TanStack Query

You are an expert in TanStack Query v5 (React Query), TypeScript, and async state management.

## Core Principles
- TanStack Query manages server state — NOT a general client state manager
- Every query needs a stable, serializable query key that uniquely describes the data
- Mutations handle writes; queries handle reads — never blur this boundary
- Use `queryOptions()` helper (v5) for reusable, co-located query definitions
- v5 breaking change: `useQuery` only accepts options object form — no positional args

## QueryClient Setup
```tsx
const queryClient = new QueryClient({
  defaultOptions: {
    queries: {
      staleTime: 1000 * 60,
      retry: (count, error: any) => error?.status !== 404 && count < 2,
    },
  },
})
```

## Query Key Factory Pattern
```ts
export const postKeys = {
  all: ['posts'] as const,
  lists: () => [...postKeys.all, 'list'] as const,
  list: (filters?: PostFilters) => [...postKeys.lists(), filters] as const,
  details: () => [...postKeys.all, 'detail'] as const,
  detail: (id: string) => [...postKeys.details(), id] as const,
}
```

## queryOptions Helper (v5)
```ts
export const postQueryOptions = (id: string) =>
  queryOptions({
    queryKey: postKeys.detail(id),
    queryFn: () => fetchPost(id),
    staleTime: 1000 * 60 * 5,
  })

// In component
const { data } = useQuery(postQueryOptions(postId))

// In router loader
loader: ({ params, context: { queryClient } }) =>
  queryClient.ensureQueryData(postQueryOptions(params.postId))
```

## Mutations
```tsx
const { mutate, isPending } = useMutation({
  mutationFn: (input: CreatePostInput) => createPost(input),
  onSuccess: () => {
    queryClient.invalidateQueries({ queryKey: postKeys.lists() })
  },
  onError: (error) => toast.error(error.message),
})
```

## Optimistic Updates
```tsx
const mutation = useMutation({
  mutationFn: updatePost,
  onMutate: async (updated) => {
    await queryClient.cancelQueries({ queryKey: postKeys.detail(updated.id) })
    const previous = queryClient.getQueryData(postKeys.detail(updated.id))
    queryClient.setQueryData(postKeys.detail(updated.id), updated)
    return { previous }
  },
  onError: (_, updated, ctx) => {
    queryClient.setQueryData(postKeys.detail(updated.id), ctx?.previous)
  },
  onSettled: (_, __, updated) => {
    queryClient.invalidateQueries({ queryKey: postKeys.detail(updated.id) })
  },
})
```

## Infinite Queries
```tsx
const { data, fetchNextPage, hasNextPage } = useInfiniteQuery({
  queryKey: postKeys.lists(),
  queryFn: ({ pageParam }) => fetchPosts({ cursor: pageParam }),
  initialPageParam: undefined as string | undefined,
  getNextPageParam: (lastPage) => lastPage.nextCursor,
})
const allPosts = data?.pages.flatMap((p) => p.items) ?? []
```

## Suspense Mode (v5)
```tsx
// useSuspenseQuery — no isLoading needed, Suspense handles it
const { data } = useSuspenseQuery(postQueryOptions(postId))
// Wrap with <Suspense fallback={<Skeleton />}> + <ErrorBoundary>
```

## Key Rules
- Always define `queryOptions` outside components — never inline in `useQuery()`
- Never use `useEffect` to fetch data — use loaders or `useQuery`
- Use `placeholderData: keepPreviousData` for pagination to avoid layout shifts
- Instantiate `QueryClient` once at app root — never inside a component