diff --git a/docs/config.json b/docs/config.json
index 5ff8e8f240..8666e2fabe 100644
--- a/docs/config.json
+++ b/docs/config.json
@@ -909,6 +909,10 @@
           "label": "onlineManager",
           "to": "reference/onlineManager"
         },
+        {
+          "label": "environmentManager",
+          "to": "reference/environmentManager"
+        },
         {
           "label": "notifyManager",
           "to": "reference/notifyManager"
diff --git a/docs/reference/environmentManager.md b/docs/reference/environmentManager.md
new file mode 100644
index 0000000000..2295a76463
--- /dev/null
+++ b/docs/reference/environmentManager.md
@@ -0,0 +1,50 @@
+---
+id: EnvironmentManager
+title: environmentManager
+---
+
+The `environmentManager` manages how TanStack Query detects whether the current runtime should be treated as server-side.
+
+By default, it uses the same server detection as the exported `isServer` utility from query-core.
+
+Use this manager to override server detection globally for runtimes that are not traditional browser/server environments (for example, extension workers).
+
+Its available methods are:
+
+- [`isServer`](#environmentmanagerisserver)
+- [`setIsServer`](#environmentmanagersetisserver)
+
+## `environmentManager.isServer`
+
+Returns whether the current runtime is treated as a server environment.
+
+```tsx
+import { environmentManager } from '@tanstack/react-query'
+
+const server = environmentManager.isServer()
+```
+
+## `environmentManager.setIsServer`
+
+Overrides the server check globally.
+
+```tsx
+import { environmentManager } from '@tanstack/react-query'
+
+// Override
+environmentManager.setIsServer(() => {
+  return typeof window === 'undefined' && !('chrome' in globalThis)
+})
+```
+
+**Options**
+
+- `isServerValue: () => boolean`
+
+To restore the default behavior, set the function back to query-core's `isServer` utility:
+
+```tsx
+import { environmentManager, isServer } from '@tanstack/react-query'
+
+environmentManager.setIsServer(() => isServer)
+```
diff --git a/packages/preact-query/src/__tests__/utils.tsx b/packages/preact-query/src/__tests__/utils.tsx
index d4f1dba05e..b37adc318b 100644
--- a/packages/preact-query/src/__tests__/utils.tsx
+++ b/packages/preact-query/src/__tests__/utils.tsx
@@ -1,4 +1,4 @@
-import * as utils from '@tanstack/query-core'
+import { environmentManager, isServer } from '@tanstack/query-core'
 import { act, render } from '@testing-library/preact'
 import type { ComponentChildren, VNode } from 'preact'
 import { useEffect, useState } from 'preact/hooks'
@@ -58,17 +58,9 @@ export function setActTimeout(fn: () => void, ms?: number) {
   }, ms)
 }
 
-// This monkey-patches the isServer-value from utils,
-// so that we can pretend to be in a server environment
-export function setIsServer(isServer: boolean) {
-  const original = utils.isServer
-  Object.defineProperty(utils, 'isServer', {
-    get: () => isServer,
-  })
-
+export function setIsServer(value: boolean) {
+  environmentManager.setIsServer(() => value)
   return () => {
-    Object.defineProperty(utils, 'isServer', {
-      get: () => original,
-    })
+    environmentManager.setIsServer(() => isServer)
   }
 }
diff --git a/packages/preact-query/src/useBaseQuery.ts b/packages/preact-query/src/useBaseQuery.ts
index 84d7567916..05ce46e7f6 100644
--- a/packages/preact-query/src/useBaseQuery.ts
+++ b/packages/preact-query/src/useBaseQuery.ts
@@ -1,4 +1,4 @@
-import { isServer, noop, notifyManager } from '@tanstack/query-core'
+import { environmentManager, noop, notifyManager } from '@tanstack/query-core'
 import type {
   QueryClient,
   QueryKey,
@@ -147,7 +147,7 @@ export function useBaseQuery<
 
