diff --git a/.changeset/signin-multisession-start.md b/.changeset/signin-multisession-start.md
new file mode 100644
index 00000000000..d8874431ba4
--- /dev/null
+++ b/.changeset/signin-multisession-start.md
@@ -0,0 +1,6 @@
+---
+'@clerk/ui': minor
+'@clerk/shared': minor
+---
+
+Add a `multiSessionStart` prop to `<SignIn />`. On multi-session instances, `'switcher'` starts a signed-in visitor on the account switcher instead of the identifier form, so flows that route through sign-in (such as OAuth authorization) can continue with an existing account. Defaults to `'form'`; ignored in single-session mode. "Add account" from the switcher now preserves the current `redirect_url`; from the switcher and the `<UserButton />` it opens the sign-in form directly instead of returning to the switcher.
diff --git a/packages/shared/src/internal/clerk-js/constants.ts b/packages/shared/src/internal/clerk-js/constants.ts
index c11db68f590..5ba7ebb8680 100644
--- a/packages/shared/src/internal/clerk-js/constants.ts
+++ b/packages/shared/src/internal/clerk-js/constants.ts
@@ -1,5 +1,8 @@
 import type { SignUpModes } from '../../types';
 
+// Set on add-account navigations so the sign-in start screen renders the identifier form instead of the account switcher.
+export const CLERK_ADD_ACCOUNT = '__clerk_add_account';
+
 // TODO: Do we still have a use for this or can we simply preserve all params?
 export const PRESERVED_QUERYSTRING_PARAMS = [
   'redirect_url',
@@ -9,6 +12,7 @@ export const PRESERVED_QUERYSTRING_PARAMS = [
   'sign_in_fallback_redirect_url',
   'sign_up_force_redirect_url',
   'sign_up_fallback_redirect_url',
+  CLERK_ADD_ACCOUNT,
 ];
 
 export const CLERK_MODAL_STATE = '__clerk_modal_state';
diff --git a/packages/shared/src/internal/clerk-js/url.ts b/packages/shared/src/internal/clerk-js/url.ts
index a9216b9a7db..de43bf8f673 100644
--- a/packages/shared/src/internal/clerk-js/url.ts
+++ b/packages/shared/src/internal/clerk-js/url.ts
@@ -4,6 +4,7 @@ import { logger } from '../../logger';
 import type { SignUpResource } from '../../types';
 import { camelToSnake } from '../../underscore';
 import { isCurrentDevAccountPortalOrigin, isLegacyDevAccountPortalOrigin } from '../../url';
+import { CLERK_ADD_ACCOUNT } from './constants';
 import { joinPaths } from './path';
 import { getQueryParams } from './querystring';
 
@@ -156,6 +157,14 @@ export function buildURL(params: BuildURLParams, options: BuildURLOptions<boolea
   return url;
 }
 
