From 76bf8efef32f7d47909a99aae0130e560dabafd7 Mon Sep 17 00:00:00 2001
From: contactjawad <contact.jawaddd@gmail.com>
Date: Wed, 19 Aug 2026 11:09:45 +0500
Subject: [PATCH 1/2] fix(query-core): notify sibling observers when one
 unsubscribes during dispatch

Query.#dispatch iterated the live observers array while onQueryUpdate() could
synchronously splice it (via removeObserver). When one observer unsubscribed a
same-query sibling during notification, forEach skipped the next observer,
leaving a still-subscribed observer with a stale `pending` result after the
query had resolved. Iterate over a snapshot of the observers instead.
---
 .../fix-query-dispatch-observer-skip.md       |  5 +++
 .../src/__tests__/queryObserver.test.tsx      | 31 +++++++++++++++++++
 packages/query-core/src/query.ts              |  4 ++-
 3 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/fix-query-dispatch-observer-skip.md

diff --git a/.changeset/fix-query-dispatch-observer-skip.md b/.changeset/fix-query-dispatch-observer-skip.md
new file mode 100644
index 0000000000..99665853b1
--- /dev/null
+++ b/.changeset/fix-query-dispatch-observer-skip.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/query-core': patch
+---
+
+Fix an observer being skipped during query notification when another observer on the same query unsubscribes mid-dispatch. `Query.#dispatch` iterated the live `observers` array while `onQueryUpdate()` could splice it, so a still-subscribed sibling could be left with a stale `pending` result after the query resolved. The notification now iterates over a snapshot of the observers.
diff --git a/packages/query-core/src/__tests__/queryObserver.test.tsx b/packages/query-core/src/__tests__/queryObserver.test.tsx
index 08fac746a4..a115b879c5 100644
--- a/packages/query-core/src/__tests__/queryObserver.test.tsx
+++ b/packages/query-core/src/__tests__/queryObserver.test.tsx
@@ -31,6 +31,37 @@ describe('queryObserver', () => {
     vi.useRealTimers()
   })
 
+  it('should notify a sibling observer when another observer on the same query unsubscribes during dispatch', async () => {
+    const key = queryKey()
+    const queryFn = vi.fn().mockImplementation(() => sleep(10).then(() => 'data'))
+
+    const observerA = new QueryObserver(queryClient, {
+      queryKey: key,
+      queryFn,
+      staleTime: Infinity,
+    })
+    const observerB = new QueryObserver(queryClient, {
+      queryKey: key,
+      queryFn,
+      staleTime: Infinity,
+    })
+
+    let unsubscribeA = () => {}
+    unsubscribeA = observerA.subscribe((result) => {
+      if (result.status === 'success') {
+        unsubscribeA()
+      }
+    })
+    const unsubscribeB = observerB.subscribe(() => undefined)
+
+    await vi.advanceTimersByTimeAsync(15)
+
+    expect(observerB.getCurrentResult().status).toBe('success')
+    expect(observerB.getCurrentResult().data).toBe('data')
+
+    unsubscribeB()
+  })
+
   it('should trigger a fetch when subscribed', () => {
     const key = queryKey()
     const queryFn = vi
diff --git a/packages/query-core/src/query.ts b/packages/query-core/src/query.ts
index 53811c8ebb..37617e538d 100644
--- a/packages/query-core/src/query.ts
+++ b/packages/query-core/src/query.ts
@@ -704,7 +704,9 @@ export class Query<
     this.state = reducer(this.state)
 
     notifyManager.batch(() => {
-      this.observers.forEach((observer) => {
+      // Iterate over a snapshot so that an observer unsubscribing during
+      // notification cannot cause a sibling observer to be skipped.
+      ;[...this.observers].forEach((observer) => {
         observer.onQueryUpdate()
       })
 

From 4f220aab1ef39d3b05f33d85d048dd135c294d7d Mon Sep 17 00:00:00 2001
From: "autofix-ci[bot]" <114827586+autofix-ci[bot]@users.noreply.github.com>
Date: Wed, 19 Aug 2026 19:20:19 +0000
Subject: [PATCH 2/2] ci: apply automated fixes

---
 packages/query-core/src/__tests__/queryObserver.test.tsx | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/packages/query-core/src/__tests__/queryObserver.test.tsx b/packages/query-core/src/__tests__/queryObserver.test.tsx
index a115b879c5..8654a28055 100644
--- a/packages/query-core/src/__tests__/queryObserver.test.tsx
+++ b/packages/query-core/src/__tests__/queryObserver.test.tsx
@@ -33,7 +33,9 @@ describe('queryObserver', () => {
 
   it('should notify a sibling observer when another observer on the same query unsubscribes during dispatch', async () => {
     const key = queryKey()
-    const queryFn = vi.fn().mockImplementation(() => sleep(10).then(() => 'data'))
+    const queryFn = vi
+      .fn()
+      .mockImplementation(() => sleep(10).then(() => 'data'))
 
     const observerA = new QueryObserver(queryClient, {
       queryKey: key,