Vercel Next JS Runtime Debug

Runtime Debug

Use this skill when reproducing runtime-bundle, module-resolution, or user-bundle inclusion regressions.

Local Repro Discipline

• Mirror CI env vars when reproducing CI failures.
• Key variables: IS_WEBPACK_TEST=1 forces webpack (turbopack is default), NEXT_SKIP_ISOLATE=1 skips packing next.js.
• For module-resolution validation, always rerun without NEXT_SKIP_ISOLATE=1.

Stack Trace Visibility

Set __NEXT_SHOW_IGNORE_LISTED=true to disable the ignore-list filtering in dev server error output. By default, Next.js collapses internal frames to at ignore-listed frames, which hides useful context when debugging framework internals. Defined in packages/next/src/server/patch-error-inspect.ts.

User-Bundle Regression Guardrail

When user next build starts bundling internal Node-only helpers unexpectedly:

1. Inspect route trace artifacts (.next/server/.../page.js.nft.json).
2. Inspect traced server chunks for forbidden internals (e.g. next/dist/server/stream-utils/node-stream-helpers.js, node:stream/promises).
3. Add a test-start-webpack assertion that reads the route trace and traced server chunks, and fails on forbidden internals. This validates user-project bundling (not publish-time runtime bundling).

Bundle Tracing / Inclusion Proof

To prove what user bundling includes, emit webpack stats from the app's next.config.js:

// next.config.js
module.exports = {
  webpack(config) {
    config.profile = true
    return config
  },
}

Then use stats.toJson({ modules: true, chunks: true, reasons: true }) and diff webpack-stats-server.json between modes. This gives concrete inclusion reasons (e.g. which module required node:stream/promises) and is more reliable than analyzer HTML alone.

Related Skills

• $flags - flag wiring (config/schema/define-env/runtime env)
• $dce-edge - DCE-safe require patterns and edge constraints
• $react-vendoring - entry-base boundaries and vendored React

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