Migrate from Next.js App Router to TanStack Start

This is a step-by-step migration checklist. Complete tasks in order.

CRITICAL: TanStack Start is isomorphic by default. ALL code runs in both environments unless you use createServerFn. This is the opposite of Next.js Server Components, where code is server-only by default.

CRITICAL: TanStack Start uses createServerFn, NOT "use server" directives. Do not carry over any "use server" or "use client" directives.

CRITICAL: Types are FULLY INFERRED in TanStack Router/Start. Never cast, never annotate inferred values.

Pre-Migration

• Create a migration branch

git checkout -b migrate-to-tanstack-start

• Install TanStack Start