+/**
+ * @internal
+ * Flags a sign-in navigation as "add account" so the start screen renders the form, not the account switcher.
+ */
+export const buildAddAccountUrl = (base: string): string => {
+  return buildURL({ base, hashSearchParams: { [CLERK_ADD_ACCOUNT]: 'true' } }, { stringify: true });
+};
+
 export function toURL(url: string | URL): URL {
   return new URL(url.toString(), window.location.origin);
 }
diff --git a/packages/shared/src/types/clerk.ts b/packages/shared/src/types/clerk.ts
index 49f7875870c..1e831ae1219 100644
--- a/packages/shared/src/types/clerk.ts
+++ b/packages/shared/src/types/clerk.ts
@@ -1902,6 +1902,14 @@ export type SignInProps = RoutingOptions & {
    * Optional for `oauth_<provider>` or `enterprise_sso` strategies. The value to pass to the [OIDC prompt parameter](https://openid.net/specs/openid-connect-core-1_0.html#:~:text=prompt,reauthentication%20and%20consent.) in the generated OAuth redirect URL.
    */
   oidcPrompt?: string;
+  /**
+   * On multi-session instances, where a signed-in visitor lands when opening the sign-in component.
+   * `'form'` renders the identifier form. `'switcher'` renders the account switcher listing the signed-in accounts,
+   * with "Add account" and "Sign out of all accounts". Ignored in single-session mode.
+   *
+   * @default 'form'
+   */
+  multiSessionStart?: 'form' | 'switcher';
 } & TransferableOption &
   SignUpForceRedirectUrl &
   SignUpFallbackRedirectUrl &
diff --git a/packages/ui/src/components/SignIn/SignInAccountSwitcher.tsx b/packages/ui/src/components/SignIn/SignInAccountSwitcher.tsx
index 3540530b6c7..9ccd4ea6ffa 100644
--- a/packages/ui/src/components/SignIn/SignInAccountSwitcher.tsx
+++ b/packages/ui/src/components/SignIn/SignInAccountSwitcher.tsx
@@ -1,3 +1,5 @@
+import { CLERK_ADD_ACCOUNT } from '@clerk/shared/internal/clerk-js/constants';
+
 import { Action, Actions } from '@/ui/elements/Actions';
 import { Card } from '@/ui/elements/Card';
 import { useCardState, withCardStateProvider } from '@/ui/elements/contexts';
@@ -9,23 +11,31 @@ import { withRedirectToAfterSignIn } from '../../common';
 import { useEnvironment, useSignInContext, useSignOutContext } from '../../contexts';
 import { Col, descriptors, Flow, localizationKeys } from '../../customizables';
 import { Add, ArrowRight } from '../../icons';
+import { useRouter } from '../../router';
 import { SignOutAllActions } from '../UserButton/SessionActions';
 import { useMultisessionActions } from '../UserButton/useMultisessionActions';
 
-const SignInAccountSwitcherInternal = () => {
+type SignInAccountSwitcherProps = {
+  // Route of the sign-in start screen relative to where the switcher is mounted.
+  addAccountPath?: string;
+};
+
+const SignInAccountSwitcherInternal = ({ addAccountPath = '..' }: SignInAccountSwitcherProps) => {
   const card = useCardState();
   const { userProfileUrl } = useEnvironment().displayConfig;
-  const { afterSignInUrl, path: signInPath, signInUrl, taskUrl } = useSignInContext();
+  const { afterSignInUrl, signInUrl, taskUrl } = useSignInContext();
   const { navigateAfterSignOut } = useSignOutContext();
-  const { handleSignOutAllClicked, handleSessionClicked, signedInSessions, handleAddAccountClicked } =
-    useMultisessionActions({
-      taskUrl,
-      navigateAfterSignOut,
-      afterSwitchSessionUrl: afterSignInUrl,
-      userProfileUrl,
-      signInUrl: signInPath ?? signInUrl,
-      user: undefined,
-    });
+  const { navigate } = useRouter();
+  const { handleSignOutAllClicked, handleSessionClicked, signedInSessions } = useMultisessionActions({
+    taskUrl,
+    navigateAfterSignOut,
+    afterSwitchSessionUrl: afterSignInUrl,
+    userProfileUrl,
+    signInUrl,
+    user: undefined,
+  });
+  const handleAddAccountClicked = () =>
+    navigate(addAccountPath, { searchParams: new URLSearchParams({ [CLERK_ADD_ACCOUNT]: 'true' }) });
 
