diff --git a/.changeset/shared-worker-navigator-server-runtimes.md b/.changeset/shared-worker-navigator-server-runtimes.md
new file mode 100644
index 00000000000..8138886d605
--- /dev/null
+++ b/.changeset/shared-worker-navigator-server-runtimes.md
@@ -0,0 +1,5 @@
+---
+'@clerk/shared': patch
+---
+
+Exclude self-identified server runtimes (`Cloudflare-Workers`, `Node.js`, `Deno`, `Bun` user agents) from the worker-scope `navigator` fallback used by `isValidBrowser`, `isBrowserOnline`, and `isValidBrowserOnline`. Today Cloudflare's workerd is excluded only because its `self` does not satisfy `instanceof WorkerGlobalScope`; this guard keeps the checks returning `false` on server-side worker runtimes even if that implementation detail changes, while real browser web/service workers (such as MV3 extension background workers) are unaffected.
diff --git a/packages/shared/src/__tests__/browser.spec.ts b/packages/shared/src/__tests__/browser.spec.ts
index 8380d0c3668..a2013c3acc7 100644
--- a/packages/shared/src/__tests__/browser.spec.ts
+++ b/packages/shared/src/__tests__/browser.spec.ts
@@ -77,6 +77,42 @@ describe('isValidBrowser', () => {
     expect(isValidBrowser()).toBe(true);
   });
 
+  it('returns false in a worker scope whose navigator identifies a server runtime (e.g. Cloudflare Workers)', () => {
+    // Today workerd's `self` fails the `instanceof WorkerGlobalScope` gate, but that is a
+    // quirk of its prototype chain. This simulates a spec-compliant workerd: full worker
+    // scope, navigator present, but the user agent self-identifies as a server runtime.
+    mockServiceWorkerScope();
+    userAgentGetter.mockReturnValue('Cloudflare-Workers');
+    webdriverGetter.mockReturnValue(false);
+
+    expect(isValidBrowser()).toBe(false);
+  });
+
+  it.each(['Node.js/24', 'Deno/2.5.0', 'Bun/1.3.9'])(
+    'returns false in a worker scope whose navigator identifies the %s server runtime',
+    userAgent => {
+      mockServiceWorkerScope();
+      userAgentGetter.mockReturnValue(userAgent);
+      webdriverGetter.mockReturnValue(false);
+
+      expect(isValidBrowser()).toBe(false);
+    },
+  );
+
+  it('returns false when WorkerGlobalScope exists but self is not an instance of it (e.g. workerd today)', () => {
+    // workerd exposes the WorkerGlobalScope constructor without linking `self` to its
+    // prototype chain, so the instanceof gate must reject it even with a browser-like UA.
+    vi.stubGlobal('WorkerGlobalScope', class {});
+    vi.stubGlobal('self', {
+      navigator: { userAgent: 'Mozilla/5.0 Chrome/120.0.0.0 Safari/537.36', onLine: true, webdriver: false },
+    });
+    const windowSpy = vi.spyOn(global, 'window', 'get');
+    // @ts-ignore - Test
+    windowSpy.mockReturnValue(undefined);
+
+    expect(isValidBrowser()).toBe(false);
+  });
+
   it('returns true if in browser, navigator is not a bot, and webdriver is not enabled', () => {
     userAgentGetter.mockReturnValue(
       'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:109.0) Gecko/20100101 Firefox/109.0',
@@ -275,4 +311,15 @@ describe('isValidBrowserOnline', () => {
 
     expect(isValidBrowserOnline()).toBe(false);
   });
+
+  it('returns FALSE in a worker scope whose navigator identifies a server runtime (e.g. Cloudflare Workers)', () => {
+    // Cloudflare Workers do not implement `navigator.onLine`, so without the server-runtime
+    // exclusion this would fall into the "onLine is not a boolean -> assume online" branch.
+    mockServiceWorkerScope();
+    userAgentGetter.mockReturnValue('Cloudflare-Workers');
+    webdriverGetter.mockReturnValue(false);
+    onLineGetter.mockReturnValue(undefined);
+
+    expect(isValidBrowserOnline()).toBe(false);
+  });
 });
diff --git a/packages/shared/src/browser.ts b/packages/shared/src/browser.ts
index a129de01366..be8eaa36855 100644
--- a/packages/shared/src/browser.ts
+++ b/packages/shared/src/browser.ts
@@ -49,6 +49,17 @@ export function userAgentIsRobot(userAgent: string): boolean {
   return !userAgent ? false : botAgentRegex.test(userAgent);
 }
 
+/**
+ * Server-side runtimes with worker-like globals self-identify in `navigator.userAgent`
+ * (`Cloudflare-Workers`, `Node.js/24`, `Deno/2.5.0`, `Bun/1.3.9`). Today workerd's `self`
+ * does not satisfy `instanceof WorkerGlobalScope` (even though it exposes the constructor),
+ * so the scope gate alone happens to exclude it, but that is an implementation detail of
+ * workerd's prototype chain, not a guarantee. Excluding self-identified server runtimes by
+ * user agent keeps these heuristics server-false even if such a runtime becomes fully
+ * spec-compliant about its worker scope.
+ */
+const serverRuntimeUserAgentRegex = /^(Cloudflare-Workers|Node\.js|Deno|Bun)\b/i;
+
 /**
  * Resolves the `Navigator` object from either the DOM `window` (standard browsers)
  * or a Web/Service Worker global scope. An MV3 extension background service worker
@@ -74,7 +85,8 @@ function getNavigator(): Navigator | null {
   if (
     typeof workerScope.WorkerGlobalScope === 'function' &&
     workerScope.self instanceof workerScope.WorkerGlobalScope &&
-    workerScope.self.navigator
+    workerScope.self.navigator &&
+    !serverRuntimeUserAgentRegex.test(workerScope.self.navigator.userAgent ?? '')
   ) {
     return workerScope.self.navigator;
   }