fix(app-router): honor reactStrictMode

[NathanDrake2406]

Fixes #2676.

Summary

goal	core change	primary files	expected impact
Match the Next.js App Router reactStrictMode contract	Publish __NEXT_STRICT_MODE_APP from resolved config and wrap the shared hydration/CSR root with StrictMode or Fragment	src/index.ts, server/app-browser-entry.ts	App Router strict checks run by default in development; reactStrictMode: false remains an opt-out

Why

Next.js enables Strict Mode by default for the App Router and preserves an explicit reactStrictMode: false. vinext already preserved the resolved boolean | null value, but the App Router build did not publish that value to its browser entry and the browser root was never wrapped. As a result, the App Router ignored both the default and explicit true configuration.

This follows Next.js's implementation: resolve the App Router default in the build define, select StrictMode or Fragment once, and use that wrapper for both hydration and the CSR error fallback. I found no dedicated Next.js behavioral test for this config; the pinned build and client sources below are the executable contract.

What changed

scenario	before	after
App Router, config unset	No Strict Mode wrapper	Strict Mode enabled
App Router, reactStrictMode: true	No Strict Mode wrapper	Strict Mode enabled
App Router, reactStrictMode: false	No Strict Mode wrapper	Fragment used; Strict Mode disabled
Pages Router	Existing behavior	Unchanged
vinext check	Reports reactStrictMode as partial	Reports it as supported

The browser regressions count client renders without scheduling a React state update. The default fixture expects two development renders; an isolated fixture with reactStrictMode: false expects one. The latter exercises the complete path from next.config.ts through Vite's define replacement and the browser root wrapper.

Validation

• Reproduced the original behavior: the App Router fixture rendered once when the test expected the default Strict Mode count of two.
• Verified the default regression fails when the completed root wrapper is forced to Fragment: expected 2, received 1.
• Verified the opt-out regression fails when the browser entry treats raw boolean false as though it were the string "false": expected 1, received 2.
• Verified config define output for unset, explicit true, and explicit false.
• Ran 381 targeted unit tests across the config, compatibility scanner, and App Router browser entry.
• Ran both App Router Playwright regressions: 2 passed.
• Built the vinext package.
• Ran scoped formatting, lint, and type checks. The pre-commit full check and knip check also passed.

Commands

vp check packages/vinext/src/check.ts packages/vinext/src/index.ts packages/vinext/src/server/app-browser-entry.ts tests/check.test.ts tests/react-strict-mode.test.ts tests/e2e/app-router/react-strict-mode.spec.ts tests/fixtures/app-basic/app/react-strict-mode/render-probe.tsx tests/fixtures/app-basic/app/react-strict-mode/page.tsx
vp check tests/e2e/app-router/react-strict-mode.spec.ts tests/fixtures/app-basic/react-strict-mode-disabled/app/layout.tsx tests/fixtures/app-basic/react-strict-mode-disabled/app/page.tsx tests/fixtures/app-basic/react-strict-mode-disabled/next.config.ts tests/fixtures/app-basic/react-strict-mode-disabled/vite.config.ts
vp test run tests/react-strict-mode.test.ts tests/check.test.ts tests/app-browser-entry.test.ts
vp run vinext#build
PLAYWRIGHT_PROJECT=app-router npx playwright test tests/e2e/app-router/react-strict-mode.spec.ts --retries=0

Risk

Low. The change is confined to the App Router client root and its build-time boolean. React Strict Mode's additional checks are development-only, while an explicit false selects Fragment. Existing Pages Router behavior and defaults are unchanged. App Router applications may now expose non-idempotent development behavior that Strict Mode is intended to detect.

References

reference	why it matters
Issue #2676	Bug report and affected configuration
Prior Pages Router work in #2433	Deliberately left the App Router root out of scope
Next.js build define	Defaults App Router Strict Mode on and preserves explicit false
Next.js client root	Selects StrictMode or Fragment
Next.js hydration and CSR wrapper	Shares the wrapped tree between both root paths
Next.js config documentation	Documents the App Router default and explicit opt-out
Browser regressions	Exercise default-on and explicit-false behavior after hydration
Explicit-false config	Drives the full opt-out path from next.config.ts
Opt-out render probe	Observes one render without scheduling another
Config regression	Covers unset, enabled, and disabled define values

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@vinext/cloudflare@2679

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npm i https://pkg.pr.new/vinext@2679

commit: 7db6b7b

[github-actions]

Performance benchmarks

Compared 7db6b7b against base 22f106e using alternating same-runner rounds. Next.js was unchanged and skipped.

0 improved · 0 regressed · 6 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	131.9 KB	132.0 KB	⚫ +0.0%
Client entry size (gzip)	vinext	119.3 KB	119.3 KB	⚫ +0.0%
Dev server cold start	vinext	2.80 s	2.81 s	⚫ +0.6%
Production build time	vinext	3.07 s	3.05 s	⚫ -0.4%
RSC entry closure size (gzip)	vinext	103.2 KB	103.2 KB	⚫ -0.0%
Server bundle size (gzip)	vinext	177.5 KB	177.5 KB	⚫ -0.0%

View detailed results and traces

_{🟢 improvement · 🔴 regression · ⚫ change below 1.5% · paired base/head}