Runtime Debug

When you're chasing down a Next.js regression where the wrong modules end up in the user bundle or resolution breaks mysteriously, this skill walks you through the debugging discipline the Vercel team uses internally. It covers the essential environment variables like IS_WEBPACK_TEST and NEXT_SKIP_ISOLATE that mirror CI conditions, plus the __NEXT_SHOW_IGNORE_LISTED flag that stops Next.js from hiding framework stack traces behind "ignore-listed frames" noise. The guardrails for catching user bundle pollution are especially handy if you've ever wondered why server-only code suddenly appeared in client builds. It's narrow in scope but solves a specific class of painful framework bugs.

skills.sh installs: 990
GitHub stars: 139.5K
last updated: May 16, 2026

View on GitHub

Install

$ npx -y skills add vercel/next.js --skill runtime-debug --agent claude-code

Installs into .claude/skills of the current project.

Files

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.