@@ -19,6 +19,18 @@
 
   Also, the server no longer sends spans of tunnel requests when span streaming is on, because these spans only show the SDK's own traffic.
 
+- **feat(nextjs): Send tunnel route requests with `credentials: 'omit'` ([#25509](https://github.com/getsentry/sentry-javascript/pull/25509))**
+
+  With `tunnelRoute` and no tunnel route handler, `@sentry/nextjs` now sends events from the browser with `credentials: 'omit'`, because the tunnel route does not need cookies.
+  If a layer in front of your app checks cookies before it lets requests through (for example Vercel Deployment Protection), you can send them again in your browser `Sentry.init` call:
+
+  ```js
+  // instrumentation-client.ts
+  Sentry.init({
+    transportOptions: { fetchOptions: { credentials: 'same-origin' } },
+  });
+  ```
+
 Work in this release was contributed by @adityareddy-dev, @jpdefo, @Jxxunnn, @philipatkinson, @Shubham-Padkonde, @mikamikasuki, and @mketiku. Thank you for your contributions!
 
 - **feat(deno): Add incoming request options to `denoServeIntegration`**
 
@@ -122,6 +122,38 @@ test('Tunnel requests should not create middleware or fetch spans', async ({ pag
   expect(sentryFetchSpans).toHaveLength(0);
 });
 
+test('Tunnel requests are sent without the cookies of the browser', async ({ page, context, baseURL }) => {
+  test.skip(process.env.NEXT_PUBLIC_E2E_TEST_TUNNEL_MODE === 'handler', 'The handler variants send the cookies');
+  await context.addCookies([{ name: 'session', value: '8f2c1d', url: baseURL }]);
+  const proxyRequestPromise = waitForRequest('nextjs-16-tunnel', ({ envelope }) =>
+    envelope[1].some(
+      ([itemHeader, item]) =>
+        itemHeader.type === 'event' && (item as Event).exception?.values?.[0]?.value === 'Tunnel cookie test',
+    ),
+  );
+
+  await page.goto('/');
+  await page.waitForFunction(() => typeof (window as { __SENTRY__?: unknown }).__SENTRY__ === 'object');
+  const tunnelRequestPromise = page.waitForRequest(
+    request =>
+      request.method() === 'POST' &&
+      new URL(request.url()).origin === baseURL &&
+      (request.postData() ?? '').includes('Tunnel cookie test'),
+  );
+  await page.evaluate(() => {
+    setTimeout(() => {
+      throw new Error('Tunnel cookie test');
+    });
+  });
+
+  const tunnelRequest = await tunnelRequestPromise;
+  const proxyRequest = await proxyRequestPromise;
+
+  expect(await page.evaluate(() => document.cookie)).toBe('session=8f2c1d');
+  expect((await tunnelRequest.allHeaders()).cookie).toBeUndefined();
+  expect(proxyRequest.rawProxyRequestHeaders.cookie).toBeUndefined();
+});
+
 test.describe('Tunnel route handler', () => {
   test.skip(process.env.NEXT_PUBLIC_E2E_TEST_TUNNEL_MODE !== 'handler', 'Only the handler variants use the handler');
 
 
@@ -36,6 +36,12 @@ export function applyTunnelRouteOption(options: BrowserOptions): void {
         tunnelPath += `&r=${regionCode}`;
       }
       options.tunnel = tunnelPath;
+      // The tunnel route is same-origin, so fetch would include credentials (cookies, HTTP authentication) that the
+      // tunnel does not need.
+      options.transportOptions = {
+        ...options.transportOptions,
+        fetchOptions: { credentials: 'omit', ...options.transportOptions?.fetchOptions },
+      };
       DEBUG_BUILD && debug.log(`Tunneling events to "${tunnelPath}"`);
     } else {
       DEBUG_BUILD && debug.warn('Provided DSN is not a Sentry SaaS DSN. Will not tunnel events.');
 
@@ -272,8 +272,8 @@ export type SentryBuildOptions = Omit<
    * - Pass `true` to auto-generate a random, ad-blocker-resistant route for each build
    * - Pass a string path (e.g., '/monitoring') to use a custom route
    *
-   * By default, the tunnel route proxies each request to Sentry with all its headers, cookies included. To forward
-   * only the event payload, create `app/sentry-tunnel/route.ts` (or `src/app/sentry-tunnel/route.ts`, not in a route
+   * By default, the tunnel route proxies each request to Sentry with its headers. To forward only the event payload,
+   * create `app/sentry-tunnel/route.ts` (or `src/app/sentry-tunnel/route.ts`, not in a route
    * group) with:
    *
    * ```ts
@@ -287,6 +287,10 @@ export type SentryBuildOptions = Omit<
    * middleware redirects or blocks unauthenticated requests, exclude the tunnel route in its `matcher` so events
    * can reach Sentry. Matchers have to be static, so use a fixed string route in that case rather than `true`.
    *
+   * Without the route handler, the browser sends tunnel requests with `credentials: 'omit'`, so without cookies or HTTP
+   * authentication. If a layer in front of your app needs them (for example Vercel Deployment Protection or HTTP Basic authentication), set
+   * `transportOptions: { fetchOptions: { credentials: 'same-origin' } }` in `Sentry.init` of `instrumentation-client.ts`.
+   *
    * NOTE: This feature only works with Next.js 11+
    */
   tunnelRoute?: string | boolean;
 
@@ -122,6 +122,53 @@ describe('Random tunnel route generation', () => {
   });
 });
 
+describe('applyTunnelRouteOption() credentials', () => {
+  const SAAS_DSN = 'https://11111111111111111111111111111111@o2222222.ingest.sentry.io/3333333';
+
+  beforeEach(() => {
+    globalWithInjectedValues._sentryRewritesTunnelPath = '/my-error-monitoring-route';
+  });
+
+  it("sends tunnel requests with `credentials: 'omit'`", () => {
+    const options: BrowserOptions = { dsn: SAAS_DSN };
+
+    applyTunnelRouteOption(options);
+
+    expect(options.transportOptions).toEqual({ fetchOptions: { credentials: 'omit' } });
+  });
+
+  it('keeps the fetch options of the user', () => {
+    const options: BrowserOptions = {
+      dsn: SAAS_DSN,
+      transportOptions: { bufferSize: 10, fetchOptions: { credentials: 'same-origin', priority: 'low' } },
+    };
+
+    applyTunnelRouteOption(options);
+
+    expect(options.transportOptions).toEqual({
+      bufferSize: 10,
+      fetchOptions: { credentials: 'same-origin', priority: 'low' },
+    });
+  });
+
+  it('does not change the transport options when the events are not tunneled', () => {
+    const options: BrowserOptions = { dsn: 'https://11111111111111111111111111111111@example.com/3333333' };
+
+    applyTunnelRouteOption(options);
+
+    expect(options.transportOptions).toBeUndefined();
+  });
+
+  it('does not change the transport options in handler mode', () => {
+    globalWithInjectedValues._sentryTunnelRouteMode = 'handler';
+    const options: BrowserOptions = { dsn: SAAS_DSN };
+
+    applyTunnelRouteOption(options);
+
+    expect(options.transportOptions).toBeUndefined();
+  });
+});
+
 describe('applyTunnelRouteOption() in handler mode', () => {
   it.each([
     ['a SaaS DSN', 'https://11111111111111111111111111111111@o2222222.ingest.us.sentry.io/3333333'],