The waitUntil could take longer than the actual request is taking, which is actually a good thing and wanted by Cloudflare. By definition of our implementation we are ending the root span at the end of the request, while the promise inside waitUntil could still generate events. These events are only send while the root span is still active ("Event 1" in the mermaid diagram), but are not send anymore when the root span ended ("Event 2") and are basically dropped.

sequenceDiagram    participant R as Request    participant W as waitUntil    participant S as Sentry Spans    Note over R: Request starts    activate R        R->>S: Root span starts    activate S        Note over R,W: User calls ctx.waitUntil(promise)    R->>W: Promise created    activate W        W->>S: Event 1 ✓    Note over S: Captured (span still active)        Note over R: Request handler returns    deactivate R        R->>S: Root span ends    deactivate S        Note over W: waitUntil continues running...        W->>S: Event 2 ✗    Note over S: LOST!<br/>Span already ended        W->>W: waitUntil completes    deactivate W

Solution

There is a way to end the root span after everything has been finished, which allows us to send everything in one transaction. In order to still have a correct span for the request itself, I had to create a dedicated span just for the request (that can be seen in the "after" picture beneath).

Right now we only send one transaction after everything is finished, which could take up to "request time" + 30 seconds (that is the maximum wait time for awaitUntil). Which means Sentry gets that data after that time. Very soon there will be the concept ofspan streaming which would make these events available earlier.

sequenceDiagram    participant R as Request    participant W as waitUntil    participant S as Root Span    Note over R: Request starts    activate R        R->>S: Root span starts    activate S        Note over R,W: User calls ctx.waitUntil(promise)    R->>W: Promise created    activate W        W->>S: Event 1 ✓        Note over R: Request handler returns    deactivate R        Note over S: Root span stays open!<br/>Waiting for waitUntil...        Note over W: waitUntil continues running...        W->>S: Event 2 ✓    Note over S: Captured!<br/>Same transaction        W->>W: waitUntil completes    deactivate W        R->>S: Root span ends (after all waitUntil done)    deactivate S

Special eyes

There is now an extra span for thefetch, which is basically the request itself. So this can be looked at with a critical eye

Comparison

before:

after:

fix(cloudflare): Missing events inside waitUntil

9b45d97

JPeer264 mentioned this pull request

Dec 12, 2025

fix(cloudflare): Missing events inside waitUntil#18475

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github-actionsbot commentedDec 12, 2025•
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size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	24.81 kB	-	-
@sentry/browser - with treeshaking flags	23.3 kB	-	-
@sentry/browser (incl. Tracing)	41.56 kB	-	-
@sentry/browser (incl. Tracing, Profiling)	46.15 kB	-	-
@sentry/browser (incl. Tracing, Replay)	79.98 kB	-	-
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	69.7 kB	-	-
@sentry/browser (incl. Tracing, Replay with Canvas)	84.65 kB	-	-
@sentry/browser (incl. Tracing, Replay, Feedback)	96.9 kB	-	-
@sentry/browser (incl. Feedback)	41.52 kB	-	-
@sentry/browser (incl. sendFeedback)	29.49 kB	-	-
@sentry/browser (incl. FeedbackAsync)	34.48 kB	-	-
@sentry/react	26.52 kB	-	-
@sentry/react (incl. Tracing)	43.76 kB	-	-
@sentry/vue	29.27 kB	-	-
@sentry/vue (incl. Tracing)	43.37 kB	-	-
@sentry/svelte	24.82 kB	-	-
CDN Bundle	27.24 kB	-	-
CDN Bundle (incl. Tracing)	42.23 kB	-	-
CDN Bundle (incl. Tracing, Replay)	78.75 kB	-	-
CDN Bundle (incl. Tracing, Replay, Feedback)	84.21 kB	-	-
CDN Bundle - uncompressed	80.04 kB	-	-
CDN Bundle (incl. Tracing) - uncompressed	125.39 kB	-	-
CDN Bundle (incl. Tracing, Replay) - uncompressed	241.42 kB	-	-
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	254.18 kB	-	-
@sentry/nextjs (client)	45.98 kB	-	-
@sentry/sveltekit (client)	41.93 kB	-	-
@sentry/node-core	51.5 kB	-	-
@sentry/node	160.31 kB	-	-
@sentry/node - without tracing	92.91 kB	-	-
@sentry/aws-serverless	108.44 kB	+0.01%	+1 B 🔺

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fixup! fix(cloudflare): Missing events inside waitUntil

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github-actionsbot commentedDec 15, 2025•
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node-overhead report 🧳

Note: This is a synthetic benchmark with a minimal express app and does not necessarily reflect the real-world performance impact in an application.

Scenario	Requests/s	% of Baseline	Prev. Requests/s	Change %
GET Baseline	8,932	-	9,114	-2%
GET With Sentry	1,770	20%	1,778	-0%
GET With Sentry (error only)	6,248	70%	6,189	+1%
POST Baseline	1,212	-	1,233	-2%
POST With Sentry	602	50%	616	-2%
POST With Sentry (error only)	1,078	89%	1,086	-1%
MYSQL Baseline	3,350	-	3,399	-1%
MYSQL With Sentry	506	15%	531	-5%
MYSQL With Sentry (error only)	2,755	82%	2,769	-1%

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fixup! fix(cloudflare): Missing events inside waitUntil

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JPeer264 commentedDec 15, 2025

This PR is on hold for now, because we add another span in here and this might not be optimal in all situations. We'll try to use the span streaming implementation and see if we can use the same behavior as before, but send thewait_until spans after the request span has ended.

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