Every module is isomorphic by default: it runs in BOTH server and client bundles, and route loaders run on both server (SSR) and client (navigation) — so DB access, filesystem, and secrets must live inside createServerFn or createServerOnlyFn, never a bare loader or module scope. process.env must be read per request, not at module scope, because module-level reads can leak into the client bundle and are undefined on edge runtimes that inject env at request time. VITE_/PUBLIC_-prefixed vars are exposed to the client, so server secrets must never carry that prefix.

Documents every environment-boundary API at a glance — createServerFn, createServerOnlyFn, createClientOnlyFn, createIsomorphicFn, , useHydrated(), and the server-only/client-only import markers — with a decision framework for which to use and complete copy-paste boundary examples.

Do NOT use this skill for initial project setup (use start-core) or for file/directory SOLID organization (use solid-tanstack-start).

TanStack Start Execution Model

Understanding where code runs is the single most important concept in Start, and the #1 source of AI-generated bugs. Targets @tanstack/react-start v1.166.2.