diff --git a/browse/test/cdp-session-cleanup.test.ts b/browse/test/cdp-session-cleanup.test.ts
new file mode 100644
index 000000000..25ca6760c
--- /dev/null
+++ b/browse/test/cdp-session-cleanup.test.ts
@@ -0,0 +1,171 @@
+import { describe, test, expect } from 'bun:test';
+import * as fs from 'fs';
+import * as path from 'path';
+import type { Page } from 'playwright';
+import { withCdpSession, getOrCreateCdpSession } from '../src/cdp-bridge';
+
+// Static-grep tripwire + behavior tests for the CDP session lifecycle
+// helpers introduced as part of the D11 EXPAND_SCOPE memory-leak fix.
+//
+// Direct calls to `page.context().newCDPSession(page)` are the leak class
+// the helpers exist to close — every direct call needs a matching
+// `session.detach()` and forgetting it leaves the Chromium-side target
+// attached until the underlying transport drops. The tripwire fails CI
+// if any source file calls `newCDPSession(` outside `cdp-bridge.ts`
+// (the file that owns the helpers).
+//
+// Pattern mirrors browse/test/terminal-agent-pid-identity.test.ts and
+// browse/test/server-sanitize-surrogates.test.ts: read source files
+// directly, assert an invariant on their contents.
+
+const SRC_DIR = path.resolve(new URL(import.meta.url).pathname, '..', '..', 'src');
+
+function readAllSourceFiles(): Array<{ file: string; content: string }> {
+  const out: Array<{ file: string; content: string }> = [];
+  for (const entry of fs.readdirSync(SRC_DIR)) {
+    if (!entry.endsWith('.ts')) continue;
+    const full = path.join(SRC_DIR, entry);
+    out.push({ file: entry, content: fs.readFileSync(full, 'utf-8') });
+  }
+  return out;
+}
+
+describe('CDP session cleanup invariant', () => {
+  test('1. no source file calls `newCDPSession(` outside cdp-bridge.ts', () => {
+    const offenders: Array<{ file: string; line: number; text: string }> = [];
+    for (const { file, content } of readAllSourceFiles()) {
+      // The helper file is the ONE allowed home for direct newCDPSession calls.
+      if (file === 'cdp-bridge.ts') continue;
+      const lines = content.split('\n');
+      for (let i = 0; i < lines.length; i++) {
+        const line = lines[i];
+        if (!/newCDPSession\s*\(/.test(line)) continue;
+        // Skip comment lines — documentation mentions are fine.
+        const trimmed = line.trim();
+        if (trimmed.startsWith('//') || trimmed.startsWith('*')) continue;
+        offenders.push({ file, line: i + 1, text: trimmed });
+      }
+    }
+    if (offenders.length > 0) {
+      const formatted = offenders
+        .map((o) => `  ${o.file}:${o.line}  ${o.text}`)
+        .join('\n');
+      throw new Error(
+        `Direct newCDPSession(...) calls found outside cdp-bridge.ts. ` +
+        `Route through withCdpSession() (one-shot, finally-detach) or ` +
+        `getOrCreateCdpSession() (cached, close-detach) instead:\n${formatted}`,
+      );
+    }
+    expect(offenders).toEqual([]);
+  });
+
+  test('2. helper file exports the two documented entry points', () => {
+    // Sanity: the tripwire is meaningless if the helpers themselves are gone.
+    expect(typeof withCdpSession).toBe('function');
+    expect(typeof getOrCreateCdpSession).toBe('function');
+  });
+});
+
+describe('withCdpSession finally-detach', () => {
+  // Fake Page surface for unit-testing the helper without spinning up a real
+  // browser. The helper only touches page.context().newCDPSession(page) and
+  // the returned session's .detach(), so this surface is enough.
+  function makeFakePage(detachSpy: { called: number; rejected?: Error }) {
+    const session = {
+      detach: async () => {
+        detachSpy.called++;
+        if (detachSpy.rejected) throw detachSpy.rejected;
+      },
+    };
+    return {
+      context: () => ({
+        newCDPSession: async (_p: unknown) => session,
+      }),
+    } as unknown as Page;
+  }
+
+  test('3. detaches on the success path', async () => {
+    const detachSpy = { called: 0 };
+    const page = makeFakePage(detachSpy);
+    const result = await withCdpSession(page, async (session) => {
+      expect(session).toBeDefined();
+      return 42;
+    });
+    expect(result).toBe(42);
+    expect(detachSpy.called).toBe(1);
+  });
+
+  test('4. detaches even when fn throws (the actual leak fix)', async () => {
+    const detachSpy = { called: 0 };
+    const page = makeFakePage(detachSpy);
+    await expect(
+      withCdpSession(page, async () => {
+        throw new Error('boom');
+      }),
+    ).rejects.toThrow('boom');
+    expect(detachSpy.called).toBe(1);
+  });
+
+  test('5. swallows detach errors so they do not mask fn errors', async () => {
+    const detachSpy = { called: 0, rejected: new Error('already detached') };
+    const page = makeFakePage(detachSpy);
+    await expect(
+      withCdpSession(page, async () => {
+        throw new Error('original');
+      }),
+    ).rejects.toThrow('original');
+    expect(detachSpy.called).toBe(1);
+  });
+
+  test('6. swallows detach errors on the success path too', async () => {
+    const detachSpy = { called: 0, rejected: new Error('target closed') };
+    const page = makeFakePage(detachSpy);
+    const result = await withCdpSession(page, async () => 'ok');
+    expect(result).toBe('ok');
+    expect(detachSpy.called).toBe(1);
+  });
+});
+
+describe('getOrCreateCdpSession close-detach', () => {
+  function makeFakePage() {
+    const closeListeners: Array<() => void> = [];
+    const session = {
+      detach: async () => {
+        session._detachCount++;
+      },
+      _detachCount: 0,
+    };
+    const page = {
+      context: () => ({
+        newCDPSession: async (_p: unknown) => session,
+      }),
+      once: (event: string, fn: () => void) => {
+        if (event === 'close') closeListeners.push(fn);
+      },
+      _fireClose: () => {
+        for (const fn of closeListeners) fn();
+      },
+    };
+    return { page: page as unknown as Page, session, fireClose: page._fireClose };
+  }
+
+  test('7. caches the session across calls', async () => {
+    const { page } = makeFakePage();
+    const cache = new WeakMap<Page, any>();
+    const s1 = await getOrCreateCdpSession(page, cache);
+    const s2 = await getOrCreateCdpSession(page, cache);
+    expect(s1).toBe(s2);
+  });
+
+  test('8. close hook detaches the session AND clears the cache', async () => {
+    const { page, session, fireClose } = makeFakePage();
+    const cache = new WeakMap<Page, any>();
+    await getOrCreateCdpSession(page, cache);
+    expect(cache.get(page)).toBeDefined();
+    fireClose();
+    // Detach runs synchronously up to the await in the close hook; let it settle.
+    await new Promise((r) => setTimeout(r, 0));
+    expect(cache.get(page)).toBeUndefined();
+    expect(session._detachCount).toBe(1);
+  });
+});