diff --git a/browse/test/security-live-playwright.test.ts b/browse/test/security-live-playwright.test.ts
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+/**
+ * Live Playwright integration — defense-in-depth contract.
+ *
+ * Loads the existing injection-combined.html fixture in a real Chromium
+ * instance and verifies BOTH module layers detect the attack independently:
+ *
+ *   L1-L3 (content-security.ts):
+ *     * Hidden element stripping removes the .sneaky div
+ *     * ARIA regex catches the aria-label injection
+ *     * URL blocklist catches webhook.site / pipedream / requestbin
+ *
+ *   L4 (security.ts via security-classifier.ts):
+ *     * ML classifier scores extracted text as INJECTION
+ *
+ * If content-security.ts ever gets refactored to remove a layer thinking
+ * "the ML classifier covers it now," this test fails — the ML signal and
+ * the deterministic signal must BOTH be present.
+ *
+ * ML portion is skipped gracefully if the model cache is absent (first-run
+ * CI). To prime: `bun run browse/src/sidebar-agent.ts` for ~30s and kill it.
+ */
+
+import { describe, test, expect, beforeAll, afterAll } from 'bun:test';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import { startTestServer } from './test-server';
+import { BrowserManager } from '../src/browser-manager';
+import {
+  markHiddenElements,
+  getCleanTextWithStripping,
+  cleanupHiddenMarkers,
+  urlBlocklistFilter,
+} from '../src/content-security';
+
+// Check if TestSavantAI model cache exists. If missing, ML tests skip.
+const MODEL_CACHE = path.join(
+  os.homedir(),
+  '.gstack',
+  'models',
+  'testsavant-small',
+  'onnx',
+  'model.onnx',
+);
+const ML_AVAILABLE = fs.existsSync(MODEL_CACHE);
+
+describe('defense-in-depth — live Playwright fixture', () => {
+  let testServer: ReturnType<typeof startTestServer>;
+  let bm: BrowserManager;
+  let baseUrl: string;
+
+  beforeAll(async () => {
+    testServer = startTestServer(0);
+    baseUrl = testServer.url;
+    bm = new BrowserManager();
+    await bm.launch();
+  });
+
+  afterAll(() => {
+    try { testServer.server.stop(); } catch {}
+    setTimeout(() => process.exit(0), 500);
+  });
+
+  test('L2 — content-security.ts hidden-element stripper detects the .sneaky div', async () => {
+    const page = bm.getPage();
+    await page.goto(`${baseUrl}/injection-combined.html`, { waitUntil: 'domcontentloaded' });
+    const stripped = await markHiddenElements(page);
+    // Expect at least the sneaky div + the ARIA-injection link
+    expect(stripped.length).toBeGreaterThanOrEqual(1);
+    const sneakyMatch = stripped.some(s =>
+      s.toLowerCase().includes('opacity') || s.toLowerCase().includes('off-screen'),
+    );
+    expect(sneakyMatch).toBe(true);
+    await cleanupHiddenMarkers(page);
+  });
+
+  test('L2b — content-security.ts ARIA regex catches the injected aria-label', async () => {
+    const page = bm.getPage();
+    await page.goto(`${baseUrl}/injection-combined.html`, { waitUntil: 'domcontentloaded' });
+    const stripped = await markHiddenElements(page);
+    const ariaHits = stripped.filter(s => s.toLowerCase().includes('aria injection'));
+    expect(ariaHits.length).toBeGreaterThanOrEqual(1);
+    await cleanupHiddenMarkers(page);
+  });
+
+  test('L3 — content-security.ts URL blocklist fires on multiple exfil domains', async () => {
+    const page = bm.getPage();
+    await page.goto(`${baseUrl}/injection-combined.html`, { waitUntil: 'domcontentloaded' });
+    const html = await page.content();
+    const result = urlBlocklistFilter(html, `${baseUrl}/injection-combined.html`, 'html');
+    expect(result.safe).toBe(false);
+    // The fixture has webhook.site, pipedream.com, AND requestbin.com — expect at least 2 distinct hits
