Experimental CPUS set at 1 with github CI

[rharkor]

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/aged-feather-tjtvm3?workspaceId=ws_2QgUDAmd5uhB7RW7KRqfHA

To Reproduce

This is a global issue that can be consistently reproduced by simply cloning the latest version of the Next.js repository. No additional setup or configuration is required.

Current vs. Expected behavior

Current Behavior

It appears that the number of CPUs is being determined at build time rather than at runtime.

Expected Behavior

The number of CPUs should be detected dynamically at execution time.

Documentation should be available explaining how CPU-related parameters and worker threads are handled.

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #21~24.04.1-Ubuntu SMP PREEMPT_DYNAMIC Mon Feb 24 16:52:15 UTC 2
  Available memory (MB): 31197
  Available CPU cores: 24
Binaries:
  Node: 22.14.0
  npm: 10.9.2
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 15.2.4 // Latest available version is detected (15.2.4).
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: N/A
Next.js Config:
  output: N/A

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

Output

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

Other (Deployed)

Additional context

I'm deploying my applications using Docker. The issue occurs when the Docker image is built via GitHub Actions but not when it's built locally on my machine.

After investigating, I discovered that the only notable difference between the two builds is the number of CPUs defined in the generated server.js file:

This raises a few questions:

• Why is the CPU count being determined at build time rather than at runtime?
• Since the build and runtime environments can differ significantly (e.g., GitHub Actions vs. production server), shouldn't the CPU configuration be dynamic?

This issue began about 3–4 days ago. Prior to that, everything was running smoothly.

I also traced the source of this behavior to the following line in the Next.js codebase, which appears to hardcode the CPU count to 1:

next.js/packages/next/src/server/config-shared.ts

Line 1209 in 5566729

cpus: Math.max(

[rharkor]

Workaround:

/* eslint-disable no-process-env */
import type { NextConfig } from "next"

const nextConfig: NextConfig = {
  output: "standalone",
  experimental: {
    cpus: process.env.ENV !== "development" ? 31 : undefined, //! This line
  },
}

export default nextConfig