diff --git a/agentex-ui/components/providers/agentex-provider.tsx b/agentex-ui/components/providers/agentex-provider.tsx
index 6b71d1a1..04b43bbb 100644
--- a/agentex-ui/components/providers/agentex-provider.tsx
+++ b/agentex-ui/components/providers/agentex-provider.tsx
@@ -73,9 +73,14 @@ export function AgentexProvider({
       updateParams(
         {
           [SearchParamKey.SGP_ACCOUNT_ID]: id,
-          // An explicit switch (not bootstrap, which passes replace) drops the open task:
-          // it's account-scoped and won't resolve under the new account.
-          ...(replace ? {} : { [SearchParamKey.TASK_ID]: null }),
+          // Explicit switch (bootstrap passes replace): drop the account-scoped task + agent
+          // in one navigation so they don't linger under the new account.
+          ...(replace
+            ? {}
+            : {
+                [SearchParamKey.TASK_ID]: null,
+                [SearchParamKey.AGENT_NAME]: null,
+              }),
         },
         replace
       );
diff --git a/agentex-ui/hooks/use-safe-search-params.test.tsx b/agentex-ui/hooks/use-safe-search-params.test.tsx
new file mode 100644
index 00000000..6c736be9
--- /dev/null
+++ b/agentex-ui/hooks/use-safe-search-params.test.tsx
@@ -0,0 +1,83 @@
+import { renderHook } from '@testing-library/react';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+import { SearchParamKey, useSafeSearchParams } from './use-safe-search-params';
+
+// Router spies + a mutable snapshot the mocked `useSearchParams` returns. Created via
+// vi.hoisted so the hoisted vi.mock factory can reference them.
+const mocks = vi.hoisted(() => ({
+  push: vi.fn<(url: string) => void>(),
+  replace: vi.fn<(url: string) => void>(),
+  snapshot: new URLSearchParams(''),
+}));
+
+vi.mock('next/navigation', () => ({
+  useRouter: () => ({ push: mocks.push, replace: mocks.replace }),
+  usePathname: () => '/',
+  useSearchParams: () => mocks.snapshot,
+}));
+
+function setLiveUrl(search: string) {
+  window.history.replaceState(null, '', `${window.location.origin}/${search}`);
+}
+
+function pushedQuery() {
+  expect(mocks.push).toHaveBeenCalledTimes(1);
+  const url = mocks.push.mock.calls[0]![0];
+  return new URLSearchParams(url.split('?')[1] ?? '');
+}
+
+afterEach(() => {
+  mocks.push.mockClear();
+  mocks.replace.mockClear();
+});
+
+describe('useSafeSearchParams.updateParams', () => {
+  it('merges against the live URL, not a stale snapshot (no write-clobber)', () => {
+    // The live URL already reflects a committed account switch: new account, task_id dropped.
+    setLiveUrl('?account_id=new&agent_name=dua-agent');
+    // But the React snapshot still shows the pre-switch state (the lag that caused the bug).
+    mocks.snapshot = new URLSearchParams(
+      'account_id=old&task_id=T&agent_name=dua-agent'
+    );
+
+    const { result } = renderHook(() => useSafeSearchParams());
+    // A second writer (the agent-validation effect) clears only agent_name.
+    result.current.updateParams({ [SearchParamKey.AGENT_NAME]: null });
+
+    // It must build from the live URL: keep account_id=new, leave task_id dropped — not
+    // resurrect account_id=old / task_id=T from the stale snapshot.
+    const qs = pushedQuery();
+    expect(qs.get('account_id')).toBe('new');
+    expect(qs.get('task_id')).toBeNull();
+    expect(qs.get('agent_name')).toBeNull();
+  });
+
+  it('sets, deletes, and preserves unlisted params in one update', () => {
+    setLiveUrl('?account_id=acc&agent_name=keep');
+    mocks.snapshot = new URLSearchParams('account_id=acc&agent_name=keep');
+
+    const { result } = renderHook(() => useSafeSearchParams());
+    result.current.updateParams({
+      [SearchParamKey.TASK_ID]: 'new-task',
+      [SearchParamKey.AGENT_NAME]: null,
+    });
+
+    const qs = pushedQuery();
+    expect(qs.get('task_id')).toBe('new-task'); // set
+    expect(qs.get('agent_name')).toBeNull(); // deleted
+    expect(qs.get('account_id')).toBe('acc'); // preserved (unlisted)
+  });
+
+  it('uses router.replace when replace=true', () => {
+    setLiveUrl('?account_id=acc');
+    mocks.snapshot = new URLSearchParams('account_id=acc');
+
+    const { result } = renderHook(() => useSafeSearchParams());
+    result.current.updateParams({ [SearchParamKey.TASK_ID]: 'x' }, true);
+
+    expect(mocks.push).not.toHaveBeenCalled();
+    expect(mocks.replace).toHaveBeenCalledTimes(1);
+    expect(mocks.replace.mock.calls[0]![0]).toContain('task_id=x');
+  });
+});
diff --git a/agentex-ui/hooks/use-safe-search-params.ts b/agentex-ui/hooks/use-safe-search-params.ts
index f6ba3fd5..dc7aaa8e 100644
--- a/agentex-ui/hooks/use-safe-search-params.ts
+++ b/agentex-ui/hooks/use-safe-search-params.ts
@@ -31,23 +31,30 @@ export function useSafeSearchParams(): SafeSearchParams {
 
   const updateParams = useCallback(
     (updates: SearchParamUpdates, replace: boolean = false): void => {
-      const params = new URLSearchParams(searchParams.toString());
+      // Merge against the live URL, not the `searchParams` snapshot: the snapshot lags a
+      // render behind router navigations, so concurrent updates would clobber each other.
+      const live =
+        typeof window !== 'undefined'
+          ? window.location.search
+          : searchParams.toString();
+      const params = new URLSearchParams(live);
 
       Object.entries(updates).forEach(([key, value]) => {
         const paramKey = key as SearchParamKey;
-        if (value !== undefined) {
-          if (value === null) {
-            params.delete(paramKey);
-          } else {
-            params.set(paramKey, value);
-          }
+        if (value === undefined) return;
+        if (value === null) {
+          params.delete(paramKey);
+        } else {
+          params.set(paramKey, value);
         }
       });
 
+      const query = params.toString();
+      const url = query ? `${pathname}?${query}` : pathname;
       if (replace) {
-        router.replace(`${pathname}?${params.toString()}`);
+        router.replace(url);
       } else {
-        router.push(`${pathname}?${params.toString()}`);
+        router.push(url);
       }
     },
     [pathname, searchParams, router]