[Instrumentation] next dev --turbopack cold metadata fetch logs unexpected root span type ResolveMetadata.generateMetadata

[reinaldosimoes]

Link to the code that reproduces this issue

https://github.com/reinaldosimoes/nextjs-resolvemetadata-root-span-repro

To Reproduce

1. Clone the reproduction repo.
2. Run npm install.
3. Run npm run dev.
4. Open http://localhost:3000/static-fetch as the first request after a fresh restart.
5. Observe this warning in the terminal:

Unexpected root span type 'ResolveMetadata.generateMetadata'. Please report this Next.js issue https://github.com/vercel/next.js

Notes:

• This repro is minimal and uses @sentry/nextjs instrumentation plus a page whose generateMetadata() performs a normal fetch(...).
• If I warm the app by hitting another route first and then visit /static-fetch, the warning does not appear.
• The second request to /static-fetch also does not warn.

Current vs. Expected behavior

Current behavior:

• In next dev --turbopack, the first cold request to /static-fetch logs:

Unexpected root span type 'ResolveMetadata.generateMetadata'. Please report this Next.js issue https://github.com/vercel/next.js
GET /static-fetch 200 in 590ms (next.js: 133ms, application-code: 457ms)

• The page still returns 200.

Expected behavior:

• A normal generateMetadata() code path should not produce a warning about an unexpected root span type.
• ResolveMetadata.generateMetadata appears to be a valid documented Next.js OTEL span type, so it seems like the root-span detection/context handling is incorrect in this cold-start path.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.3.0: Wed Jan 28 20:54:46 PST 2026; root:xnu-12377.91.3~2/RELEASE_ARM64_T6000
  Available memory (MB): 16384
  Available CPU cores: 8
Binaries:
  Node: 25.5.0
  npm: 11.8.0
  Yarn: 1.22.19
  pnpm: 10.28.2
Relevant Packages:
  next: 16.2.1-canary.5 // Latest available version is detected (16.2.1-canary.5).
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3
Next.js Config:
  output: N/A

Also reproduced on stable next@16.2.1.

Which area(s) are affected? (Select all that apply)

• Instrumentation
• Metadata
• Turbopack

Which stage(s) are affected? (Select all that apply)

• next dev (local)

Additional context

• The public repro also reproduces on next@16.2.1-canary.5, not just stable 16.2.1.
• I first saw this in a real app on next@16.2.0, then confirmed it still happened after upgrading to 16.2.1.
• In the real app, the warning appeared on a localized weather page whose generateMetadata() did async work with fetch(...).
• I narrowed the public repro down to the smallest case I could find:
  • App Router page
  • generateMetadata() does a normal await fetch("https://example.com")
  • @sentry/nextjs instrumentation active
  • next dev --turbopack
  • first request after restart
• I also checked @sentry/nextjs@10.45.0; the warning still reproduces there.
• It did not reproduce in my reduced repro when the metadata fetch used cache: "no-store".
• This may be related to the recent OTEL parent/root-span work in:
  • [otel] fix missing http.route attribute #90832
  • [otel] set parent span name #90840
• Next.js OTEL docs list ResolveMetadata.generateMetadata as a valid span type:
  https://nextjs.org/docs/14/app/building-your-application/optimizing/open-telemetry