| name | flags |
| description | How to add or modify Next.js experimental feature flags end-to-end. Use when editing config-shared.ts, config-schema.ts, define-env-plugin.ts, next-server.ts, export/worker.ts, or module.compiled.js. Covers type declaration, zod schema, build-time injection, runtime env plumbing, and the decision between runtime env-var branching vs separate bundle variants.
|
| metadata | {"internal":true} |
Feature Flags
Use this skill when adding or changing framework feature flags in Next.js internals.
Required Wiring
All flags need: config-shared.ts (type) → config-schema.ts (zod). If the flag is consumed in user-bundled code (client components, edge routes, app-page.ts template), also add it to define-env.ts for build-time injection. Runtime-only flags consumed exclusively in pre-compiled bundles can skip define-env.ts.
Where the Flag Is Consumed
Client/bundled code only (e.g. __NEXT_PPR in client components): define-env.ts is sufficient. Webpack/Turbopack replaces process.env.X at the user's build time.
Pre-compiled runtime bundles (e.g. code in app-render.tsx): The flag must also be set as a real process.env var at runtime, because app-render.tsx runs from pre-compiled bundles where define-env.ts doesn't reach. Two approaches:
- Runtime env var: Set in
next-server.ts + export/worker.ts. Both code paths stay in one bundle. Simple but increases bundle size.
- Separate bundle variant: Add DefinePlugin entry in
next-runtime.webpack-config.js (scoped to bundleType === 'app'), new taskfile tasks, update module.compiled.js selector, and still set env var in next-server.ts + export/worker.ts for bundle selection. Eliminates dead code but adds build complexity.
For runtime flags, also add the field to the NextConfigRuntime Pick type in config-shared.ts.
Runtime-Bundle Model
- Runtime bundles are built by
next-runtime.webpack-config.js (rspack) via taskfile.js bundle tasks.
- Bundle selection occurs at runtime in
src/server/route-modules/app-page/module.compiled.js based on process.env vars.
- Variants:
{turbo/webpack} × {experimental/stable/nodestreams/experimental-nodestreams} × {dev/prod} = up to 16 bundles per route type.
define-env.ts affects user bundling, not pre-compiled runtime internals.process.env.X checks in app-render.tsx are either replaced by DefinePlugin at runtime-bundle-build time, or read as actual env vars at server startup. They are NOT affected by the user's defines from define-env.ts.- Gotcha: DefinePlugin entries in
next-runtime.webpack-config.js must be scoped to the correct bundleType (e.g. app only, not server) to avoid replacing assignment targets in next-server.ts.
Related Skills
$dce-edge - DCE-safe require patterns and edge constraints$react-vendoring - entry-base boundaries and vendored React$runtime-debug - reproduction and verification workflow
