fix(browser): Skip browserTracingIntegration setup for bot user agents (#19708)

closes #19670

When browserTracingIntegration initializes, it creates a 30-second
setTimeout (idle span final timeout), multiple PerformanceObserver
instances, and various other timers. These keep the JS event loop
active, which prevents Googlebot's headless Chromium renderer from
considering the page "idle" — resulting in incomplete or broken page
snapshots in Google Search Console.

This PR detects known bot/crawler user agents and skips the tracing
setup entirely, so no timers or observers are created. Error monitoring
via other integrations is unaffected.

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: chargome

.size-limit.js

@@ -82,7 +82,7 @@ module.exports = [
     path: 'packages/browser/build/npm/esm/prod/index.js',
     import: createImport('init', 'browserTracingIntegration', 'replayIntegration', 'replayCanvasIntegration'),
     gzip: true,
-    limit: '86 KB',
+    limit: '87 KB',
   },
   {
     name: '@sentry/browser (incl. Tracing, Replay, Feedback)',
@@ -255,7 +255,7 @@ module.exports = [
     path: createCDNPath('bundle.tracing.logs.metrics.min.js'),
     gzip: false,
     brotli: false,
-    limit: '131 KB',
+    limit: '132 KB',
   },
   {
     name: 'CDN Bundle (incl. Replay, Logs, Metrics) - uncompressed',
@@ -269,7 +269,7 @@ module.exports = [
     path: createCDNPath('bundle.tracing.replay.min.js'),
     gzip: false,
     brotli: false,
-    limit: '245 KB',
+    limit: '246 KB',
   },
   {
     name: 'CDN Bundle (incl. Tracing, Replay, Logs, Metrics) - uncompressed',
@@ -308,7 +308,7 @@ module.exports = [
     import: createImport('init'),
     ignore: ['$app/stores'],
     gzip: true,
-    limit: '43 KB',
+    limit: '44 KB',
   },
   // Node-Core SDK (ESM)
   {

packages/browser/src/tracing/browserTracingIntegration.ts

@@ -54,6 +54,22 @@ import { defaultRequestInstrumentationOptions, instrumentOutgoingRequests } from
 
 export const BROWSER_TRACING_INTEGRATION_ID = 'BrowserTracing';
 
+/**
+ * We don't want to start a bunch of idle timers and PerformanceObservers
+ * for web crawlers, as they may prevent the page from being seen as "idle"
+ * by the crawler's rendering engine (e.g. Googlebot's headless Chromium).
+ */
+const BOT_USER_AGENT_RE =
+  /Googlebot|Google-InspectionTool|Storebot-Google|Bingbot|Slurp|DuckDuckBot|Baiduspider|YandexBot|Facebot|facebookexternalhit|LinkedInBot|Twitterbot|Applebot/i;
+
+function _isBotUserAgent(): boolean {
+  const nav = WINDOW.navigator as Navigator | undefined;
+  if (!nav?.userAgent) {
+    return false;
+  }
+  return BOT_USER_AGENT_RE.test(nav.userAgent);
+}
+
 interface RouteInfo {
   name: string | undefined;
   source: TransactionSource | undefined;
@@ -384,6 +400,8 @@ export const browserTracingIntegration = ((options: Partial<BrowserTracingOption
     ...options,
   };
 
+  const _isBot = _isBotUserAgent();
+
   let _collectWebVitals: undefined | (() => void);
   let lastInteractionTimestamp: number | undefined;
 
@@ -484,6 +502,11 @@ export const browserTracingIntegration = ((options: Partial<BrowserTracingOption
   return {
     name: BROWSER_TRACING_INTEGRATION_ID,
     setup(client) {
+      if (_isBot) {
+        DEBUG_BUILD && debug.log('[Tracing] Skipping browserTracingIntegration setup for bot user agent.');
+        return;
+      }
+
       registerSpanErrorInstrumentation();
 