   if (
     defaultedOptions.experimental_prefetchInRender &&
-    !isServer &&
+    !environmentManager.isServer() &&
     willFetch(result, isRestoring)
   ) {
     const promise = isNewCacheEntry
diff --git a/packages/query-core/src/__tests__/environmentManager.test.tsx b/packages/query-core/src/__tests__/environmentManager.test.tsx
new file mode 100644
index 0000000000..8d428d7be0
--- /dev/null
+++ b/packages/query-core/src/__tests__/environmentManager.test.tsx
@@ -0,0 +1,29 @@
+import { afterEach, describe, expect, test } from 'vitest'
+import { environmentManager, isServer } from '..'
+
+describe('environmentManager', () => {
+  afterEach(() => {
+    environmentManager.setIsServer(() => isServer)
+  })
+
+  test('should use the default isServer detection', () => {
+    expect(environmentManager.isServer()).toBe(isServer)
+  })
+
+  test('should allow overriding isServer globally', () => {
+    environmentManager.setIsServer(() => true)
+    expect(environmentManager.isServer()).toBe(true)
+
+    environmentManager.setIsServer(() => false)
+    expect(environmentManager.isServer()).toBe(false)
+  })
+
+  test('should allow overriding isServer with a function', () => {
+    let server = true
+    environmentManager.setIsServer(() => server)
+    expect(environmentManager.isServer()).toBe(true)
+
+    server = false
+    expect(environmentManager.isServer()).toBe(false)
+  })
+})
diff --git a/packages/query-core/src/__tests__/focusManager.test.tsx b/packages/query-core/src/__tests__/focusManager.test.tsx
index 3783361fb0..5900b9b202 100644
--- a/packages/query-core/src/__tests__/focusManager.test.tsx
+++ b/packages/query-core/src/__tests__/focusManager.test.tsx
@@ -1,6 +1,5 @@
 import { afterEach, beforeEach, describe, expect, it, test, vi } from 'vitest'
 import { FocusManager } from '../focusManager'
-import { setIsServer } from './utils'
 
 describe('focusManager', () => {
   let focusManager: FocusManager
@@ -55,17 +54,26 @@ describe('focusManager', () => {
   })
 
   test('cleanup (removeEventListener) should not be called if window is not defined', () => {
-    const restoreIsServer = setIsServer(true)
-
+    const windowSpy = vi.spyOn(globalThis, 'window', 'get')
+    windowSpy.mockImplementation(
+      () => undefined as unknown as Window & typeof globalThis,
+    )
     const removeEventListenerSpy = vi.spyOn(globalThis, 'removeEventListener')
 
-    const unsubscribe = focusManager.subscribe(() => undefined)
+    const subscribe = () => focusManager.subscribe(() => undefined)
+    let firstUnsubscribe: (() => void) | undefined
 
-    unsubscribe()
+    expect(() => {
+      firstUnsubscribe = subscribe()
+    }).not.toThrow()
+    const secondUnsubscribe = subscribe()
+
+    firstUnsubscribe?.()
+    secondUnsubscribe()
 
     expect(removeEventListenerSpy).not.toHaveBeenCalled()
 
-    restoreIsServer()
+    windowSpy.mockRestore()
   })
 
   test('cleanup (removeEventListener) should not be called if window.addEventListener is not defined', () => {
diff --git a/packages/query-core/src/__tests__/onlineManager.test.tsx b/packages/query-core/src/__tests__/onlineManager.test.tsx
index 20a9439de9..79fbb697ee 100644
--- a/packages/query-core/src/__tests__/onlineManager.test.tsx
+++ b/packages/query-core/src/__tests__/onlineManager.test.tsx
@@ -1,6 +1,5 @@
 import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest'
 import { OnlineManager } from '../onlineManager'
-import { setIsServer } from './utils'
 
