Calling `error` in browser layout `load` function does not work

[hyunbinseo]

Describe the bug

I am trying to trigger an error page if the user-agent is not supported.

if (browser && !checkBrowserFeatures()) error(400);

However, calling error() in a browser layout load function does not work:

// src\routes\+layout.ts

import { browser } from '$app/environment';
import { error } from '@sveltejs/kit';

export const load = () => {
  if (browser) error(400);
};

Uncaught (in promise) Object { status: 400, body: {…} }

Renaming +layout.ts to +page.ts fixes the issue, rendering a 400 error page.

Reproduction

https://github.com/hyunbinseo/svelte-kit-14678

Logs

System Info

System:
  OS: Windows 11 10.0.26100
  CPU: (8) x64 Intel(R) Core(TM) Ultra 7 258V
  Memory: 5.31 GB / 31.48 GB
Binaries:
  Node: 24.10.0 - C:\Users\hyunb\AppData\Local\fnm_multishells\2436_1760085385841\node.EXE
  Yarn: 1.22.22 - C:\Users\hyunb\AppData\Local\fnm_multishells\2436_1760085385841\yarn.CMD
  npm: 11.6.1 - C:\Users\hyunb\AppData\Local\fnm_multishells\2436_1760085385841\npm.CMD
  pnpm: 10.18.1 - C:\Users\hyunb\AppData\Local\fnm_multishells\2436_1760085385841\pnpm.CMD
  Deno: 2.4.0 - C:\Users\hyunb\.deno\bin\deno.EXE
Browsers:
  Chrome: 141.0.7390.66
  Edge: Chromium (140.0.3485.54)
  Firefox: 143.0.4 - C:\Program Files\Mozilla Firefox\firefox.exe
npmPackages:
  @sveltejs/adapter-auto: ^6.1.0 => 6.1.1
  @sveltejs/kit: ^2.43.2 => 2.46.4
  @sveltejs/vite-plugin-svelte: ^6.2.0 => 6.2.1
  svelte: ^5.39.5 => 5.39.11
  vite: ^7.1.7 => 7.1.9

Severity

annoyance

Additional Information

I know this technically isn't an HTTP error, thus the status code is ambiguous.

Still there should be a valid way to programmatically trigger an error page on the client.

Imagine a client-side navigation to a page without 1) server load functions 2) HTTP requests. The JS file including the load function is simply loaded and executed on the browser, but you can still call error(400).