diff --git a/lib/audit/dns-engine.ts b/lib/audit/dns-engine.ts
index a064a3c2..2b360876 100644
--- a/lib/audit/dns-engine.ts
+++ b/lib/audit/dns-engine.ts
@@ -1,9 +1,9 @@
 import Anthropic from "@anthropic-ai/sdk";
-import { zodOutputFormat } from "@anthropic-ai/sdk/helpers/zod";
-// zod/v4 — the SDK's zodOutputFormat calls z.toJSONSchema(), a v4-only API.
+// zod/v4: backendAi and the SDK both call z.toJSONSchema(), a v4-only API.
 // See the note in claude-engine.ts.
 import { z } from "zod/v4";
 import { env } from "../env";
+import { generateStructuredOutput, hasBackendAiTextProvider } from "../lx/backendAi";
 import { collectDnsRecords, dnsBaselineFindings, type DnsRecords } from "./dns";
 import { scoreFindings } from "./score";
 import type { ClaudeAuditResult } from "./claude-engine";
@@ -97,26 +97,19 @@ export async function dnsAudit(targetUrl: string): Promise<ClaudeAuditResult> {
 
   // AI step — best effort. Fall back to the deterministic baseline on any
   // failure so this free scan never hard-fails the worker.
-  if (env.anthropicApiKey) {
+  if (hasBackendAiTextProvider({ anthropicApiKey: env.anthropicApiKey, openaiApiKey: env.openaiApiKey })) {
     try {
-      const client = new Anthropic({ apiKey: env.anthropicApiKey });
-      const stream = client.messages.stream({
-        model: "claude-sonnet-4-6",
-        max_tokens: 16000,
-        thinking: { type: "disabled" },
-        output_config: {
-          effort: "medium",
-          // eslint-disable-next-line @typescript-eslint/no-explicit-any
-          format: zodOutputFormat(ResultSchema as any),
-        },
-        system: [
-          { type: "text", text: SYSTEM_PROMPT, cache_control: { type: "ephemeral" } },
-        ],
-        messages: [{ role: "user", content: buildUserPrompt(records) }],
-      });
-      const response = await stream.finalMessage();
-      const parsed = response.parsed_output;
-      if (!parsed) throw new Error(`no parsed output (stop_reason=${response.stop_reason})`);
+      const { output: parsed } = await generateStructuredOutput({
+        name: "dns_audit",
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        schema: ResultSchema as any,
+        system: SYSTEM_PROMPT,
+        user: buildUserPrompt(records),
+        maxTokens: 16000,
+        anthropicModel: "claude-sonnet-4-6",
+        anthropic: env.anthropicApiKey ? new Anthropic({ apiKey: env.anthropicApiKey }) : null,
+        anthropicCacheSystemPrompt: true,
+      }) as { output: z.infer<typeof ResultSchema> };
 
       const aiFindings: Finding[] = parsed.findings.map(
         (f: z.infer<typeof FindingSchema>) => ({
diff --git a/tests/dns-engine.test.ts b/tests/dns-engine.test.ts
new file mode 100644
index 00000000..5b9e7cb1
--- /dev/null
+++ b/tests/dns-engine.test.ts
@@ -0,0 +1,50 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const calls = vi.hoisted(() => ({ args: [] as any[], answer: null as null | (() => unknown) }));
+
+vi.mock("@/lib/lx/backendAi", () => ({
+  hasBackendAiTextProvider: () => true,
+  generateStructuredOutput: async (args: unknown) => {
+    calls.args.push(args);
+    return calls.answer!();
+  },
+}));
+vi.mock("@/lib/env", () => ({ env: { anthropicApiKey: "", openaiApiKey: "" } }));
+vi.mock("@/lib/audit/dns", () => ({
+  collectDnsRecords: async () => ({ domain: "example.com", a: [], aaaa: [], mx: [], ns: [], txt: [], dkim: [] }),
+  dnsBaselineFindings: () => [
+    { section: "DNS", check_key: "dns.inventory", status: "pass", title: "Inventory", priority: 5 },
+  ],
+}));
+
+import { dnsAudit } from "@/lib/audit/dns-engine";
+
+describe("dnsAudit AI step", () => {
+  beforeEach(() => {
+    calls.args = [];
+  });
+
+  it("runs with no paid key and takes c0mpute's answer", async () => {
+    calls.answer = () => ({
+      provider: "c0mpute",
+      output: {
+        score: 80,
+        findings: [{ check_key: "spf", status: "warn", title: "SPF is soft", priority: 2 }],
+        markdown: "# DNS",
+      },
+    });
+    const result = await dnsAudit("https://example.com");
+    expect(calls.args).toHaveLength(1);
+    expect(calls.args[0].anthropic).toBeNull();
+    expect(result.score).toBe(80);
+    expect(result.findings.map((f) => f.check_key)).toEqual(["dns.inventory", "dns.spf"]);
+  });
+
+  it("keeps the deterministic baseline when every provider fails", async () => {
+    calls.answer = () => {
+      throw new Error("Backend AI generation failed");
+    };
+    const result = await dnsAudit("https://example.com");
+    expect(result.findings.map((f) => f.check_key)).toEqual(["dns.inventory"]);
+  });
+});