 describe('onlineManager', () => {
   let onlineManager: OnlineManager
@@ -64,17 +63,20 @@ describe('onlineManager', () => {
   })
 
   test('cleanup (removeEventListener) should not be called if window is not defined', () => {
-    const restoreIsServer = setIsServer(true)
-
+    const windowSpy = vi.spyOn(globalThis, 'window', 'get')
+    windowSpy.mockImplementation(
+      () => undefined as unknown as Window & typeof globalThis,
+    )
     const removeEventListenerSpy = vi.spyOn(globalThis, 'removeEventListener')
 
     const unsubscribe = onlineManager.subscribe(() => undefined)
+    expect(unsubscribe).toBeInstanceOf(Function)
 
     unsubscribe()
 
     expect(removeEventListenerSpy).not.toHaveBeenCalled()
 
-    restoreIsServer()
+    windowSpy.mockRestore()
   })
 
   test('cleanup (removeEventListener) should not be called if window.addEventListener is not defined', () => {
diff --git a/packages/query-core/src/__tests__/utils.ts b/packages/query-core/src/__tests__/utils.ts
index f9ef89d765..6396d341ea 100644
--- a/packages/query-core/src/__tests__/utils.ts
+++ b/packages/query-core/src/__tests__/utils.ts
@@ -1,6 +1,5 @@
 import { vi } from 'vitest'
-import { onlineManager } from '..'
-import * as utils from '../utils'
+import { environmentManager, isServer, onlineManager } from '..'
 import type { MockInstance } from 'vitest'
 import type { MutationOptions, QueryClient } from '..'
 
@@ -21,17 +20,9 @@ export function executeMutation<TVariables>(
     .execute(variables)
 }
 
-// This monkey-patches the isServer-value from utils,
-// so that we can pretend to be in a server environment
-export function setIsServer(isServer: boolean) {
-  const original = utils.isServer
-  Object.defineProperty(utils, 'isServer', {
-    get: () => isServer,
-  })
-
+export function setIsServer(value: boolean) {
+  environmentManager.setIsServer(() => value)
   return () => {
-    Object.defineProperty(utils, 'isServer', {
-      get: () => original,
-    })
+    environmentManager.setIsServer(() => isServer)
   }
 }
diff --git a/packages/query-core/src/environmentManager.ts b/packages/query-core/src/environmentManager.ts
new file mode 100644
index 0000000000..9da5caafd0
--- /dev/null
+++ b/packages/query-core/src/environmentManager.ts
@@ -0,0 +1,25 @@
+import { isServer } from './utils'
+
+export type IsServerValue = () => boolean
+
+/**
+ * Manages environment detection used by TanStack Query internals.
+ */
+export const environmentManager = (() => {
+  let isServerFn: IsServerValue = () => isServer
+
+  return {
+    /**
+     * Returns whether the current runtime should be treated as a server environment.
+     */
+    isServer(): boolean {
+      return isServerFn()
+    },
+    /**
+     * Overrides the server check globally.
+     */
+    setIsServer(isServerValue: IsServerValue): void {
+      isServerFn = isServerValue
+    },
+  }
+})()
diff --git a/packages/query-core/src/focusManager.ts b/packages/query-core/src/focusManager.ts
index cb0d859871..21be2612d3 100644
--- a/packages/query-core/src/focusManager.ts
+++ b/packages/query-core/src/focusManager.ts
@@ -1,5 +1,4 @@
 import { Subscribable } from './subscribable'
-import { isServer } from './utils'
 
 type Listener = (focused: boolean) => void
 
@@ -18,7 +17,7 @@ export class FocusManager extends Subscribable<Listener> {
     this.#setup = (onFocus) => {
       // addEventListener does not exist in React Native, but window does
       // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
-      if (!isServer && window.addEventListener) {
+      if (typeof window !== 'undefined' && window.addEventListener) {
         const listener = () => onFocus()
         // Listen to visibilitychange
         window.addEventListener('visibilitychange', listener, false)
diff --git a/packages/query-core/src/index.ts b/packages/query-core/src/index.ts
index a7763cf648..a4267aabc9 100644
--- a/packages/query-core/src/index.ts
+++ b/packages/query-core/src/index.ts
@@ -1,6 +1,7 @@
 /* istanbul ignore file */
 
