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Unsampled pageload span is profiled when profileLifecycle is 'trace' (browser UI profiling)

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@Jxxunnn is already working on this.

Since Oct 19, 2026.

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
82/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
javascript, typescript
Domain
frontend, performance

Research direction

Start with packages/browser/src/profiling/UIProfiler.ts and compare notifyRootSpanActive() with the spanStart listener in _setupTraceLifecycleListeners(). Check the existing profiling tests for coverage of an already active, unsampled root span at setup. Done means that case does not start the profiler or send a profile chunk, while sampled root spans still start profiling.

Written by the indexing model from the issue text.

Description

Browser Waiting for: Product Owner
Is there an existing issue for this?
How do you use Sentry?

Sentry Saas (sentry.io)

Which SDK are you using?

@sentry/browser

SDK Version

@sentry/browser 11.8.0 (also reproduced on 11.4.0; UIProfiler.ts on develop is unchanged as of 2026-10-19)

Framework Version

No response

Link to Sentry event

No response

Reproduction Example/SDK Setup
import * as Sentry from '@sentry/browser';

Sentry.init({
  dsn: '<dsn>',
  integrations: [Sentry.browserTracingIntegration(), Sentry.browserProfilingIntegration()],
  tracesSampleRate: 0, // pageload span NOT sampled -> expect NO profiling
  profileSessionSampleRate: 1,
  profileLifecycle: 'trace',
});

The page is served with Document-Policy: js-profiling.

Steps to Reproduce
  1. Initialize the SDK with profileLifecycle: 'trace', profileSessionSampleRate: 1 and tracesSampleRate: 0 (see SDK setup above).
  2. Load the page. The pageload span is not sampled (isRecording() === false, traceFlags === 0).
  3. Observe that new Profiler(...) is called once and a profile_chunk envelope is sent when the pageload span ends.
Expected Result

The pageload span is not sampled, so trace lifecycle profiling should not start. This is how root spans started after setup are already handled.

Actual Result

The profiler starts for the unsampled pageload span and a profile_chunk is sent.

I checked this in Chromium with a window.Profiler stub that counts constructor calls and an in-memory transport:

  • tracesSampleRate: 0, profileSessionSampleRate: 1: profiler started once, profile_chunk sent
  • tracesSampleRate: 0, profileSessionSampleRate: 0: profiler not started (control)
  • tracesSampleRate: 1, profileSessionSampleRate: 1: profiler started once, profile_chunk and the pageload span sent (expected)
  • tracesSampleRate: 0, profileSessionSampleRate: 1, pageload instrumentation off, an unsampled root span started after init: profiler not started

The last case shows that only the root span that is already active at setup skips the sampling check.

Additional Context
Root cause (source walkthrough)

In packages/browser/src/profiling/integration.ts (develop):

  • After profiler.initialize(client), setup calls profiler.notifyRootSpanActive(rootSpan) for the active root span. The comment says "If there is an active, sampled root span already, notify the profiler".
  • Setup calls it again in a setTimeout(0). This is the path that picks up the pageload span that browserTracingIntegration starts in afterAllSetup.

In packages/browser/src/profiling/UIProfiler.ts:

  • The spanStart listener registered by _setupTraceLifecycleListeners() returns early when !span.isRecording() ("Only count sampled root spans").
  • notifyRootSpanActive() only checks the lifecycle mode and _sessionSampled. It then registers the span and calls _beginProfiling() without checking rootSpan.isRecording().

So an unsampled root span that is already active at setup starts the profiler. The unsampled span still emits spanEnd, so the profile runs for the length of the pageload.

#17619, which added the trace lifecycle, describes it as "Starts when the first sampled root span starts".

Impact

With profileSessionSampleRate: 1, unsampled pageload spans can still start UI profiling in supported browsers. Reducing tracesSampleRate, even to 0, does not prevent this startup path. This can add unintended profiling overhead and profile usage, without a sampled pageload span to correlate with the resulting chunks.

Related

#22927 was the same kind of gap in the Node trace lifecycle, there for the session sampling decision.

Suggested fix

Return early in notifyRootSpanActive() when !rootSpan.isRecording(), matching the spanStart listener. Happy to open a PR.

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