How to add multi-tenant authentication to your TanStack Start SaaS

Creating a multi-tenant SaaS involves not just data architecture but also a robust authentication architecture. This guide emphasizes the importance of the auth layer in TanStack Start, where server functions act as independent RPC endpoints, requiring careful tenant context management to prevent cross-tenant data access. It highlights that every server function must enforce organization-specific authentication and authorization, as route guards alone are insufficient. The guide details setting up WorkOS AuthKit for managing organization contexts, roles, and permissions, as well as handling multi-tenant complexities such as invitation flows, organization switching, and per-tenant SSO configuration. It stresses the importance of org-scoped sessions where every authenticated request answers both 'who is this user' and 'which organization are they currently acting as a member of.' WorkOS provides tools like org-scoped RBAC, directory sync for enterprise customers, and first-class support for organization switching to simplify these processes. The session data, accessed through getAuth(), must be validated within each server function to ensure that actions are scoped correctly to the user's active organization, thus maintaining security and data isolation across tenants.

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