Aborting a fetch call to an app router API endpoint does not abort the request when streaming under Vercel's Node Runtime

[acrognale]

Link to the code that reproduces this issue

https://github.com/acrognale/vercel-next-abort-repro

To Reproduce

I've included a standard Next app in the repository that you can start with npm run dev.

Current vs. Expected behavior

When running locally, the request properly aborts. Here is the expected server output:

   ▲ Next.js 15.2.4 (Turbopack)
   - Local:        http://localhost:3000
   - Network:      http://192.168.128.250:3000

 ✓ Starting...
 ✓ Ready in 533ms
 ○ Compiling / ...
 ✓ Compiled / in 744ms
 GET / 200 in 860ms
 ✓ Compiled /favicon.ico in 145ms
 GET /favicon.ico?favicon.45db1c09.ico 200 in 169ms
 ✓ Compiled /api/stream in 125ms
count: 1
count: 2
 GET /api/stream 200 in 2738ms
abort

and on the frontend:

Message 1 at 2025-04-03T16:52:59.275Z
Message 2 at 2025-04-03T16:53:00.276Z
Stream was cancelled!

But when running on Vercel under the Node Runtime, we receive the following:

server:

count: 1
count: 2
count: 3
count: 4
count: 5
count: 6
count: 7
count: 8
count: 9

(and then the function times out because the max duration is 10s.

On the frontend:

Message 1 at 2025-04-03T16:55:04.790Z
Message 2 at 2025-04-03T16:55:05.790Z
Stream was cancelled!

So while the browser is canceling the request, the function on the Node Runtime continues to stream.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.1.0: Thu Oct 10 21:06:57 PDT 2024; root:xnu-11215.41.3~3/RELEASE_ARM64_T6041
  Available memory (MB): 36864
  Available CPU cores: 14
Binaries:
  Node: 22.14.0
  npm: 10.9.2
  Yarn: 1.22.22
  pnpm: 10.6.5
Relevant Packages:
  next: 15.2.4 // Latest available version is detected (15.2.4).
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.8.2
Next.js Config:
  output: N/A

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

Connection

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

Vercel (Deployed)

Additional context

No response