   return (
     <Flow.Part part='accountSwitcher'>
diff --git a/packages/ui/src/components/SignIn/SignInStart.tsx b/packages/ui/src/components/SignIn/SignInStart.tsx
index a95040b8465..6e8ff6f986b 100644
--- a/packages/ui/src/components/SignIn/SignInStart.tsx
+++ b/packages/ui/src/components/SignIn/SignInStart.tsx
@@ -1,5 +1,5 @@
 import { getAlternativePhoneCodeProviderData } from '@clerk/shared/alternativePhoneCode';
-import { ERROR_CODES, SIGN_UP_MODES } from '@clerk/shared/internal/clerk-js/constants';
+import { CLERK_ADD_ACCOUNT, ERROR_CODES, SIGN_UP_MODES } from '@clerk/shared/internal/clerk-js/constants';
 import { clerkInvalidFAPIResponse } from '@clerk/shared/internal/clerk-js/errors';
 import { getClerkQueryParam, removeClerkQueryParam } from '@clerk/shared/internal/clerk-js/queryParams';
 import { useClerk } from '@clerk/shared/react';
@@ -45,6 +45,7 @@ import {
   SIGN_IN_RESET_PASSWORD_INTENT_PARAM,
   useHandleAuthenticateWithPasskey,
 } from './shared';
+import { SignInAccountSwitcher } from './SignInAccountSwitcher';
 import { SignInAlternativePhoneCodePhoneNumberCard } from './SignInAlternativePhoneCodePhoneNumberCard';
 import { SignInSocialButtons } from './SignInSocialButtons';
 import {
@@ -797,6 +798,21 @@ const InstantPasswordRow = ({
   );
 };
 
-export const SignInStart = withRedirectToSignInTask(
-  withRedirectToAfterSignIn(withCardStateProvider(SignInStartInternal)),
-);
+const SignInStartCard = withRedirectToSignInTask(withRedirectToAfterSignIn(withCardStateProvider(SignInStartInternal)));
+
+export const SignInStart = () => {
+  const clerk = useClerk();
+  const { authConfig } = useEnvironment();
+  const { multiSessionStart } = useSignInContext();
+  const { queryParams } = useRouter();
+  // Snapshot on mount: the sign-in POST adds a session before setActive navigates; keep the form until then.
+  const [hadSignedInSessions] = useState(() => clerk.client.signedInSessions.length > 0);
+
+  const showSwitcher =
+    multiSessionStart === 'switcher' &&
+    !authConfig.singleSessionMode &&
+    hadSignedInSessions &&
+    queryParams[CLERK_ADD_ACCOUNT] === undefined;
+
+  return showSwitcher ? <SignInAccountSwitcher addAccountPath='.' /> : <SignInStartCard />;
+};
diff --git a/packages/ui/src/components/SignIn/__tests__/SignInAccountSwitcher.test.tsx b/packages/ui/src/components/SignIn/__tests__/SignInAccountSwitcher.test.tsx
index 54a8cd799de..f00d87128e6 100644
--- a/packages/ui/src/components/SignIn/__tests__/SignInAccountSwitcher.test.tsx
+++ b/packages/ui/src/components/SignIn/__tests__/SignInAccountSwitcher.test.tsx
@@ -36,12 +36,20 @@ describe('SignInAccountSwitcher', () => {
     expect(fixtures.clerk.setActive).toHaveBeenCalled();
   });
 
-  // this one uses the windowNavigate method. we need to mock it correctly
-  it.skip('navigates to SignInStart component if user clicks on "Add account" button', async () => {
+  it('navigates to the sign-in start screen with the add-account param when "Add account" is clicked', async () => {
     const { wrapper, fixtures } = await createFixtures(initConfig);
     const { userEvent, getByText } = render(<SignInAccountSwitcher />, { wrapper });
     await userEvent.click(getByText('Add account'));
-    expect(fixtures.router.navigate).toHaveBeenCalled();
+    expect(fixtures.router.navigate).toHaveBeenCalledWith('..', {
+      searchParams: new URLSearchParams({ __clerk_add_account: 'true' }),
+    });
+  });
+
+  it('navigates to the given start screen path when rendered in place', async () => {
+    const { wrapper, fixtures } = await createFixtures(initConfig);
+    const { userEvent, getByText } = render(<SignInAccountSwitcher addAccountPath='.' />, { wrapper });
+    await userEvent.click(getByText('Add account'));
+    expect(fixtures.router.navigate).toHaveBeenCalledWith('.', expect.anything());
   });
 
