Regression: Since 15.3 environment variables are not available in client components using standalone output mode

[hood]

Link to the code that reproduces this issue

https://github.com/hood/nextjs-env-repro

To Reproduce

1. Build the application using the build command.
2. Run the output with node .next/standalone/server.js.

Current vs. Expected behavior

Client components can’t seem to access environment variables that I’m 100% sure are defined (other components can access them just fine). I’d expect them to be able to access NEXT_PUBLIC_* environment variables.

Let’s refer to my MOCK_ENV_VAR environment variable:

• I’ve set it in the environment where the process is running.
• I’ve set both MOCK_ENV_VAR and NEXT_PUBLIC_MOCK_ENV_VAR in an .env.production file which is correctly getting consumed.
• Component Child is not able to access the environment variable at all, seeing MOCK_ENV_VAR: undefined, NEXT_PUBLIC_MOCK_ENV_VAR: undefined.

I’m using app router, deploying my Next.js application as a standalone server, built with the following command: next build --experimental-build-mode compile, with all caching mechanisms disabled (export const dynamic = "force-dynamic”; on all pages). I basically build all my pages afresh on every hit.
This issue only seems to affect my client components, server components are unaffected.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.5.0: Tue Apr 22 19:54:49 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6000
  Available memory (MB): 32768
  Available CPU cores: 10
Binaries:
  Node: 24.1.0
  npm: 11.3.0
  Yarn: 1.22.22
  pnpm: 10.11.0
Relevant Packages:
  next: 15.3.3 // Latest available version is detected (15.3.3).
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.8.3
Next.js Config:
  output: standalone

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

Dynamic Routes, Output

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

Other (Deployed)

Additional context

An interesting thing is that I see the NEXT_PUBLIC_* environment variables consumed by my client components as they should be set under the window global, not inlined as the docs say:
(Excerpt from the output bundle)

"p",
  {
    children: "Called by: "
      .concat("child", " -> MOCK_ENV_VAR=")
      .concat(n.env.MOCK_ENV_VAR, " | NEXT_PUBLIC_MOCK_ENV_VAR=")
      .concat(window.NEXT_PUBLIC_MOCK_ENV_VAR),
  };

I’ve done a quick check rolling Next.js back to 15.2, and the NEXT_PUBLIC_ -prefixed environment variables are back working correctly in my client code.

[hood]

@ijjk #76990 This MR may have introduced the issue, but I don’t get where it stemmed from so I don’t get if it was an inteded change or not.

I’ve tried running next build --experimental-build-mode=compile && next build --experimental-build-mode=generate (at least this is what I figured out was the new intended way), but generate breaks trying to fetch data for my sitemap.ts, whereas I was using the command to avoid data fetching completely during my builds.

I’ve tried running generate-env instead of generate, which successfully completes the build, but I then get errors like this breaking my pages, caused by the output code getting 404s when trying to fetch some chunks:

Uncaught ChunkLoadError: Loading chunk 296 failed.

Given that I am navigating the darkness while attempting to reconstruct the correct procedure from limited comments in GitHub discussions, could you kindly provide a recommended method for successfully getting this setup to work again?