Build fails with vanilla-extract and edge runtime

[miktirr]

Link to the code that reproduces this issue

https://github.com/miktirr/nextjs-build-fail-reproduction

To Reproduce

1. Install dependencies with pnpm install
2. Run pnpm build
3. The build fails to compile: “Conflict: Multiple assets emit different content to the same filename middleware-manifest.json”

Current vs. Expected behavior

The expectation is that build succeeds with every run. Currently a clean build fails.

Provide environment information

Operating System:
  Platform: darwin
  Arch: x64
  Version: Darwin Kernel Version 24.3.0: Thu Jan  2 20:22:00 PST 2025; root:xnu-11215.81.4~3/RELEASE_X86_64
  Available memory (MB): 32768
  Available CPU cores: 16
Binaries:
  Node: 23.9.0
  npm: 10.9.2
  Yarn: 1.22.22
  pnpm: 10.6.1
Relevant Packages:
  next: 15.2.2-canary.4 // Latest available version is detected (15.2.2-canary.4).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.8.2
Next.js Config:
  output: N/A

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

Webpack

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

next build (local)

Additional context

The problem seems to be introduced by version 15.2.0.

There is a related issue open in vanilla-extract’s repo: vanilla-extract-css/vanilla-extract#1554

[samcx]

@miktirr Can you clarify why you need to use Edge Runtime in Layouts?

[miktirr]

@samcx We use Edge Runtime for the whole application (our deploy target is edge network). Having the runtime definition in a low level layout has been economical as it has set the runtime for all the pages.

[samcx]

I think the documentation needs to be updated.

The sentence states Middleware is the only supported option for this, but then it lists the different files you can use it in.

We recommend using the Node.js runtime for rendering your application

There shouldn't be a difference in performance if you just use the default runtime (Node.js) for your Layouts and Pages.

[miktirr]

I’m not sure I follow. Next.js has two server runtimes for applications, right? The Edge Runtime is just the default (and until recently the only) option for Middleware.

We deploy our app to edge network where Node.js and Node.js APIs are not available. This is why we use Edge Runtime not only for the Middleware but for the whole app.

[samcx]

We deploy our app to edge network where Node.js and Node.js APIs are not available.

Is it possible to clarify with edge network you are referring to?

[miktirr]

We currently use Cloudflare’s edge network.

[miktirr]

Is there information we could provide to help resolving the problem? This is currently preventing us from updating past the version 15.1.7.

I checked that the problem exists still in v15.3.0-canary.20. Moving the edge runtime declaration from the layout to the page doesn’t change the situation.

For production build it is possible to circumvent the problem by building twice (the first build fails but seconds succeeds). But the error also happens with the dev mode:

[Karibash]

Hi @samcx,
This issue is not related to whether EdgeRuntime is used or not.
The root cause is a change made to the Webpack plugin in the following PR:
#75927
I’ve created a PR to fix the issue. Could you take a look?
#78011

[Karibash]

Hi @samcx,
It's been about two weeks since I opened the PR, and I haven't heard anything back. Is there anything I can do on my end?