   it('signs out when user clicks on "Sign out of all accounts"', async () => {
diff --git a/packages/ui/src/components/SignIn/__tests__/SignInStart.test.tsx b/packages/ui/src/components/SignIn/__tests__/SignInStart.test.tsx
index 01bd5d4e3a0..e5b2ec061bd 100644
--- a/packages/ui/src/components/SignIn/__tests__/SignInStart.test.tsx
+++ b/packages/ui/src/components/SignIn/__tests__/SignInStart.test.tsx
@@ -1,7 +1,7 @@
 import { ClerkAPIResponseError, ClerkWebAuthnError } from '@clerk/shared/error';
-import { CAPTCHA_ELEMENT_ID } from '@clerk/shared/internal/clerk-js/constants';
+import { CAPTCHA_ELEMENT_ID, CLERK_ADD_ACCOUNT } from '@clerk/shared/internal/clerk-js/constants';
 import { OAUTH_PROVIDERS } from '@clerk/shared/oauth';
-import type { SignInResource } from '@clerk/shared/types';
+import type { SignedInSessionResource, SignInResource } from '@clerk/shared/types';
 import { waitFor } from '@testing-library/react';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 
@@ -66,6 +66,93 @@ describe('SignInStart', () => {
     screen.getAllByText(/sign in to .*/i);
   });
 
+  describe('multi-session start', () => {
+    const withSignedInSessions = createFixtures.config(f => {
+      f.withEmailAddress();
+      f.withMultiSessionMode();
+      f.withUser({ email_addresses: ['test1@clerk.com'] });
+    });
+    const { createFixtures: createFixturesWithAddAccount } = bindCreateFixtures('SignIn', {
+      router: { queryParams: { [CLERK_ADD_ACCOUNT]: 'true' } },
+    });
+    const expectForm = () => {
+      screen.getAllByText(/sign in to .*/i);
+      expect(screen.queryByText('Add account')).toBeNull();
+    };
+    const expectSwitcher = () => {
+      screen.getByText('Add account');
+      expect(screen.queryByText(/sign in to .*/i)).toBeNull();
+    };
+
+    it('renders the identifier form when the prop is unset and signed-in sessions exist', async () => {
+      const { wrapper } = await createFixtures(withSignedInSessions);
+      render(<SignInStart />, { wrapper });
+      expectForm();
+    });
+
+    it('renders the account switcher when the prop is "switcher" and signed-in sessions exist', async () => {
+      const { wrapper, fixtures, props } = await createFixtures(withSignedInSessions);
+      props.setProps({ multiSessionStart: 'switcher' });
+      render(<SignInStart />, { wrapper });
+      expectSwitcher();
+      expect(fixtures.router.navigate).not.toHaveBeenCalled();
+    });
+
+    it('renders the identifier form when the prop is "switcher" and no signed-in sessions exist', async () => {
+      const { wrapper, props } = await createFixtures(f => {
+        f.withEmailAddress();
+        f.withMultiSessionMode();
+      });
+      props.setProps({ multiSessionStart: 'switcher' });
+      render(<SignInStart />, { wrapper });
+      expectForm();
+    });
+
+    it('keeps the identifier form when a session appears after mount', async () => {
+      const { wrapper, fixtures, props } = await createFixtures(f => {
+        f.withEmailAddress();
+        f.withMultiSessionMode();
+      });
+      props.setProps({ multiSessionStart: 'switcher' });
+      const { rerender } = render(<SignInStart />, { wrapper });
+      vi.spyOn(fixtures.clerk.client, 'signedInSessions', 'get').mockReturnValue([
+        { id: 'sess_1' } as unknown as SignedInSessionResource,
+      ]);
+      rerender(<SignInStart />);
+      expectForm();
+    });
+
+    it('renders the identifier form when the add-account param is set', async () => {
+      const { wrapper, props } = await createFixturesWithAddAccount(withSignedInSessions);
+      props.setProps({ multiSessionStart: 'switcher' });
+      render(<SignInStart />, { wrapper });
+      expectForm();
+    });
+
+    it('carries the add-account param through the OAuth callback URL', async () => {
+      const { wrapper, fixtures } = await createFixturesWithAddAccount(f => {
+        f.withMultiSessionMode();
+        f.withSocialProvider({ provider: 'google' });
+      });
+      const { userEvent } = render(<SignInStart />, { wrapper });
+      await userEvent.click(screen.getByText('Continue with Google'));
+      expect(fixtures.signIn.authenticateWithRedirect).toHaveBeenCalledWith(
+        expect.objectContaining({ redirectUrl: expect.stringContaining('__clerk_add_account=true') }),
+      );
+    });
+
+    it('ignores the prop in single-session mode', async () => {
+      const { wrapper, fixtures, props } = await createFixtures(f => {
+        f.withEmailAddress();
+        f.withUser({ email_addresses: ['test1@clerk.com'] });
+      });
+      props.setProps({ multiSessionStart: 'switcher' });
+      render(<SignInStart />, { wrapper });
+      expect(screen.queryByText('Add account')).toBeNull();
+      expect(fixtures.router.navigate).toHaveBeenCalledWith('/');
+    });
+  });
+
   describe('Login Methods', () => {
     it('enables login with email address', async () => {
       const { wrapper } = await createFixtures(f => {
diff --git a/packages/ui/src/components/UserButton/__tests__/UserButton.test.tsx b/packages/ui/src/components/UserButton/__tests__/UserButton.test.tsx
index 06d19655215..b8c9009eba4 100644
--- a/packages/ui/src/components/UserButton/__tests__/UserButton.test.tsx
+++ b/packages/ui/src/components/UserButton/__tests__/UserButton.test.tsx
@@ -1,12 +1,15 @@
 import { UNSAFE_PortalProvider } from '@clerk/shared/react';
 import React from 'react';
-import { describe, expect, it } from 'vitest';
+import { describe, expect, it, vi } from 'vitest';
 