 export { focusManager } from './focusManager'
+export { environmentManager } from './environmentManager'
 export {
   defaultShouldDehydrateMutation,
   defaultShouldDehydrateQuery,
diff --git a/packages/query-core/src/onlineManager.ts b/packages/query-core/src/onlineManager.ts
index daf77d5a4c..6c59a14b90 100644
--- a/packages/query-core/src/onlineManager.ts
+++ b/packages/query-core/src/onlineManager.ts
@@ -1,5 +1,4 @@
 import { Subscribable } from './subscribable'
-import { isServer } from './utils'
 
 type Listener = (online: boolean) => void
 type SetupFn = (setOnline: Listener) => (() => void) | undefined
@@ -15,7 +14,7 @@ export class OnlineManager extends Subscribable<Listener> {
     this.#setup = (onOnline) => {
       // addEventListener does not exist in React Native, but window does
       // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
-      if (!isServer && window.addEventListener) {
+      if (typeof window !== 'undefined' && window.addEventListener) {
         const onlineListener = () => onOnline(true)
         const offlineListener = () => onOnline(false)
         // Listen to online
diff --git a/packages/query-core/src/queryObserver.ts b/packages/query-core/src/queryObserver.ts
index 463407a073..d6b3817b82 100644
--- a/packages/query-core/src/queryObserver.ts
+++ b/packages/query-core/src/queryObserver.ts
@@ -1,10 +1,10 @@
 import { focusManager } from './focusManager'
+import { environmentManager } from './environmentManager'
 import { notifyManager } from './notifyManager'
 import { fetchState } from './query'
 import { Subscribable } from './subscribable'
 import { pendingThenable } from './thenable'
 import {
-  isServer,
   isValidTimeout,
   noop,
   replaceData,
@@ -358,7 +358,11 @@ export class QueryObserver<
       this.#currentQuery,
     )
 
-    if (isServer || this.#currentResult.isStale || !isValidTimeout(staleTime)) {
+    if (
+      environmentManager.isServer() ||
+      this.#currentResult.isStale ||
+      !isValidTimeout(staleTime)
+    ) {
       return
     }
 
@@ -389,7 +393,7 @@ export class QueryObserver<
     this.#currentRefetchInterval = nextInterval
 
     if (
-      isServer ||
+      environmentManager.isServer() ||
       resolveEnabled(this.options.enabled, this.#currentQuery) === false ||
       !isValidTimeout(this.#currentRefetchInterval) ||
       this.#currentRefetchInterval === 0
diff --git a/packages/query-core/src/removable.ts b/packages/query-core/src/removable.ts
index 8642ab36ec..68545f7438 100644
--- a/packages/query-core/src/removable.ts
+++ b/packages/query-core/src/removable.ts
@@ -1,5 +1,6 @@
 import { timeoutManager } from './timeoutManager'
-import { isServer, isValidTimeout } from './utils'
+import { environmentManager } from './environmentManager'
+import { isValidTimeout } from './utils'
 import type { ManagedTimerId } from './timeoutManager'
 
 export abstract class Removable {
@@ -24,7 +25,7 @@ export abstract class Removable {
     // Default to 5 minutes (Infinity for server-side) if no gcTime is set
     this.gcTime = Math.max(
       this.gcTime || 0,
-      newGcTime ?? (isServer ? Infinity : 5 * 60 * 1000),
+      newGcTime ?? (environmentManager.isServer() ? Infinity : 5 * 60 * 1000),
     )
   }
 
diff --git a/packages/query-core/src/retryer.ts b/packages/query-core/src/retryer.ts
index f4ada851c9..0b1bb1833a 100644
--- a/packages/query-core/src/retryer.ts
+++ b/packages/query-core/src/retryer.ts
@@ -1,7 +1,8 @@
 import { focusManager } from './focusManager'
 import { onlineManager } from './onlineManager'
 import { pendingThenable } from './thenable'
-import { isServer, sleep } from './utils'
+import { environmentManager } from './environmentManager'
+import { sleep } from './utils'
 import type { Thenable } from './thenable'
 import type { CancelOptions, DefaultError, NetworkMode } from './types'
 
