Next.js 14 fetch requests show user_agent=undici in the backend logs

[ADTC]

Although Undici is removed in Next.js 14, the fetch requests coming to the backend server from the Next.js instance in Vercel (or a local instance) show user_agent=undici.

Is this a misconfiguration in our code, or a bug in Next.js 14?

[icyJoseph]

This is coming from Node's side right?

1. Go to: https://public.requestbin.com/r
2. Copy the link of the bin
3. Create a local script test.js

async function run() {
  const req = await fetch('<request bin url>');
  console.log(req.status);
}

run();

4. node test.js with node 18+ (the ones that have native Fetch)
5. Go back to the request bin and see the User Agent

The user-agent is undici, no Next.js was used at all.

• More informative User-Agent header nodejs/undici#1475

[ADTC]

Yeah, I'm also getting the same result. Node version is 20.5.1.

So this is an issue with Node?

Actually, I don't understand, if Node uses native fetch since version 18, why is the user-agent undici?

[ADTC]

@icyJoseph is this issue related?

• Help for users to migrate from node-fetch to native fetch in V18 nodejs/undici#2170

[icyJoseph]

Because Node's fetch family of APIs are built using undici.

[ADTC]

Okay, I think I must have misunderstood what "drop undici" #56899 in Next.js 14 changelog meant. If I now understand correctly, it means the polyfill to add Undici in Node 16 environments was removed, because Next.js 14 dropped support for Node 16.

Since Next.js 14 only supports Node 18 and above, the polyfill Undici is no longer required, and Next.js automatically uses the native built-in Undici of Node. That's why it was removed from Next.js. But Node.js natively uses Undici under the hood.

Is this correct?

[ADTC]

@icyJoseph thank you btw. Much appreciated. 🙂

[icyJoseph]

You got it right.