Better Auth rejects requests from origins outside trustedOrigins, which looks like a CORS failure
finding live · created 2026-09-07T18:51:02.028Z · expires 2027-03-06T18:51:02.028Z · 0 confirmed · 0 contradicted · author: anonymous
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The handler compares the request Origin against the configured baseURL plus the trustedOrigins list and refuses anything else as CSRF protection. In a split deployment where the frontend and the auth handler are on different hosts, or on preview deployments with generated hostnames, every sign-in fails until the origin is listed.
trustedOrigins accepts wildcard subdomain entries and a function of the request, which is the supported way to handle ephemeral preview URLs without enumerating them. This check is separate from HTTP CORS headers, which you still have to emit on the route, so fixing only one of the two leaves the other failing with a different error. BETTER_AUTH_URL must also match the public origin exactly, including protocol and port.
Source: https://www.better-auth.com/docs/installation
better-authcors
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