+    const distinctDomains = new Set(
+      result.warnings.map(w => (w.match(/(webhook\.site|pipedream\.com|requestbin\.com)/) ?? [])[1]).filter(Boolean),
+    );
+    expect(distinctDomains.size).toBeGreaterThanOrEqual(2);
+  });
+
+  test('L1 — stripped clean text excludes the .sneaky injection', async () => {
+    const page = bm.getPage();
+    await page.goto(`${baseUrl}/injection-combined.html`, { waitUntil: 'domcontentloaded' });
+    await markHiddenElements(page);
+    const cleanText = await getCleanTextWithStripping(page);
+    // Visible content preserved
+    expect(cleanText).toContain('Premium Widget');
+    // Hidden injection removed
+    expect(cleanText).not.toContain('SYSTEM OVERRIDE');
+    expect(cleanText).not.toContain('webhook.site/malicious');
+    await cleanupHiddenMarkers(page);
+  });
+
+  test('defense-in-depth — all deterministic layers (L1-L3) fire on the combined fixture', async () => {
+    // Pins the contract: removing any one content-security.ts layer breaks
+    // at least one signal in this test. This is the E5 regression anchor.
+    const page = bm.getPage();
+    await page.goto(`${baseUrl}/injection-combined.html`, { waitUntil: 'domcontentloaded' });
+    const stripped = await markHiddenElements(page);
+    const html = await page.content();
+    const urlResult = urlBlocklistFilter(html, `${baseUrl}/injection-combined.html`, 'html');
+
+    // L2: hidden element stripper
+    const hiddenCount = stripped.filter(s =>
+      s.toLowerCase().includes('opacity') || s.toLowerCase().includes('off-screen'),
+    ).length;
+    expect(hiddenCount).toBeGreaterThanOrEqual(1);
+
+    // L2b: ARIA regex
+    const ariaCount = stripped.filter(s => s.toLowerCase().includes('aria injection')).length;
+    expect(ariaCount).toBeGreaterThanOrEqual(1);
+
+    // L3: URL blocklist
+    expect(urlResult.safe).toBe(false);
+
+    await cleanupHiddenMarkers(page);
+  });
+
+  // L4 ML tests — skipped if model cache is absent
+  test.skipIf(!ML_AVAILABLE)('L4 — security.ts ML classifier flags the combined fixture text', async () => {
+    const page = bm.getPage();
+    await page.goto(`${baseUrl}/injection-combined.html`, { waitUntil: 'domcontentloaded' });
+    // Use RAW text (not stripped) so the ML layer sees what Claude would see
+    // in a naive pipeline — content-security.ts strips hidden content, but
+    // we want to assert the ML layer would ALSO catch it independently.
+    const rawText = await page.evaluate(() => document.body.innerText);
+
+    const { loadTestsavant, scanPageContent } = await import('../src/security-classifier');
+    await loadTestsavant();
+    const signal = await scanPageContent(rawText);
+    // Expect the classifier to flag some confidence > 0 (INJECTION label).
+    // The combined fixture has instruction-heavy content which TestSavantAI
+    // reliably flags at >= 0.5.
+    expect(signal.confidence).toBeGreaterThan(0);
+    expect(signal.layer).toBe('testsavant_content');
+  }, 60000); // allow WASM cold-start up to 60s
+
+  test.skipIf(!ML_AVAILABLE)('L4 — ML classifier does NOT flag the benign product description alone', async () => {
+    const benign = 'Premium Widget. $29.99. High-quality widget with premium features. Add to Cart.';
+    const { loadTestsavant, scanPageContent } = await import('../src/security-classifier');
+    await loadTestsavant();
+    const signal = await scanPageContent(benign);
+    // Product-catalog content should score low. Give generous headroom
+    // to avoid flakiness on model version drift — the contract is just
+    // "doesn't false-positive on obviously-clean ecommerce copy."
+    expect(signal.confidence).toBeLessThan(0.5);
+  }, 60000);
+});