Node v24 errors with "TypeError: RequestInit: Expected signal to be an instance of AbortSignal."

[amchappell9]

The problem

When running tests with Node version 24.5.0, all tests fail with the following error:

 TypeError: RequestInit: Expected signal ("CustomAbortControllerSignal [EventTarget] {
      aborted: false,
      _onabort: null,
      _reason: undefined,
      _nativeCtrl: undefined,
      Symbol(events.maxEventTargetListeners): 0
    }") to be an instance of AbortSignal.

      at new NextRequest (node_modules/next/src/server/web/spec-extension/request.ts:38:10)
      at node_modules/next-test-api-route-handler/dist/src/index.js:138:57
      at handleNodeRequestAndResponse (node_modules/@whatwg-node/server/cjs/createServerAdapter.js:174:16)
      at node_modules/@whatwg-node/server/cjs/createServerAdapter.js:183:25
      at Promise.then (node_modules/@whatwg-node/promise-helpers/cjs/index.js:48:40)
      at requestListener (node_modules/@whatwg-node/server/cjs/createServerAdapter.js:183:14)
      at genericRequestHandler (node_modules/@whatwg-node/server/cjs/createServerAdapter.js:273:20)
      at Object.apply (node_modules/@whatwg-node/server/cjs/createServerAdapter.js:366:20)
      at Server.<anonymous> (node_modules/next-test-api-route-handler/dist/src/index.js:223:9)

Reproduction steps

Reproduction repo

Clone repo, then run npm install. Then run npm run test while using Node version 24 to reproduce error.

[Xunnamius]

Thanks for the report. Will investigate

[Xunnamius]

@amchappell9 Hmm. Although I am getting a strange "module already linked" error with node@24.6, which seems unrelated, I am not able to replicate this with node@24.5.

Do you have a minimum reproducible example? Ah! I read too fast and didn't notice you shared a repro already! Thanks, that is super helpful. Investigation ongoing...

[Xunnamius]

I see now. @whatwg-node/server, which expects globalThis.Request to be a certain thing, ends up passing a Proxy object that wraps a custom AbortSignal instance when globalThis.Request is not said thing. NextRequest undici's Request class doesn't like this.

This seems to be happening because globalThis.Request is being affected by the presence of jest-fixed-jsdom, a package that attempts to workaround certain issues with jsdom proper (e.g. by polyfilling globalThis.Request and globalThis.AbortSignal). Removing the line testEnvironment: 'jest-fixed-jsdom' from your jest.config.js allows the test to pass.

So the fix here is to not run your API-specific tests using jsdom. This makes sense, since your API code will never* run in a "DOM" environment (like a browser) in production.

Still, I think I'll wrap the maybe-a-proxy signal instance coming from @whatwg-node/server with an actual AbortSignal instance, which should paper over this incompatibility. That turned out to be much harder than it seemed. Above fix is probably the best we can do for now.

[amchappell9]

Ah, specifying that the test should run in a node test environment does resolve the issue:

/**
 * @jest-environment node
 */
import { testApiHandler } from "next-test-api-route-handler";
import * as handler from "../../app/api/example/route";

describe("/api/example", () => {
// ...

The jest-fixed-jsdom specification came from a MSW workaround when using Jest. Not sure how common that is, but hopefully that helps someone else.

Thanks for taking the time to look into it, I appreciate it!