From 131504a8908b391abb009e2c475835d5ca472d21 Mon Sep 17 00:00:00 2001
From: Scott Cooper <scttcper@gmail.com>
Date: Wed, 19 Aug 2026 17:36:23 -0700
Subject: [PATCH] fix(query-core): Keep observer notifications stable

Removing an observer in place can shift the active notification loop and skip the next observer. Iterate over a shallow copy so synchronous unsubscriptions keep the old behavior without giving up the faster removal path from #11214.

Co-Authored-By: Codex <noreply@openai.com>
---
 .changeset/stable-observers-notify.md         |  5 +++++
 .../query-core/src/__tests__/query.test.tsx   | 21 +++++++++++++++++++
 packages/query-core/src/query.ts              |  4 +++-
 3 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/stable-observers-notify.md

diff --git a/.changeset/stable-observers-notify.md b/.changeset/stable-observers-notify.md
new file mode 100644
index 0000000000..d53bbf22b3
--- /dev/null
+++ b/.changeset/stable-observers-notify.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/query-core': patch
+---
+
+Notify every query observer when another observer unsubscribes synchronously during the same query update.
diff --git a/packages/query-core/src/__tests__/query.test.tsx b/packages/query-core/src/__tests__/query.test.tsx
index f4edc0d0c1..bc637fcc0f 100644
--- a/packages/query-core/src/__tests__/query.test.tsx
+++ b/packages/query-core/src/__tests__/query.test.tsx
@@ -824,6 +824,27 @@ describe('query', () => {
     notifySpy.mockRestore()
   })
 
+  it('should notify remaining observers when one unsubscribes during an update', () => {
+    const key = queryKey()
+    const options = { queryKey: key, enabled: false }
+    const firstObserver = new QueryObserver(queryClient, options)
+    const secondObserver = new QueryObserver(queryClient, options)
+    const secondListener = vi.fn()
+
+    let unsubscribeFirst: () => void = () => undefined
+    unsubscribeFirst = firstObserver.subscribe(() => {
+      unsubscribeFirst()
+    })
+    const unsubscribeSecond = secondObserver.subscribe(secondListener)
+
+    queryClient.setQueryData(key, 'data')
+
+    expect(secondListener).toHaveBeenCalledTimes(1)
+    expect(secondObserver.getCurrentResult().data).toBe('data')
+
+    unsubscribeSecond()
+  })
+
   it('should not change state on invalidate() if already invalidated', async () => {
     const key = queryKey()
 
diff --git a/packages/query-core/src/query.ts b/packages/query-core/src/query.ts
index 53811c8ebb..245ec28378 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) => {
+      // Keep the current iteration stable if an observer unsubscribes
+      // synchronously while it is being notified.
+      this.observers.slice().forEach((observer) => {
         observer.onQueryUpdate()
       })