How do you perform optimistic updates on complex objects in TanStack Query?

Perform optimistic updates on complex objects using TanStack Query mutation callbacks and cache manipulation. In onMutate, snapshot current cache state and update it optimistically: queryClient.setQueryData(queryKey, (old) => ({ ...old, field: newValue })). For nested objects, use immutable update patterns or immer for cleaner syntax. Return snapshot from onMutate for rollback in onError. Handle arrays by updating specific items: data.items.map(item => item.id === id ? updated : item). Use context returned from onMutate to restore state on failure. Implement pessimistic fallback when mutations fail. For complex relationships, invalidate related queries after mutation succeeds. Use optimistic updates for user-facing actions requiring immediate feedback. Avoid optimistic updates for critical operations where accuracy matters more than speed. Test complex update logic thoroughly as bugs create confusing UI states. Validate JSON structures with our JSON Editor at jsonconsole.com/json-editor before implementing updates. Proper optimistic updates make applications feel instant while maintaining data consistency through automatic rollback on errors.