Route Handlers Buffer Incoming Request Streams Instead of Processing Chunk-by-Chunk

### Link to the code that reproduces this issue https://github.com/PatrickRogg/nextjs-stream-issue ### To Reproduce 1. Install dependencies: `npm install` 2. Start the Next.js development server: `npm run dev` 3. Go to `http://localhost:3000` 4. Click the button `Start Stream` 5. Look at the logs: This is the output: ``` [CLIENT] Starting POST to http://localhost:3000/api/stream [CLIENT] Enqueuing chunk 1 [CLIENT] Enqueuing chunk 2 [CLIENT] Enqueuing chunk 3 [CLIENT] Closing the stream. --- [SERVER] Request handler invoked at 2025-07-18T09:15:15.340Z --- [SERVER] Chunk #1 received at 2025-07-18T09:15:15.341Z: "This is chunk one. This is the second chunk. This is the final chunk." --- [SERVER] Stream finished at 2025-07-18T09:15:15.341Z --- ``` Shows that the chunks are batched before the API handler receives it. I created the same setup in a Node.js and Express environment, there the chunks are processed one by one, so I assume somewhere in the route handler logic in Next the batching happens. ### Current vs. Expected behavior Current Behavior: Incoming chunks are batched and only once all chunks are received and stream is closed the API handler function receives the chunks. Logs: ``` (a 2-second pause) Chunk #1 received at 2024-09-12T10:00:00.000Z: "This is chunk one.\n" Chunk #2 received at 2024-09-12T10:00:01.000Z: "This is the second chunk.\n" Chunk #3 received at 2024-09-12T10:00:02.000Z: "This is the final chunk.\n" ``` Expected Behavior: Chunks in API route are received right when they are sent and not in a batch at the end. Logs expected: ``` Chunk #1 received at 2024-09-12T10:00:00.000Z: "This is chunk one.\n" (a 1-second pause) Chunk #2 received at 2024-09-12T10:00:01.000Z: "This is the second chunk.\n" (a 1-second pause) Chunk #3 received at 2024-09-12T10:00:02.000Z: "This is the final chunk.\n" ``` ### Provide environment information ```bash Operating System: Platform: linux Arch: x64 Version: #1 SMP Tue Nov 5 00:21:55 UTC 2024 Available memory …