[BUG] `FormData` sent via Service Binding is serialized as `text/plain` in local development

[xiandew]

Describe the bug

When sending a FormData object (containing a File) from a Next.js app (using @opennextjs/cloudflare) to another Worker via a Service Binding, the request body is received as text/plain instead of multipart/form-data.

This appears to happen in the local Node.js environment (using next dev).

Steps to reproduce

Minimal reproduction link: https://github.com/xiandew/opennext-binding-repro

1. Clone the reproduction repository.
2. Install dependencies: pnpm install.
3. Start the backend worker: cd apps/backend && pnpm dev.
4. Start the frontend: cd apps/frontend && pnpm run dev.
5. Click the "Trigger Upload" button on the frontend.
6. Observe the logs in the backend terminal.

Expected behavior

The backend worker should receive a request with Content-Type: multipart/form-data; boundary=... and be able to parse the FormData.

Actual Behavior:
The backend worker receives a request with Content-Type: text/plain;charset=UTF-8.
The body contains the string representation of the file/data rather than the proper multipart binary stream.

@opennextjs/cloudflare version

1.14.3

Wrangler version

4.53.0

next info output

frontend % npx next info                              

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.1.0: Mon Oct 20 19:33:36 PDT 2025; root:xnu-12377.41.6~2/RELEASE_ARM64_T6030
  Available memory (MB): 36864
  Available CPU cores: 12
Binaries:
  Node: 22.3.0
  npm: 10.8.1
  Yarn: 1.22.22
  pnpm: 10.8.0
Relevant Packages:
  next: 15.5.7 // An outdated version detected (latest is 16.0.7), upgrade is highly recommended!
  eslint-config-next: 15.0.3
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.9.3
Next.js Config:
  output: N/A
 ⚠ An outdated version detected (latest is 16.0.7), upgrade is highly recommended!
   Please try the latest canary version (`npm install next@canary`) to confirm the issue still exists before creating a new issue.
   Read more - https://nextjs.org/docs/messages/opening-an-issue

Additional context

No response

[vicb]

@xiandew I won't have time to look at the details here before next week.

One thing that could be worth investigating before that is to try and reproduce outside of the context of (Open) Next. That use a plain worker for the frontend instead of an Open Next app.

If the issue persists, please create a bug at https://github.com/cloudflare/workers-sdk instead.

Thanks!

[xiandew]

All good, thanks for the suggestion. I actually tested that before creating the issue. I’ve added frontend-plain to the repro repo to show that the plain worker can send FormData just fine.

[vicb]

This appears to happen in the local workerd environment (using next dev or opennextjs-cloudflare preview).

Could you clarify this, I only see a failure with next dev vs opennextjs-cloudflare preview working.

-> %  pnpm opennextjs-cloudflare preview                           

 ⛅️ wrangler 4.53.0
───────────────────
Your Worker has access to the following bindings:
Binding                          Resource      Mode
env.BACKEND (repro-backend)      Worker        local [connected]
env.ASSETS                       Assets        local


[...]

Sending FormData via Service Binding...
Response status: 200
Response body: Success: Received file "test.txt" with size 12 and type text/plain
[wrangler:info] POST / 200 OK (45990ms)

--

-> % pnpm dev

> frontend@0.0.0 dev /Users/vberchet/code/work/opennext-binding-repro/apps/frontend
> next dev

   ▲ Next.js 15.5.7
   - Local:        http://localhost:3000
   - Network:      http://127.0.2.2:3000

[...]

Sending FormData via Service Binding...
Response status: 400
Response body: Error: Expected multipart/form-data, got text/plain;charset=UTF-8
 POST / 200 in 60ms

[xiandew]

- This appears to happen in the local `workerd` environment (using `next dev` or `opennextjs-cloudflare preview`).
+ This appears to happen in the local Node.js environment (using `next dev`).

Yes, you're right. I’ve updated the description. It looks like the issue only occurs with next dev and not with opennextjs-cloudflare preview.

[vicb]

For some reason it looks like Next does not use native FormData so a fix would be:

    const payload = new Response(fd);

    const res = await env.BACKEND.fetch('http://internal/upload', {
      method: 'POST',
      headers: payload.headers,
      body: payload.body,
      duplex: 'half',
    });