basePath doesn't affect the callback URI

[Unaty498]

Here's my code :

import { SvelteKitAuth } from "@auth/sveltekit";
import { env } from '$env/dynamic/private';

export const { handle } = SvelteKitAuth({
    providers: [
        {
            id: "******", // signIn("my-provider") and will be part of the callback URL
            name: "******", // optional, used on the default login page as the button text.
            type: "oidc", // or "oauth" for OAuth 2 providers
            issuer: env.AUTH_ISSUER, // to infer the .well-known/openid-configuration URL
            clientId: env.AUTH_CLIENT_ID, // from the provider's dashboard
            clientSecret: env.AUTH_CLIENT_SECRET, // from the provider's dashboard
            authorization: {
                scope: "openid profile email",
            },
        },
    ],
    trustHost: env.AUTH_TRUSTED_HOST === 'true',
    secret: env.AUTH_SECRET,
    basePath: '/dev/auth'
});

My website is hosted behind a nginx server, and uses a proxy-pass behind the /dev/ route (so the base url is https://domainname/dev/)
In order to account for that, I juste use resolve from $app/path with relative: true in svelte.config.js

But even with basePath: '/dev/auth' instead of the default /auth, the callback URI when logging onto the authentication service is still https://domainname/auth/callback

Am I missing something ?