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 .