.setServers(['1.1.1.1', '8.8.8.8']);
+
+  return new Promise((resolve) => {
+    let settled = false;
+    const timer = setTimeout(() => {
+      if (settled) return;
+      settled = true;
+      resolver.cancel();
+      resolve({ ok: false, error: `DNS lookup timed out after ${timeoutMs}ms` });
+    }, timeoutMs);
+
+    resolver.resolveNs(target, (err, addresses) => {
+      if (settled) return;
+      settled = true;
+      clearTimeout(timer);
+      if (err) {
+        resolve({ ok: false, error: err.message });
+        return;
+      }
+      resolve({ ok: true, nameServers: addresses });
+    });
+  });
+}
+
+export interface ZoneDelegationInput {
+  id: string;
+  name: string;
+  authoritativeNameServers: unknown;
+}
+
+export interface ZoneDelegationResult {
+  zoneId: string;
+  zoneName: string;
+  mismatch: boolean;
+  error?: string;
+  missingFromLive: string[];
+  extraInLive: string[];
+}
+
+/**
+ * Check live NS delegation for a batch of zones, bounded to at most
+ * `opts.concurrency` (default 5) concurrent lookups so a large zone list
+ * does not open hundreds of concurrent UDP sockets (T-24-14).
+ *
+ * Zones whose authoritativeNameServers normalizes to an empty list are
+ * skipped (unjudgeable). A lookup failure yields `{ mismatch: false, error }`
+ * — an unreachable resolver is an infrastructure problem, not evidence of
+ * delegation drift, and must never be reported as a mismatch.
+ */
+export async function checkAllZoneDelegations(
+  zones: ZoneDelegationInput[],
+  opts?: { concurrency?: number },
+): Promise<ZoneDelegationResult[]> {
+  const concurrency = opts?.concurrency ?? 5;
+  const judgeable = zones.filter((z) => normalizeNsList(z.authoritativeNameServers).length > 0);
+  const results: ZoneDelegationResult[] = [];
+
+  let cursor = 0;
+  async function worker(): Promise<void> {
+    while (cursor < judgeable.length) {
+      const idx = cursor;
+      cursor += 1;
+      const zone = judgeable[idx];
+      const live = await resolveLiveNs(zone.name);
+      if (!live.ok) {
+        results.push({
+          zoneId: zone.id,
+          zoneName: zone.name,
+          mismatch: false,
+          error: live.error,
+          missingFromLive: [],
+          extraInLive: [],
+        });
+        continue;
+      }
+      const comparison = compareNsDelegation(zone.authoritativeNameServers, live.nameServers);
+      results.push({
+        zoneId: zone.id,
+        zoneName: zone.name,
+        mismatch: comparison.mismatch,
+        missingFromLive: comparison.missingFromLive,
+        extraInLive: comparison.extraInLive,
+      });
+    }
+  }
+
+  const workers = Array.from({ length: Math.min(concurrency, judgeable.length) }, () => worker());
+  await Promise.all(workers);
+  return results;
+}