+server.ts API endpoint fails to parse body

Describe the bug

The only file new file in the blank svelte-kit project is the +server.ts located in src/routes/api/+server.ts. It just echoes the incoming JSON. Hitting http://localhost:5173/api (with content-type and accept headers set to application/json and body { "sample": "test"} ) will return HTTP 400 Bad request -> and the request never hits the API endpoint.

If I remove the content-type and accept headers it will enter the API endpoint but it would error out on await request.json() with:

Unexpected end of JSON input
SyntaxError: Unexpected end of JSON input

It might be a Windows issue but honestly I am not sure and can’t test atm.

Reproduction

https://github.com/callmeberzerker/body-not-working

Run pnpm install and then pnpm dev.

Logs

Sadly there are none.

System Info

System:
    OS: Windows 10 10.0.22621
    CPU: (16) x64 Intel(R) Core(TM) i9-9900K CPU @ 3.60GHz
    Memory: 16.30 GB / 31.92 GB
  Binaries:
    Node: 19.0.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.19 - C:\Program Files\nodejs\yarn.CMD
    npm: 8.19.2 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Spartan (44.22621.674.0), Chromium (105.0.1343.42)
    Internet Explorer: 11.0.22621.1
  npmPackages:
    vite: ^3.2.0 => 3.2.0

Severity

blocking all usage of SvelteKit

Additional Information

N/A

[Conduitry]

Yeah, when I override to 5.11.0, things seem to work fine. The obvious change in 5.12.0 that would be causing this is https://github.com/nodejs/undici/pull/1681 but I haven’t dug any further into what this was intended to do or whether this is a new bug there or what.

[Conduitry]

Until #7412 is released, the workaround is to use overrides to force undici@5.11.0.