App Router ignores `reactStrictMode` in `next.config` (Pages Router only today)

[314systems]

Summary

reactStrictMode appears to be applied only for the Pages Router in vinext, but not for the App Router.

Based on current code/comments, this seems known internally (reactStrictMode is marked as partial), but I couldn't find a dedicated issue tracking App Router parity.

Current behavior

• reactStrictMode: true enables StrictMode behavior for Pages Router client rendering.
• App Router is not wrapped/enforced the same way, so StrictMode checks are not activated there.

Expected behavior

For Next.js parity, App Router should respect strict mode behavior as well (or document a clear intentional divergence).

Evidence

In packages/vinext/src/check.ts, reactStrictMode is explicitly marked as partial with detail similar to:

enforced for the Pages Router ... App Router is not yet wrapped (Next.js defaults App Router strict mode on)

Reproduction (high level)

1. Create/use an App Router app on vinext.
2. Set reactStrictMode: true in next.config.
3. Add a small component that makes StrictMode dev behavior obvious (e.g. effect/log patterns).
4. Compare with equivalent Pages Router setup and/or upstream Next.js behavior.

Questions

1. Is this a known parity gap to be fixed?
2. Should we track this as a dedicated compatibility issue?
3. If intentional for now, can we document it more prominently in compatibility docs/check output?