       _collectWebVitals = startTrackingWebVitals({
@@ -630,6 +653,10 @@ export const browserTracingIntegration = ((options: Partial<BrowserTracingOption
     },
 
     afterAllSetup(client) {
+      if (_isBot) {
+        return;
+      }
+
       let startingUrl: string | undefined = getLocationHref();
 
       if (linkPreviousTrace !== 'off') {

packages/browser/test/tracing/browserTracingIntegration.test.ts

@@ -86,6 +86,68 @@ describe('browserTracingIntegration', () => {
     Object.defineProperty(WINDOW, 'history', { value: originalGlobalHistory });
   });
 
+  describe('bot user agent detection', () => {
+    let originalNavigator: Navigator;
+
+    beforeEach(() => {
+      originalNavigator = WINDOW.navigator;
+    });
+
+    afterEach(() => {
+      Object.defineProperty(WINDOW, 'navigator', { value: originalNavigator, writable: true, configurable: true });
+    });
+
+    function setUserAgent(ua: string): void {
+      Object.defineProperty(WINDOW, 'navigator', {
+        value: { userAgent: ua },
+        writable: true,
+        configurable: true,
+      });
+    }
+
+    it.each([
+      'Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)',
+      'Mozilla/5.0 (Linux; Android 6.0.1; Nexus 5X Build/MMB29P) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/W.X.Y.Z Mobile Safari/537.36 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)',
+      'Mozilla/5.0 AppleWebKit/537.36 (KHTML, like Gecko; compatible; Bingbot/2.0; +http://www.bing.com/bingbot.htm) Chrome/W.X.Y.Z Safari/537.36',
+      'Mozilla/5.0 (compatible; YandexBot/3.0; +http://yandex.com/bots)',
+      'facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)',
+      'LinkedInBot/1.0 (compatible; Mozilla/5.0)',
+      'Twitterbot/1.0',
+      'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_5) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1.1 Safari/605.1.15 (Applebot/0.1)',
+      'Mozilla/5.0 (compatible; Google-InspectionTool/1.0)',
+    ])('skips tracing setup for bot user agent: %s', ua => {
+      setUserAgent(ua);
+
+      const client = new BrowserClient(
+        getDefaultBrowserClientOptions({
+          tracesSampleRate: 1,
+          integrations: [browserTracingIntegration()],
+        }),
+      );
+      setCurrentClient(client);
+      client.init();
+
+      expect(getActiveSpan()).toBeUndefined();
+    });
+
+    it('does not skip tracing setup for normal user agents', () => {
+      setUserAgent(
+        'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36',
+      );
+
+      const client = new BrowserClient(
+        getDefaultBrowserClientOptions({
+          tracesSampleRate: 1,
+          integrations: [browserTracingIntegration()],
+        }),
+      );
+      setCurrentClient(client);
+      client.init();
+
+      expect(getActiveSpan()).toBeDefined();
+    });
+  });
+
   it('works with tracing enabled', () => {
     const client = new BrowserClient(
       getDefaultBrowserClientOptions({

packages/nextjs/test/config/conflictingDebugOptions.test.ts

@@ -20,7 +20,15 @@ describe('debug: true + removeDebugLogging warning', () => {
   let originalLocation: unknown;
   let originalAddEventListener: unknown;
 
-  beforeAll(() => {
+  beforeAll(async () => {
+    // Pre-warm V8 compilation cache for the large SDK module graphs.
+    // Without this, the first dynamic import after vi.resetModules() can hang
+    // because vitest needs to compile the entire module graph from scratch.
+    await import('../../src/client/index.js');
+    await import('../../src/server/index.js');
+    await import('../../src/edge/index.js');
+    vi.resetModules();
+
     dom = new JSDOM('<!doctype html><html><head></head><body></body></html>', { url: 'https://example.com/' });
 
     originalDocument = (globalThis as any).document;