TanStack Query v5 - Expert Troubleshooting

Assumption: You know useQuery basics. This covers what breaks in production.

Arguments

• $ARGUMENTS: Query bug, migration issue, or caching decision to analyze
  • Example: /tanstack-query infinite refetch loop on dashboard
  • Example: /tanstack-query v4 to v5 cacheTime issue
  • If empty: ask which TanStack Query issue is in scope

Before Using React Query: Strategic Assessment

When NOT to Use React Query

Need data fetching?
│
├─ Data from URL (search params, path) → DON'T use queries
│   └─ Use framework loaders (Next.js, Remix)
│      WHY: Queries cache by key, URL is already your cache key
│
├─ Derived/computed data → DON'T use queries
│   └─ Use useMemo or Zustand
│      WHY: No server, no stale data, no refetch needed
│
├─ Form state → DON'T use queries
│   └─ Use React Hook Form or controlled state
│
├─ WebSocket/realtime (> 1/sec) → DON'T use queries
│   └─ Use Zustand; queries are designed for request/response, not streaming
│
└─ REST/GraphQL server state → USE queries ✅

The trap: Developers use React Query for everything. It's a server cache, not a state manager.

staleTime Selection

Breaking Changes: v4 → v5 Migration Gotchas

❌ #1: cacheTime Renamed to gcTime

Failure mode: Silent — code runs, TypeScript doesn't error, cache garbage-collects immediately.

// WRONG - silently ignored in v5
useQuery({ queryKey: ['todos'], queryFn: fetchTodos, cacheTime: 10 * 60 * 1000 })

// CORRECT
useQuery({ queryKey: ['todos'], queryFn: fetchTodos, gcTime: 10 * 60 * 1000 })

Debug signal: DevTools shows 0ms gcTime despite setting 10 minutes.

❌ #2: isLoading Removed → Use isPending

Failure mode: if (isLoading) evaluates falsy (undefined), spinner never shows.

// WRONG - isLoading is undefined in v5
const { isLoading } = useQuery(...)

// CORRECT
const { isPending } = useQuery(...)

Semantic difference: isPending stays true during refetches with cached data — isLoading did not. Causes "stale data + spinner simultaneously" if naively swapped.

❌ #3: keepPreviousData → placeholderData

Failure mode: Pagination flickers on page change.

// WRONG
useQuery({ queryKey: ['todos', page], keepPreviousData: true })

// CORRECT - function form required
useQuery({
  queryKey: ['todos', page],
  placeholderData: (previousData) => previousData,
})

❌ #4: Query Functions Must Return Non-Void

Failure mode: Silent runtime error when using any types.

// WRONG - void return
queryFn: async () => { await api.deleteTodo(id) }

// CORRECT
queryFn: async () => { await api.deleteTodo(id); return { success: true } }

Performance Pitfalls

❌ Infinite Refetch Loop

Cause: Object or array reference in queryKey — new reference on every render triggers new query.

// WRONG - object in key = new reference each render = infinite loop
useQuery({ queryKey: ['user', user], queryFn: () => fetchUser(user.id) })

// CORRECT - use stable primitives
useQuery({ queryKey: ['user', user.id], queryFn: () => fetchUser(user.id) })

Detection: Network tab shows identical requests >10/sec. React DevTools Profiler shows constant re-renders.

Fallback (when key must contain object):

const stableKey = useMemo(() => ['user', user], [user.id])
useQuery({ queryKey: stableKey, queryFn: () => fetchUser(user.id), structuralSharing: false })

❌ Stale Data Trap

Cause: staleTime: Infinity — data never marked stale regardless of server changes.

Detection: Network tab shows zero requests after initial load. Users report "data doesn't update" but devs can't reproduce (devs refresh frequently, clearing cache).

Fix: Use reasonable staleTime. If still stale: queryClient.invalidateQueries({ queryKey: ['your-key'] }).

❌ Over-Invalidation

Cause: queryClient.invalidateQueries() with no filter nukes entire cache → all queries refetch.

// WRONG - refetches 100 queries on every mutation
onSuccess: () => { queryClient.invalidateQueries() }

// CORRECT - targeted
onSuccess: () => { queryClient.invalidateQueries({ queryKey: ['user', userId] }) }

Decision Frameworks

Optimistic Updates vs Invalidation

Mutation completes...
│
├─ Simple list append/prepend → Optimistic (useMutationState)
│   └─ Add todo, add comment — no complex logic needed
│
├─ Complex computed data → Invalidation
│   └─ Aggregates, filters, sorts — let server compute
│
├─ Risk of conflicts (multi-user) → Invalidation
│   └─ Optimistic update may be wrong; let server resolve
│
└─ Must feel instant → Optimistic + rollback on error
    └─ Toggle like, toggle favorite

React Query vs SWR

SSR Hydration (Next.js App Router)

❌ Mismatch Pattern

Server renders "Loading...", client has cached data → hydration error.

✅ Prefetch Pattern

// app/page.tsx (Server Component)
import { dehydrate, HydrationBoundary, QueryClient } from '@tanstack/react-query'

export default async function Page() {
  const queryClient = new QueryClient()
  await queryClient.prefetchQuery({ queryKey: ['todos'], queryFn: fetchTodos })
  return (
    <HydrationBoundary state={dehydrate(queryClient)}>
      <TodoList />
    </HydrationBoundary>
  )
}

// components/TodoList.tsx ('use client')
export function TodoList() {
  const { data } = useQuery({ queryKey: ['todos'], queryFn: fetchTodos })
  // No isPending check — data guaranteed from server prefetch
  return <div>{data.map(...)}</div>
}

Hydration mismatch fallback: Pass as initialData via props instead of prefetch.

Debugging Commands

// Find refetch loops — add to QueryClient defaultOptions
onSuccess: (data, query) => { console.count(`Refetch: ${query.queryKey}`) }
// Count > 10 in 1 second = infinite loop

// Check cache state
const state = queryClient.getQueryState(['todos'])
console.log(state?.isInvalidated)

// Nuclear cache clear
queryClient.removeQueries({ queryKey: ['your-key'] })
queryClient.refetchQueries({ queryKey: ['your-key'] })
// Or: queryClient.clear()

# Find v4 property names still in codebase
grep -r "cacheTime\|isLoading\|keepPreviousData" src/

Add <ReactQueryDevtools initialIsOpen={false} /> to visualize cache state, refetch counts, and staleness.

When to Load Full Reference

READ references/v5-features.md when using 3+ v5-specific features simultaneously (useMutationState, throwOnError, infinite queries, suspense mode).

READ references/migration-guide.md when migrating a codebase with 10+ query usages or running codemods.

Do NOT load references for single breaking change fixes, basic troubleshooting, or simple optimistic updates — all covered above.

Resources

• Official Docs: https://tanstack.com/query/latest