Link to the code that reproduces this issue

https://github.com/rubenferreira97/next-manifest-issue

To Reproduce

1. Clone the repository (next template without changes) and install dependencies with canary.
2. Start the application in development mode with Turbopack (e.g., next dev).
3. Open the application in your browser.
4. Open the developer tools and check the Network tab or the Console.
5. Observe the 404 errors for the following manifest files:
   • /_next/static%5Cdevelopment%5C_clientMiddlewareManifest.js
   • /_next/static%5Cdevelopment%5C_buildManifest.js
   • /_next/static%5Cdevelopment%5C_ssgManifest.js

Current vs. Expected behavior

Current behavior: When running Next.js Canary with Turbopack on Windows, there are 404 errors for the manifest files. The dynamically generated URL paths passed to the client contain URL-encoded backslashes (%5C) instead of forward slashes (/). If the %5C are manually replaced with / in the URL, the files are successfully found.

Expected behavior: Client-facing URLs should always use forward slashes (/) regardless of the host operating system. The manifest files should load successfully without 404 errors.

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Education
  Available memory (MB): 16237
  Available CPU cores: 12
Binaries:
  Node: 25.2.1
  npm: 11.6.2
  Yarn: N/A
  pnpm: 10.29.3
Relevant Packages:
  next: 16.2.0-canary.58 // Latest available version is detected (16.2.0-canary.58).
  eslint-config-next: N/A
  react: 19.2.3
  react-dom: 19.2.3
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Turbopack

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

next dev (local), next build (local), next start (local)

Additional context

The bug stems from packages/next/src/shared/lib/turbopack/manifest-loader.ts.

When constructing the paths for buildManifestPath, ssgManifestPath, and clientMiddlewareManifestPath, the file uses Node's native path.join(). On Windows, path.join() inherently uses \ as the path separator.

These dynamically generated paths are subsequently included in the payload passed to the client (for example via lowPriorityFiles), causing the Next.js client router to fetch the URLs using literal backslashes, which then get URL-encoded into %5C.

Relevant code snippets in manifest-loader.ts: In writeClientBuildManifest():

buildManifestPath = join(CLIENT_STATIC_FILES_PATH, this.buildId, '_buildManifest.js');
ssgManifestPath = join(CLIENT_STATIC_FILES_PATH, this.buildId, '_ssgManifest.js');

In writeMiddlewareManifest():

let clientMiddlewareManifestPath = this.deploymentId && !this.dev 
  ? join(CLIENT_STATIC_FILES_PATH, TURBOPACK_CLIENT_MIDDLEWARE_MANIFEST) 
  : join(CLIENT_STATIC_FILES_PATH, this.buildId, TURBOPACK_CLIENT_MIDDLEWARE_MANIFEST);