Interceptors returning null in React native caused by breaking change in msw 2.14.0

[williamsaar]

Prerequisites

• I confirm my issue is not in the opened issues
• I confirm the Frequently Asked Questions didn't contain the answer to my issue

Environment check

• I'm using the latest msw version
• I'm using Node.js version 20 or higher

Node.js version

24

Reproduction repository

https://github.com/williamsaar

Reproduction steps

I am using msw for tests in React native. We're using RTK Query for fetching data and msw to intercept those requests with mocked data for our tests. When updating msw from 2.12.4 to 2.14.6 the tests no longer worked because RTK Query received null for all requests, instead of our mocked data.

1. React Native project using MSW for tests (e.g. Jest + RNTL).
2. Handler returning JSON: http.get('/api/foo', () => HttpResponse.json({ ok: true })).
3. Consumer reads body: const data = await response.json() or via RTK Query transformResponse.
4. With MSW ≤2.12.x: body is received correctly.
5. With MSW ≥2.14.0: body is null / "".

This was caused by an update to the @mswjs/interceptors lib PR #778 - released as v0.41.7 on April 29, 2026, the same day as MSW 2.14.0.

Root cause

@mswjs/interceptors PR #778 - released as v0.41.7 on April 29, 2026, the same day as MSW 2.14.0.

What changed in interceptors v0.41.7

In FetchInterceptor's respondWith, the body construction changed from:

// BEFORE (≤v0.41.6) — safe for React Native
const response = decompressedStream === null
  ? rawResponse                                        // ← passes rawResponse as-is
  : new FetchResponse(decompressedStream, rawResponse)
FetchResponse.setUrl(request.url, response)

to:

// AFTER (v0.41.7+) — breaks React Native
const response = new FetchResponse(
  decompressedStream || rawResponse.body,              // ← reads .body
  { url: request.url, status: ..., statusText: ..., headers: ... }
)

Why it breaks React Native

React Native's whatwg-fetch polyfill stores the body as _bodyText and doesn't expose a .body getter returning a ReadableStream. So rawResponse.body is undefined, new FetchResponse(undefined, ...) gets a null body, and .text() returns "" - which RTK Query's transformResponse sees as null.

The connection to MSW versions

• MSW 2.12.14 -> USed "@mswjs/interceptors": "^0.41.2", which resolved to ≤0.41.3, which still used the safe rawResponse passthrough.
• MSW 2.14.0 -> Used "@mswjs/interceptors": "^0.41.3", released the same day as interceptors v0.41.7, so a fresh install resolves to 0.41.7, pulling in the breaking body change.

Workaround

Adding this to the msw.polyfill.js patches the Response.prototype.body top return _bodyText:

if (
  typeof global.Response !== "undefined" &&
  !Object.getOwnPropertyDescriptor(global.Response.prototype, "body")
) {
  Object.defineProperty(global.Response.prototype, "body", {
    get() {
      const text = this._bodyText;
      return text != null ? text : null;
    },
    configurable: true,
  });
}

Full msw.polyfill.js for reference:

import "fast-text-encoding";
import "react-native-url-polyfill/auto";

function defineMockGlobal(name) {
  if (typeof global[name] === "undefined") {
    global[name] = class {
      constructor(type, eventInitDict) {
        this.type = type;
        Object.assign(this, eventInitDict);
      }
    };
  }
}

["MessageEvent", "Event", "EventTarget", "BroadcastChannel"].forEach(defineMockGlobal);

if (
  typeof global.Response !== "undefined" &&
  !Object.getOwnPropertyDescriptor(global.Response.prototype, "body")
) {
  Object.defineProperty(global.Response.prototype, "body", {
    get() {
      const text = this._bodyText;
      return text != null ? text : null;
    },
    configurable: true,
  });
}

Current behavior

rawResponse.body is undefined in React Native's polyfill, so new FetchResponse(undefined, ...) is constructed with a null body.

Expected behavior

Mocked response body is forwarded correctly to the React Native fetch consumer regardless of whether the underlying Response polyfill exposes .body as a ReadableStream.

[kettanaito]

Hi @williamsaar. Thanks for reporting this.

React Native not having a public .body property on the Response is a huge violation of the spec and a bug in whichever response implementation they are using. If that's the case, this isn't something MSW can/should address.