Next.js 16: file uploads via Server Actions lose file on first call when using proxy.ts (dev only; mp3/mp4)

[nghiepdev]

Link to the code that reproduces this issue

https://github.com/nghiepdev/next16-server-action-proxy-uploadfile-mp34-bug

To Reproduce

1. Run next dev.
2. Select a file.
3. Click Upload.
4. Check the terminal — the file is missing on the first attempt.
5. Click Upload again.
6. The terminal now logs the file.

Current vs. Expected behavior

In development mode, when ‎proxy.ts is configured, the first call to a server action that uploads an mp3/mp4 results in the uploaded file being missing or empty. Subsequent calls work. Image uploads do not exhibit this problem.

Expected: All file types (including mp3/mp4) should be consistently available to server actions on the first call in development, regardless of proxy configuration.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.0.0: Wed Sep 17 21:41:50 PDT 2025; root:xnu-12377.1.9~141/RELEASE_ARM64_T6030
  Available memory (MB): 36864
  Available CPU cores: 12
Binaries:
  Node: 24.10.0
  npm: 11.6.1
  Yarn: N/A
  pnpm: 10.18.0
Relevant Packages:
  next: 16.0.0 // Latest available version is detected (16.0.0).
  eslint-config-next: N/A
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Server Actions, Middleware

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

next dev (local)

Additional context

Problem occurs only in development. If I remove proxy.ts, the bug no longer appears.
Also, the issue does not occur with JPEG files.

[Hunter-Matata]

I have the same problem, except that it happens more than once at startup in production.

[vonschappler]

I have the same issue, but this happens with me with images uploads and it can happen if doing multiple uploads - both in dev and production.

[LouisCuvelier]

Same problem here also but with PDFs and in production only.

[Gouttfi]

Same problem here, but for me it seems more stable in production although I haven't done a lot of testing yet. I'm uploading audio files, and it seems that the larger the file, the more likely it is to fail. The FormData type variable is null.

[Ged-Fenir]

Bump

Same problem here with 16.0.1, both dev and production.
Renaming proxy.ts to middleware.ts fixes the problem for now.

[Gouttfi]

Bump

Same problem here with 16.0.1, both dev and production. Renaming proxy.ts to middleware.ts fixes the problem for now.

Did you downgrade your version to 15 or did you stay at 16.0.1?

[LouisCuvelier]

Might be related to #85416 and the PR #85418