| name | runtime-debug |
| description | Debug and verification workflow for runtime-bundle and module-resolution regressions. Use when diagnosing unexpected module inclusions, bundle size regressions, or CI failures related to NEXT_SKIP_ISOLATE, nft.json traces, or runtime bundle selection (module.compiled.js). Covers CI env mirroring, full stack traces via __NEXT_SHOW_IGNORE_LISTED, route trace inspection, and webpack stats diffing.
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| metadata | {"internal":true} |
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:
- Inspect route trace artifacts (
.next/server/.../page.js.nft.json).
- Inspect traced server chunks for forbidden internals (e.g.
next/dist/server/stream-utils/node-stream-helpers.js, node:stream/promises).
- 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:
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
