after upgrading 15.5.3 -> .6, types for [Next]Response seem off?

[gcb]

Summary

I had some routes.ts returning new Response( buffer_array, { ... and new Reponse( read_stream, {......

Now both body values, using either Response or NextResponse, cause:

Type error: Argument of type 'Buffer<ArrayBufferLike>' is not assignable to parameter of type 'BodyInit | null | undefined'. Type 'Buffer<ArrayBufferLike>' is missing the following properties from type 'URLSearchParams': size, append, delete, get, and 2 more.

or

Argument of type 'ReadStream' is not assignable to parameter of type 'BodyInit | null | undefined'.

If I follow the type for BodyInit, i can see ReadableStream

(still collecting info, but posting in case this is not some mistake on our side)

Additional information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Mon, 24 Nov 2025 15:21:09 +0000
  Available memory (MB): 31397
  Available CPU cores: 12
Binaries:
  Node: 25.2.1
  npm: 11.6.4
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 15.5.6 // An outdated version detected (latest is 16.0.4), upgrade is highly recommended!
  eslint-config-next: 15.1.7
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.9.3

builds codebase without errors:

 $ npm list
[email protected] /home/gcb/code
├── @chakra-ui/[email protected]
├── @emnapi/[email protected] extraneous
├── @eslint/[email protected]
├── @prisma/[email protected]
├── @types/[email protected]
├── @types/[email protected]
├── @types/[email protected]
├── @types/[email protected]
├── @types/[email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
├── [email protected]
└── [email protected]

diff versions before the upgrade that broke the build

Example

No response

[icyJoseph]

How could I repro this? I have a create-next-app fresh, w/

import { NextRequest, NextResponse } from "next/server";

export async function GET(request: NextRequest) {
  const buffer = Buffer.from("Hello World", "utf-8");

  return new Response(buffer, {
    headers: {
      "Content-Type": "text/plain",
      "Content-Length": buffer.length.toString(),
    },
  });
}

Builds fine in 15.5.6 and 16

[gcb]

i'm halfway to publish a minimal project. will get to it tomorrow morning if nothing else blows up.

[gcb]

see my next comment. I think this is a eslint types issue only.

But, I was going to use only code from the nextjs manual on the reproduction project, and realized there's zero documentation on how to serve anything but text/json :)

most "official" source I could find was this issue #15453
(and even that doesn't have what i was using, which was to pass the result of fsPromises.readFile() directly. Though I should have probably been using that readableWebStream one.)

your code doesn't trigger the bug, but this does:

//const buffer = Buffer.from("Hello World", "utf-8"); // pass
const f = await fs.open('/tmp/1', 'r');
const buffer = f.readableWebStream(); //fail
const buffer = f.read(); //fail

[icyJoseph]

But, I was going to use only code from the nextjs manual on the reproduction project, and realized there's zero documentation on how to serve anything but text/json :)

Note taken, I'll add some stuff (eventually) to the BFF guide or route handler examples. For what is worth, even in 15.5.3 I get the error. Likely a node types update? or something like that triggered it for you.

import { NextRequest, NextResponse } from "next/server";
import fs from "fs/promises";
import path from "path";

export async function GET(request: NextRequest) {
  // const buffer = Buffer.from("Hello World", "utf-8");
  const f = await fs.open(
    path.resolve(process.cwd(), "public", "sample.txt"),
    "r"
  );
  const buffer = f.readableWebStream();

  const reader = buffer.getReader();
  const stream = new ReadableStream({
    async pull(controller) {
      const { value, done } = await reader.read();

      if (done) {
        controller.close();
        f.close();
      } else {
        controller.enqueue(value);
      }
    },
  });

  return new Response(stream, {
    headers: {
      "Content-Type": "text/plain",
      "Content-Length": (await f.stat()).size.toString(),
    },
  });
}

Using this from the docs, https://nextjs.org/docs/app/api-reference/file-conventions/route#streaming

[gcb]

yes, it's definitely a change on typescript/types only. Let me try with the readable stream you suggested. As I said, we should have been using that pattern, but currently we just send a Buffer<ArrayBufferLike> as the body parameter.

[gcb]

let a script running to upgrade one dependency at a time and build (like a weird npm upgrade bissect :)

[email protected] -> 9.39.1 - OK
[email protected] -> 15.5.6 - OK
[email protected] -> 4.20.6 - OK
[email protected] -> 5.9.3 - OK
[email protected] -> 8.48.0 - FAIL

(the upgrades are cumulative, if it pass, upgrade the next and build)

EDIT: read the logs from my hacky script wrong... the failure actually occurs on [email protected] -> 5.9.3 upgrade. If one keeps [email protected], typescript-eslint can be upgraded to any version.