fix: respect pageExtensions in prerender and exclude _ prefixed api files

[james-elicx]

Summary

• run-prerender.ts: pagesRouter/apiRouter were called without pageExtensions, so the prerender phase always scanned with default extensions (tsx,ts,jsx,js) regardless of next.config.js. Now passes config.pageExtensions.
• cli.ts: Hoisted resolveNextConfig so nextConfig.pageExtensions is forwarded to printBuildReport (previously the result was discarded inline).
• pages-router.ts (scanApiRoutes): Added _-prefix exclude to match Next.js behaviour — pages/api/_helpers.ts was becoming a /api/_helpers route.
• report.ts: Tightened printBuildReport signature — pageExtensions is now required (not optional), making callers explicit.

Testing

• New test in tests/page-extensions-routing.test.ts: verifies _-prefixed files in pages/api/ are excluded.
• Existing tests in tests/build-report.test.ts (added in prior work): verify printBuildReport respects pageExtensions for both App and Pages Router.
• All 157 affected tests passing locally.

Notes

With default pageExtensions: ["tsx","ts","jsx","js"], a file like home.spec.tsx in pages/ will still become a route — same as Next.js itself. The correct user-side fix for colocation is pageExtensions: ["page.tsx"], which vinext now respects end-to-end.

[pkg-pr-new]

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npm i https://pkg.pr.new/vinext@591

commit: 7c668a8

[github-actions]

Example	Preview	Production	Original
app-router-cloudflare	preview	production
pages-router-cloudflare	preview	production
app-router-playground	preview	production	original
realworld-api-rest	preview	production
nextra-docs-template	preview	production
benchmarks	preview	production
hackernews	preview	production	original

[james-elicx]

/bigbonk review

[ask-bonk]

Review

Good bug fix — the pageExtensions plumbing was clearly broken in the prerender and build-report paths, and the _-prefix exclusion for API routes is a correct parity fix. The tests are well-structured with proper cleanup. One issue:

Bug: appRouter in run-prerender.ts still missing pageExtensions

The PR fixes pagesRouter/apiRouter calls in run-prerender.ts (lines 227-228) but the appRouter call on line 195 still reads:

const routes = await appRouter(appDir);

This should be:

const routes = await appRouter(appDir, config.pageExtensions);

Without this, the App Router prerender phase still scans with default extensions — the exact same class of bug this PR fixes for Pages Router. A project with pageExtensions: ["page.tsx", "page.ts"] in next.config.js would get phantom app routes during prerender.

Everything else looks correct

• cli.ts: Hoisting resolveNextConfig is clean — avoids the double-resolve and makes the config available to printBuildReport. The diff is minimal and correct.
• report.ts: Making pageExtensions required (not optional) is the right call — forces callers to be explicit.
• pages-router.ts: The _-prefix exclude in scanApiRoutes is consistent with how scanPageRoutes already handles it (line 76). Node.js glob's exclude callback applies to both files and directory entries, so _helpers.ts (file) and _utils/ (directory) are both correctly excluded.
• Tests: Good coverage — the build-report.test.ts additions verify the end-to-end printBuildReport path with both routers and both include/exclude cases. The page-extensions-routing.test.ts addition specifically tests the _-prefix exclusion. Proper cache invalidation and tmp cleanup throughout.

Minor: afterEach cleanup in build-report.test.ts only invalidates app route cache

In the printBuildReport respects pageExtensions describe block, afterEach calls invalidateAppRouteCache() but not invalidateRouteCache(pagesDir). The pages router tests do call invalidateRouteCache in their finally blocks, so this isn't a correctness issue in practice — but if a pages router test were to fail before reaching finally, the cache wouldn't be cleaned up by afterEach. Consider adding invalidateRouteCache for the pages dir in afterEach too for robustness.

[ask-bonk]

Review posted successfully on PR #591. Here's a summary of the findings:

Key issue found: run-prerender.ts:195 — the appRouter(appDir) call is still missing config.pageExtensions, which means the App Router prerender phase has the same bug this PR is fixing for Pages Router. This should be appRouter(appDir, config.pageExtensions).

Everything else is correct:

• cli.ts hoisting of resolveNextConfig is clean and avoids a double-resolve
• report.ts making pageExtensions required is the right API tightening
• pages-router.ts _-prefix exclusion in scanApiRoutes is consistent with how scanPageRoutes already works
• Tests provide good coverage for both the pageExtensions and _-prefix behaviors

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