diff --git a/extensions/browser/src/browser/pw-session.dialogs.test.ts b/extensions/browser/src/browser/pw-session.dialogs.test.ts
index 66750ebd41a..8ae1ece4da1 100644
--- a/extensions/browser/src/browser/pw-session.dialogs.test.ts
+++ b/extensions/browser/src/browser/pw-session.dialogs.test.ts
@@ -99,6 +99,28 @@ describe("observed browser dialogs", () => {
     observed.cleanup();
   });
 
+  it("uses the default arm-next-dialog timeout for non-finite timeoutMs", async () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(0);
+    const { page, emit } = createPageHarness();
+    ensurePageState(page);
+    const dialog = createDialog({ type: "alert", message: "Still armed" });
+    const observed = createObservedDialogAbortSignalForPage({ page });
+
+    armObservedDialogResponseOnPage({ page, accept: false, timeoutMs: Number.NaN });
+    await vi.advanceTimersByTimeAsync(119_999);
+    emit("dialog", dialog);
+    await Promise.resolve();
+
+    expect(observed.signal.aborted).toBe(false);
+    expect(dialog.dismiss).toHaveBeenCalledOnce();
+    expect(getObservedBrowserStateForPage(page).dialogs.pending).toEqual([]);
+    expect(getObservedBrowserStateForPage(page).dialogs.recent).toMatchObject([
+      { id: "d1", type: "alert", closedBy: "armed" },
+    ]);
+    observed.cleanup();
+  });
+
   it("aborts in-flight actions while keeping unarmed dialogs pending", async () => {
     const { page, emit } = createPageHarness();
     ensurePageState(page);
diff --git a/extensions/browser/src/browser/pw-session.ts b/extensions/browser/src/browser/pw-session.ts
index e061c652dfd..cc4e5eae7f8 100644
--- a/extensions/browser/src/browser/pw-session.ts
+++ b/extensions/browser/src/browser/pw-session.ts
@@ -1,5 +1,6 @@
 import crypto from "node:crypto";
 import path from "node:path";
+import { parseFiniteNumber } from "openclaw/plugin-sdk/number-runtime";
 import { normalizeOptionalString } from "openclaw/plugin-sdk/string-coerce-runtime";
 import type {
   Browser,
@@ -182,6 +183,11 @@ const connectingByCdpUrl = new Map<string, Promise<ConnectedBrowser>>();
 const blockedTargetsByCdpUrl = new Set<string>();
 const blockedPageRefsByCdpUrl = new Map<string, WeakSet<Page>>();
 
+function resolveObservedDialogTimeoutMs(timeoutMs: number | undefined): number {
+  const parsed = parseFiniteNumber(timeoutMs);
+  return Math.max(1, Math.floor(parsed ?? OBSERVED_DIALOG_TIMEOUT_MS));
+}
+
 function normalizeCdpUrl(raw: string) {
   return raw.replace(/\/$/, "");
 }
@@ -784,7 +790,7 @@ export function armObservedDialogResponseOnPage(opts: {
 }): void {
   const state = ensurePageState(opts.page);
   clearArmedDialogResponse(state);
-  const timeoutMs = Math.max(1, Math.floor(opts.timeoutMs ?? OBSERVED_DIALOG_TIMEOUT_MS));
+  const timeoutMs = resolveObservedDialogTimeoutMs(opts.timeoutMs);
   const response: ArmedDialogResponse = {
     accept: opts.accept,
     expiresAt: Date.now() + timeoutMs,