Turbopack passes "development" to SWC plugins during next build

[jantimon]

Link to the code that reproduces this issue

https://github.com/jantimon/repro-turbopack-swc-plugin-environment

To Reproduce

1. Clone the repo (contains a minimal SWC plugin that reads TransformPluginMetadataContextKind::Env and replaces ENV_CHECK with the value)
2. Build the SWC plugin: cd swc-plugin-env-check && cargo build --target wasm32-wasip1 --release && cd ..
3. npm install
4. Run next build (uses turbopack by default in Next 16)
5. Check the output: grep -o 'strong>[^<]*</strong' .next/server/app/index.html
6. You'll see development instead of production
7. Compare with next build --webpack which correctly outputs production

Current vs. Expected behavior

Current: next build --turbopack passes "development" as the env to SWC plugins via TransformPluginMetadataContextKind::Env, regardless of whether it's a dev or prod build.

Expected: next build --turbopack should pass "production", same as webpack does.

Mode	Bundler	Env received	Correct?
Dev	Turbopack	"development"	Yes
Dev	Webpack	"development"	Yes
Prod	Webpack	"production"	Yes
Prod	Turbopack	"development"*	No

The root cause looks like a hardcoded "development" in turbopack:

next.js/turbopack/crates/turbopack-ecmascript-plugins/src/transform/swc_ecma_transform_plugins.rs

Lines 240 to 241 in b5da7b7

	//[TODO]: Support env-related variable injection, i.e process.env.NODE_ENV
	"development".to_string(),

//[TODO]: Support env-related variable injection, i.e process.env.NODE_ENV
"development".to_string(),

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.3.0

Binaries:
  Node: 24.13.0
  npm: 11.6.2
  Yarn: 1.22.22
  pnpm: 10.17.1

Relevant Packages:
  next: 16.2.1-canary.28
  eslint-config-next: N/A
  react: 19.2.5
  react-dom: 19.2.5
  typescript: 6.0.2

Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Turbopack, SWC

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

Tested on both 16.2.3 (stable) and 16.2.1-canary.28 (latest canary) — same behavior.

We hit this migrating galaxus.ch to turbopack. Our translation SWC plugin uses the env to split imports per-key in prod vs bundling all translations in dev. With turbopack always reporting dev, prod builds ship the full dictionary +1.4 MB JS transferred.

[Vishwaspatel2401]

Hi @jantimon, I would like to work on this.