Stop tracing Next.js control-flow errors as span errors#438
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The `trace()` helper marked spans as errored via `span.setStatus({ code: 2, message })` for every rejection of the traced function. This included errors that Next.js uses for control flow and that the evaluation layer deliberately re-throws via `isInternalNextError`: redirects, notFound, and the rejected hanging promises of aborted prerenders (`HANGING_PROMISE_REJECTION`).
The hanging-promise case is the noisy one in practice. Whenever a runtime prefetch prerender is aborted while a flag evaluation awaits `connection()` or `cookies()` (for example through an `identify` function that reads the request), the evaluation promise rejects with the `HANGING_PROMISE_REJECTION` digest, and every affected flag span reported "During prerendering, `connection()` rejects when the prerender is complete" as its status description. On heavily prefetched routes this produces a large volume of error-annotated spans for behavior that is entirely expected.
The tracing helper now skips the error status for internal Next.js errors in both the promise rejection path and the synchronous throw path, while still ending the span and preserving span-context attributes. Control flow is unchanged: the error keeps propagating to the caller. `isInternalNextError` moves from `src/next/` to `src/lib/` so the shared tracing module can use it without depending on the Next.js entrypoint; it is a plain digest check with no Next.js imports.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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eps1lon
marked this pull request as ready for review
July 22, 2026 16:41
dferber90
reviewed
Jul 23, 2026
| message: error instanceof Error ? error.message : undefined, | ||
| }); | ||
| span.setStatus({ | ||
| code: 2, // 2 = Error |
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Span status is now UNSET for ignored errors. When
isInternalNextErroris true you skipsetStatusentirely, so the span ends with no status rather than OK. That matches the changeset ("end without an error status") and is defensible — a redirect isn't really "Ok" either — but if you'd rather these read as successful in trace UIs, setcode: 1. Worth a deliberate decision rather than falling out of the control flow.
@eps1lon wdyt, should hat set code: 1?
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Yeah all of these are ok. We just use them for control flow. So if code:1 shows as successful in the UI, we should use that.
dferber90
approved these changes
Jul 23, 2026
When `isIgnoredError` classifies an error as control flow, the traced function completed as intended, so the span now gets an explicit Ok status instead of ending with an unset status. This keeps ignored control-flow rejections (like Next.js hanging promise rejections of aborted prerenders) clearly distinguishable from both errored spans and spans that were never finalized. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
AndyBitz
approved these changes
Jul 24, 2026
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Follow-up to #397. Flags SDK was still reporting errored OTEL spans even though the error is considered handled from Next.js' perspective. The OTEL spans with errors could create alerts even though everything would be working correctly.