# Next.js cookies headers async request APIs synchronous usage runtime error Next 15 official docs

Status: search-observed
Product: Next.js
Last verified: 2026-06-25
Canonical HTML: https://gitdocs.org/fix/next-js-cookies-headers-async-request-apis-synchronous-usage-runtime-error-next-15-official-docs
Machine JSON: https://gitdocs.org/api/fixes/next-js-cookies-headers-async-request-apis-synchronous-usage-runtime-error-next-15-official-docs.json

## Exact Symptom

See the observed Codex queries below.

## Diagnosis

Local code often type-checks under both old and new framework semantics; the failure depends on the installed framework version and current runtime defaults.

## Fix

```
Verifier checks behavior across multiple requests/build modes, not merely compilation. Check request count, response freshness, and render behavior over multiple calls.
```

## Avoid

- Changing local code without checking the current external contract.
- Retrying the same install, build, or API call with no version/source change.
- Applying a broad unsafe bypass when a narrow compatibility fix is available.

## Observed Codex Queries

- Next.js cookies headers async request APIs synchronous usage runtime error Next 15 official docs
- site:nextjs.org/docs/messages/sync-dynamic-apis cookies headers async
- https://nextjs.org/docs/messages/sync-dynamic-apis
- 'Async Request APIs' in https://nextjs.org/docs/app/guides/upgrading/version-15

## Sources

- Next.js 16: https://nextjs.org/blog/next-16
- Official reference opened by Codex: https://nextjs.org/docs/messages/sync-dynamic-apis