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name: tanstack-server-functions description: Use this when implementing TanStack Start server functions and middleware (validation, auth, redirects, request context) in the Cloudflare Workers runtime.

TanStack Start server functions + middleware

Use this skill when you need server-only logic in apps/user-application that’s callable from routes/components, with full type-safety.

Where things go (this repo)

• Server functions: apps/user-application/src/core/functions/*
• Middleware: apps/user-application/src/core/middleware/*
• Server entry (Cloudflare Worker): apps/user-application/src/server.ts

Server function basics (upstream)

Server functions are defined with createServerFn from @tanstack/react-start.

• GET (default): createServerFn().handler(...)
• POST (mutations): createServerFn({ method: "POST" }).handler(...)

They accept a single input parameter (usually passed as { data: ... } from the client) and should validate input at the boundary.

Validation

Upstream supports passing a schema directly:

• createServerFn({ method: "POST" }).inputValidator(MyZodSchema).handler(...)

This repo also commonly uses:

• .inputValidator((data: z.infer<typeof Schema>) => Schema.parse(data))

Validated input is available as ctx.data inside .handler(...).

Middleware composition

Use createMiddleware from @tanstack/react-start.

• type: "function" applies to specific server functions.
• type: "request" applies more broadly (request lifecycle).

Pattern used in this repo:

• Create middleware in apps/user-application/src/core/middleware/* that adds context.
• Create a baseFunction = createServerFn().middleware([...]).
• Build functions off the base.

Examples already in-repo:

• Auth middleware: apps/user-application/src/core/middleware/auth.ts
• Polar SDK middleware: apps/user-application/src/core/middleware/polar.ts
• Example server function: apps/user-application/src/core/functions/example-functions.ts

Auth + request access (this repo)

• Better Auth is wired in the Cloudflare worker entry: apps/user-application/src/server.ts.
• Use getRequest() from @tanstack/react-start/server in middleware to read the current request.
• Auth middleware throws on missing session (see protectedFunctionMiddleware).

Errors, redirects, not-found (upstream)

• Throw new Error("...") for serializable errors.
• Throw redirect({ to: "/login" }) from @tanstack/react-router for auth flows.
• Throw notFound() from @tanstack/react-router when a resource is missing.

Request utilities (upstream)

From @tanstack/react-start/server:

• getRequest() / getRequestHeader() for request inspection
• setResponseHeader() / setResponseStatus() for custom responses
• getRequestIP() for IP (already used in apps/user-application/src/core/functions/payments.ts)

Static server functions (upstream, experimental)

Use staticFunctionMiddleware from @tanstack/start-static-server-functions:

• createServerFn({ method: "GET" }).middleware([staticFunctionMiddleware]).handler(...)

Important: staticFunctionMiddleware must be the final middleware.

Cloudflare bindings

• Access bindings via import { env } from "cloudflare:workers".
• Use .dev.vars for local secrets in apps/user-application.

Client integration (TanStack Query)

Server functions can be passed directly to TanStack Query:

• useMutation({ mutationFn: myServerFn })
• useQuery({ queryFn: () => myServerFn({ data: ... }) })

This repo includes a demo at apps/user-application/src/components/demo/middleware-demo.tsx.