Build fails with vanilla-extract and edge runtime

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: https://github.com/vanilla-extract-css/vanilla-extract/issues/1554

Start with the linked reproduction: run pnpm install and pnpm build, confirming the middleware-manifest.json conflict on the reported Next.js version. Compare the behavior around the 15.2.0 change and the related vanilla-extract issue; done means clean builds succeed consistently without the conflicting asset error.