fix(browser): filter implausible lcp values

Co-Authored-By: GPT-5 <noreply@anthropic.com>

Author: logaretm

packages/browser-utils/src/metrics/browserMetrics.ts

@@ -22,7 +22,7 @@ import {
   addTtfbInstrumentationHandler,
   type PerformanceLongAnimationFrameTiming,
 } from './instrument';
-import { trackLcpAsStandaloneSpan } from './lcp';
+import { isValidLcpMetric, trackLcpAsStandaloneSpan } from './lcp';
 import { resourceTimingToSpanAttributes } from './resourceTiming';
 import { getBrowserPerformanceAPI, isMeasurementValue, msToSec, startAndEndSpan } from './utils';
 import { getActivationStart } from './web-vitals/lib/getActivationStart';
@@ -283,7 +283,7 @@ function _trackCLS(): () => void {
 function _trackLCP(): () => void {
   return addLcpInstrumentationHandler(({ metric }) => {
     const entry = metric.entries[metric.entries.length - 1];
-    if (!entry) {
+    if (!entry || !isValidLcpMetric(metric.value)) {
       return;
     }
 

packages/browser-utils/src/metrics/lcp.ts

@@ -15,6 +15,15 @@ import { addLcpInstrumentationHandler } from './instrument';
 import type { WebVitalReportEvent } from './utils';
 import { listenForWebVitalReportEvents, msToSec, startStandaloneWebVitalSpan, supportsWebVital } from './utils';
 
+/**
+ * 60 seconds is the maximum for a plausible LCP value.
+ */
+export const MAX_PLAUSIBLE_LCP_DURATION = 60_000;
+
+export function isValidLcpMetric(lcpValue: number | undefined): lcpValue is number {
+  return lcpValue != null && lcpValue >= 0 && lcpValue <= MAX_PLAUSIBLE_LCP_DURATION;
+}
+
 /**
  * Starts tracking the Largest Contentful Paint on the current page and collects the value once
  *
@@ -34,7 +43,7 @@ export function trackLcpAsStandaloneSpan(client: Client): void {
 
   const cleanupLcpHandler = addLcpInstrumentationHandler(({ metric }) => {
     const entry = metric.entries[metric.entries.length - 1] as LargestContentfulPaint | undefined;
-    if (!entry) {
+    if (!entry || !isValidLcpMetric(metric.value)) {
       return;
     }
     standaloneLcpValue = metric.value;
@@ -56,6 +65,10 @@ export function _sendStandaloneLcpSpan(
   pageloadSpanId: string,
   reportEvent: WebVitalReportEvent,
 ) {
+  if (!isValidLcpMetric(lcpValue)) {
+    return;
+  }
+
   DEBUG_BUILD && debug.log(`Sending LCP span (${lcpValue})`);
 
   const startTime = msToSec((browserPerformanceTimeOrigin() || 0) + (entry?.startTime || 0));

packages/browser-utils/test/metrics/lcp.test.ts

@@ -0,0 +1,104 @@
+import * as SentryCore from '@sentry/core';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { _sendStandaloneLcpSpan, isValidLcpMetric, MAX_PLAUSIBLE_LCP_DURATION } from '../../src/metrics/lcp';
+import * as WebVitalUtils from '../../src/metrics/utils';
+
+vi.mock('@sentry/core', async () => {
+  const actual = await vi.importActual('@sentry/core');
+  return {
+    ...actual,
+    browserPerformanceTimeOrigin: vi.fn(),
+    getCurrentScope: vi.fn(),
+    htmlTreeAsString: vi.fn(),
+  };
+});
+
+describe('isValidLcpMetric', () => {
+  it('returns true for plausible lcp values', () => {
+    expect(isValidLcpMetric(0)).toBe(true);
+    expect(isValidLcpMetric(2_500)).toBe(true);
+    expect(isValidLcpMetric(MAX_PLAUSIBLE_LCP_DURATION)).toBe(true);
+  });
+
+  it('returns false for implausible lcp values', () => {
+    expect(isValidLcpMetric(undefined)).toBe(false);
+    expect(isValidLcpMetric(-1)).toBe(false);
+    expect(isValidLcpMetric(MAX_PLAUSIBLE_LCP_DURATION + 1)).toBe(false);
+  });
+});
+
+describe('_sendStandaloneLcpSpan', () => {
+  const mockSpan = {
+    addEvent: vi.fn(),
+    end: vi.fn(),
+  };
+
+  const mockScope = {
+    getScopeData: vi.fn().mockReturnValue({
+      transactionName: 'test-transaction',
+    }),
+  };
+
+  beforeEach(() => {
+    vi.mocked(SentryCore.getCurrentScope).mockReturnValue(mockScope as any);
+    vi.mocked(SentryCore.browserPerformanceTimeOrigin).mockReturnValue(1000);
+    vi.mocked(SentryCore.htmlTreeAsString).mockImplementation((node: any) => `<${node?.tagName || 'div'}>`);
+    vi.spyOn(WebVitalUtils, 'startStandaloneWebVitalSpan').mockReturnValue(mockSpan as any);
+  });
+
+  afterEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('sends a standalone lcp span with entry data', () => {
+    const lcpValue = 1_234;
+    const mockEntry: LargestContentfulPaint = {
+      name: 'largest-contentful-paint',
+      entryType: 'largest-contentful-paint',
+      startTime: 100,
+      duration: 0,
+      id: 'image',
+      url: 'https://example.com/image.png',
+      size: 1234,
+      loadTime: 95,
+      renderTime: 100,
+      element: { tagName: 'img' } as Element,
+      toJSON: vi.fn(),
+    };
+
+    _sendStandaloneLcpSpan(lcpValue, mockEntry, '123', 'navigation');
+
+    expect(WebVitalUtils.startStandaloneWebVitalSpan).toHaveBeenCalledWith({
+      name: '<img>',
+      transaction: 'test-transaction',
+      attributes: {
+        'sentry.origin': 'auto.http.browser.lcp',
+        'sentry.op': 'ui.webvital.lcp',
+        'sentry.exclusive_time': 0,
+        'sentry.pageload.span_id': '123',
+        'sentry.report_event': 'navigation',
+        'lcp.element': '<img>',
+        'lcp.id': 'image',
+        'lcp.url': 'https://example.com/image.png',
+        'lcp.loadTime': 95,
+        'lcp.renderTime': 100,
+        'lcp.size': 1234,
+      },
+      startTime: 1.1,
+    });
+
+    expect(mockSpan.addEvent).toHaveBeenCalledWith('lcp', {
+      'sentry.measurement_unit': 'millisecond',
+      'sentry.measurement_value': lcpValue,
+    });
+    expect(mockSpan.end).toHaveBeenCalledWith(1.1);
+  });
+
+  it('does not send a standalone lcp span for implausibly large values', () => {
+    _sendStandaloneLcpSpan(MAX_PLAUSIBLE_LCP_DURATION + 1, undefined, '123', 'pagehide');
+
+    expect(WebVitalUtils.startStandaloneWebVitalSpan).not.toHaveBeenCalled();
+    expect(mockSpan.addEvent).not.toHaveBeenCalled();
+    expect(mockSpan.end).not.toHaveBeenCalled();
+  });
+});