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feat(browser): Record standalone LCP spans #16591

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feat(browser): Record standalone LCP spans #16591

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@Lms24 Lms24 commented Jun 16, 2025 •
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This PR implements sending standalone LCP spans as an opt-in feature.

Behaviour-wise, it's mostly aligned with our prior implementation of sending CLS standalone spans (#13056):

  • add an _experiments.enableStandaloneLcpSpans option and treat it as opt-in
  • keep collecting LCP values until users soft-navigate or the page is hidden
  • then, send the LCP span once
  • adds all lcp.* span attributes as well as the lcp measurement to the span

(depending on if we merge #16589 or this first, we might need to readjust size limit)

closes #13063

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github-actions bot commented Jun 16, 2025 •
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size-limit report 📦

Path Size % Change Change
@sentry/browser 23.99 kB - -
@sentry/browser - with treeshaking flags 23.76 kB - -
⛔️ @sentry/browser (incl. Tracing) (max: 39 kB) 39.02 kB +0.47% +179 B 🔺
@sentry/browser (incl. Tracing, Replay) 77.16 kB +0.23% +174 B 🔺
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags 70.25 kB +0.27% +185 B 🔺
@sentry/browser (incl. Tracing, Replay with Canvas) 81.93 kB +0.23% +188 B 🔺
@sentry/browser (incl. Tracing, Replay, Feedback) 94.02 kB +0.21% +191 B 🔺
@sentry/browser (incl. Feedback) 40.75 kB - -
@sentry/browser (incl. sendFeedback) 28.7 kB - -
@sentry/browser (incl. FeedbackAsync) 33.59 kB - -
@sentry/react 25.76 kB - -
@sentry/react (incl. Tracing) 41 kB +0.39% +159 B 🔺
@sentry/vue 28.36 kB - -
@sentry/vue (incl. Tracing) 40.86 kB +0.39% +155 B 🔺
@sentry/svelte 24.01 kB - -
CDN Bundle 25.5 kB - -
CDN Bundle (incl. Tracing) 39.11 kB +0.5% +191 B 🔺
CDN Bundle (incl. Tracing, Replay) 75 kB +0.23% +168 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback) 80.46 kB +0.21% +162 B 🔺
CDN Bundle - uncompressed 74.5 kB - -
CDN Bundle (incl. Tracing) - uncompressed 116.63 kB +1.05% +1.21 kB 🔺
CDN Bundle (incl. Tracing, Replay) - uncompressed 230.68 kB +0.53% +1.21 kB 🔺
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed 243.5 kB +0.5% +1.21 kB 🔺
@sentry/nextjs (client) 42.63 kB +0.38% +161 B 🔺
@sentry/sveltekit (client) 39.49 kB +0.4% +155 B 🔺
@sentry/node 154.29 kB - -
@sentry/node - without tracing 98.64 kB - -
@sentry/aws-serverless 124.4 kB - -

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@Lms24 Lms24 self-assigned this Jun 16, 2025
Lms24
Lms24 commented Jun 16, 2025
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packages/browser-utils/src/metrics/cls.ts
Comment on lines -61 to -62
// TODO: Figure out if we can switch to using whenIdleOrHidden instead of onHidden
// use pagehide event from web-vitals
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@Lms24 Lms24 Jun 16, 2025 •
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we most definitely don't want to switch to this, because idle fires too early. Removed the comment I added in #16310

edwardgou-sentry
edwardgou-sentry approved these changes Jun 17, 2025
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Thanks!

cursor[bot]
cursor bot reviewed Jun 27, 2025
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Bug: LCP Span Attribute Handling Mismatch

The standalone LCP span's attribute handling is inconsistent with the existing pageload span implementation. It unconditionally sets lcp.* attributes (e.g., lcp.id, lcp.url, lcp.element) without null/truthy checks. Additionally, it does not trim or truncate the lcp.url attribute to 200 characters. The pageload span, however, performs these checks and truncates the URL, leading to different attribute reporting for the same LCP event and potentially very long URLs.

packages/browser-utils/src/metrics/lcp.ts#L106-L114

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

Lines 106 to 114 in a204305

if (entry) {
attributes['lcp.element'] = htmlTreeAsString(entry.element);
attributes['lcp.id'] = entry.id;
attributes['lcp.url'] = entry.url;
attributes['lcp.loadTime'] = entry.loadTime;
attributes['lcp.renderTime'] = entry.renderTime;
attributes['lcp.size'] = entry.size;
}

Fix in Cursor

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@cursor cursor[bot] cursor[bot] left review comments

@s1gr1d s1gr1d s1gr1d approved these changes

@edwardgou-sentry edwardgou-sentry edwardgou-sentry approved these changes

@mydea mydea Awaiting requested review from mydea

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Send Standalone LCP spans
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