@@ -166,7 +167,7 @@ export function createRetryer<TData = unknown, TError = DefaultError>(
         }
 
         // Do we need to retry the request?
-        const retry = config.retry ?? (isServer ? 0 : 3)
+        const retry = config.retry ?? (environmentManager.isServer() ? 0 : 3)
         const retryDelay = config.retryDelay ?? defaultRetryDelay
         const delay =
           typeof retryDelay === 'function'
diff --git a/packages/query-core/src/utils.ts b/packages/query-core/src/utils.ts
index 27d44bc98b..b29e8ded45 100644
--- a/packages/query-core/src/utils.ts
+++ b/packages/query-core/src/utils.ts
@@ -86,6 +86,9 @@ export type QueryTypeFilter = 'all' | 'active' | 'inactive'
 
 // UTILS
 
+/** @deprecated
+ * use `environmentManager.isServer()` instead.
+ */
 export const isServer = typeof window === 'undefined' || 'Deno' in globalThis
 
 export function noop(): void
diff --git a/packages/react-query/src/__tests__/utils.tsx b/packages/react-query/src/__tests__/utils.tsx
index 3734c1caf6..ae8e2fb23f 100644
--- a/packages/react-query/src/__tests__/utils.tsx
+++ b/packages/react-query/src/__tests__/utils.tsx
@@ -1,7 +1,7 @@
 import { vi } from 'vitest'
 import * as React from 'react'
 import { act, render } from '@testing-library/react'
-import * as utils from '@tanstack/query-core'
+import { environmentManager, isServer } from '@tanstack/query-core'
 import { QueryClientProvider, onlineManager } from '..'
 import type { QueryClient } from '..'
 import type { MockInstance } from 'vitest'
@@ -56,17 +56,9 @@ export function setActTimeout(fn: () => void, ms?: number) {
   }, ms)
 }
 
-// This monkey-patches the isServer-value from utils,
-// so that we can pretend to be in a server environment
-export function setIsServer(isServer: boolean) {
-  const original = utils.isServer
-  Object.defineProperty(utils, 'isServer', {
-    get: () => isServer,
-  })
-
+export function setIsServer(value: boolean) {
+  environmentManager.setIsServer(() => value)
   return () => {
-    Object.defineProperty(utils, 'isServer', {
-      get: () => original,
-    })
+    environmentManager.setIsServer(() => isServer)
   }
 }
diff --git a/packages/react-query/src/useBaseQuery.ts b/packages/react-query/src/useBaseQuery.ts
index 2a151fe113..a88f7d40fb 100644
--- a/packages/react-query/src/useBaseQuery.ts
+++ b/packages/react-query/src/useBaseQuery.ts
@@ -1,7 +1,7 @@
 'use client'
 import * as React from 'react'
 
-import { isServer, noop, notifyManager } from '@tanstack/query-core'
+import { environmentManager, noop, notifyManager } from '@tanstack/query-core'
 import { useQueryClient } from './QueryClientProvider'
 import { useQueryErrorResetBoundary } from './QueryErrorResetBoundary'
 import {
@@ -148,7 +148,7 @@ export function useBaseQuery<
 
   if (
     defaultedOptions.experimental_prefetchInRender &&
-    !isServer &&
+    !environmentManager.isServer() &&
     willFetch(result, isRestoring)
   ) {
     const promise = isNewCacheEntry