Tanstack Start Auth

TanStack Start + Better Auth

Single skill for library configuration and Start-specific wiring in FMC apps (TILDA, Trassenscout migration, etc.).

When to apply

• Creating or changing betterAuth() / createAuthClient() config
• Mounting /api/auth/*, tanstackStartCookies() plugin, OAuth sign-in routes
• beforeLoad gates, endpointAuth, session in server functions or API handlers
• Public vs protected server functions (public*.functions.ts naming)
• API key + session dual auth on automation endpoints

References (read as needed)

Live docs: better-auth.com/docs · llms.txt

Related skills

• tanstack-router-conventions — validateSearch, search serialization, loaders + Query
• tanstack-start-conventions — layout, .server.ts / .functions.ts, boundaries, SSR
• playwright-skill — E2E stubbed auth (TILDA)

Non-negotiable rules

1. Session helpers always receive headers: Headers from the current request.
2. Route beforeLoad does not call DB/session directly — server functions + getRequestHeaders().
3. Use official tanstackStartCookies() as the last plugin; verify no client-bundle leak after better-auth version changes (auth.md).
4. /api/auth/$ and other handler-only API routes: ssr: false (selective-ssr.md).
5. API routes: auth inside each handler (no route-level beforeLoad).
6. Admin/role checks with cookie cache: getSession({ query: { disableCookieCache: true } }).
7. Compare API secrets with timing-safe equality.
8. Two layers: beforeLoad = route UX only; endpointAuth / guards = data boundary. Never rely on beforeLoad alone.
9. No auth middleware for route guarding — use layout beforeLoad. Global auth middleware breaks public RPCs (public*.functions.ts, getSessionForRouteFn) and is not the FMC auth-route pattern.

Quick patterns

// beforeLoad (via server fn)
await routeSessionFn({ data: location })

// API handler
const session = await endpointAuth.session(request.headers)

// Server function
const headers = getRequestHeaders()
await endpointAuth.admin(headers)

Setup snapshot

1. betterAuth() in auth.server.ts (Prisma/Drizzle adapter, plugins)
2. createAuthClient() with matching client plugins
3. /api/auth/$ → auth.handler(request) (cookies via tanstackStartCookies() plugin)
4. session.server.ts helpers taking Headers
5. CLI migrate/generate after plugin changes — details in better-auth-config.md