fix: don't update dataUpdateCount when setting initial data

.changeset/eight-webs-buy.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/query-core': patch
+---
+
+Fixed isFetchedAfterMount in cases where initialData is applied

packages/query-core/src/__tests__/query.test.tsx

@@ -1304,4 +1304,47 @@ describe('query', () => {
       data: 'data1',
     })
   })
+
+  test('should not increment dataUpdateCount when setting initialData on prefetched query', async () => {
+    const key = queryKey()
+    const queryFn = vi.fn().mockImplementation(() => 'fetched-data')
+
+    // First prefetch the query (creates query without data)
+    queryClient.prefetchQuery({
+      queryKey: key,
+      queryFn,
+    })
+
+    const query = queryCache.find({ queryKey: key })!
+    expect(query.state.data).toBeUndefined()
+    expect(query.state.dataUpdateCount).toBe(0)
+
+    // Now create an observer with initialData
+    const observer = new QueryObserver(queryClient, {
+      queryKey: key,
+      queryFn,
+      initialData: 'initial-data',
+    })
+
+    // The query should now have the initial data but dataUpdateCount should still be 0
+    // since this was not fetched data but initial data
+    expect(query.state.data).toBe('initial-data')
+    expect(query.state.dataUpdateCount).toBe(0)
+
+    // Get the initial state as captured by the observer
+    const result = observer.getCurrentResult()
+    expect(result.data).toBe('initial-data')
+    expect(result.isFetchedAfterMount).toBe(false) // This should be false since no actual fetch occurred
+
+    // Now trigger a refetch through the observer to simulate real-world usage
+    await observer.refetch()
+
+    // After actual fetch, dataUpdateCount should increment
+    expect(query.state.dataUpdateCount).toBe(1)
+    expect(query.state.data).toBe('fetched-data')
+
+    // And isFetchedAfterMount should now be true
+    const updatedResult = observer.getCurrentResult()
+    expect(updatedResult.isFetchedAfterMount).toBe(true)
+  })
 })

packages/query-core/src/query.ts

@@ -210,10 +210,9 @@ export class Query<
     if (this.state && this.state.data === undefined) {
       const defaultState = getDefaultState(this.options)
       if (defaultState.data !== undefined) {
-        this.setData(defaultState.data, {
-          updatedAt: defaultState.dataUpdatedAt,
-          manual: true,
-        })
+        this.setState(
+          successState(defaultState.data, defaultState.dataUpdatedAt),
+        )
         this.#initialState = defaultState
       }
     }
@@ -635,12 +634,8 @@ export class Query<
         case 'success':
           const newState = {
             ...state,
-            data: action.data,
+            ...successState(action.data, action.dataUpdatedAt),
             dataUpdateCount: state.dataUpdateCount + 1,
-            dataUpdatedAt: action.dataUpdatedAt ?? Date.now(),
-            error: null,
-            isInvalidated: false,
-            status: 'success' as const,
             ...(!action.manual && {
               fetchStatus: 'idle' as const,
               fetchFailureCount: 0,
@@ -710,6 +705,16 @@ export function fetchState<
   } as const
 }
 
+function successState<TData>(data: TData | undefined, dataUpdatedAt?: number) {
+  return {
+    data,
+    dataUpdatedAt: dataUpdatedAt ?? Date.now(),
+    error: null,
+    isInvalidated: false,
+    status: 'success' as const,
+  }
+}
+
 function getDefaultState<
   TQueryFnData,
   TError,