start-core/server-functions

Use createServerFn from @tanstack/react-start, optionally pass { method: "POST" } (GET is default), chain inputValidator for typed input, and define logic in handler. Call it from loaders, components, or other server functions like a normal async function.

Pass a Zod schema directly to inputValidator, such as z.object({ name: z.string().min(1) }). The handler receives the parsed, type-safe data, and invalid input is rejected before the handler runs.

No. Loaders are isomorphic and run on both client and server, so database queries and secret keys in loaders leak to the client bundle. Wrap server-only logic in createServerFn and call that function from the loader.

No. TanStack Start does not use "use server" directives or getServerSideProps. All server-side logic is defined exclusively through createServerFn, which the build replaces with RPC stubs in client bundles.

createServerFn returns a callable function, not a Promise, so you must invoke it with parentheses: await getItems() or await getItems({ data: { id: "1" } }). Awaiting the reference without calling it returns the function itself.

Import getRequest, getRequestHeader, setResponseHeaders, and setResponseStatus from @tanstack/react-start/server. These utilities work inside server function handlers to read incoming request details and control the outgoing response.