 import { bindCreateFixtures } from '@/test/create-fixtures';
 import { render, screen, waitFor } from '@/test/utils';
+import { clerkWindowNavigate } from '@/ui/utils/windowNavigate';
 
 import { UserButton } from '../';
 
+vi.mock('@/ui/utils/windowNavigate', () => ({ clerkWindowNavigate: vi.fn() }));
+
 const { createFixtures } = bindCreateFixtures('UserButton');
 
 describe('UserButton', () => {
@@ -87,8 +90,6 @@ describe('UserButton', () => {
     expect(fixtures.router.navigate).toHaveBeenCalledWith('/');
   });
 
-  it.todo('navigates to sign in url when "Add account" is clicked');
-
   describe('UserButton with PortalProvider', () => {
     it('passes getContainer to openUserProfile when wrapped in PortalProvider', async () => {
       const container = document.createElement('div');
@@ -157,6 +158,17 @@ describe('UserButton', () => {
       expect(getByText('First3 Last3')).toBeDefined();
     });
 
+    it('navigates to the sign-in URL with the add-account param when "Add account" is clicked', async () => {
+      const { wrapper } = await createFixtures(initConfig);
+      const { getByText, getByRole, userEvent } = render(<UserButton />, { wrapper });
+      await userEvent.click(getByRole('button', { name: 'Open user menu' }));
+      await userEvent.click(getByText('Add account'));
+      expect(clerkWindowNavigate).toHaveBeenLastCalledWith(
+        expect.anything(),
+        expect.stringContaining('__clerk_add_account=true'),
+      );
+    });
+
     it('changes the active session when clicking another session', async () => {
       const { wrapper, fixtures } = await createFixtures(initConfig);
       fixtures.clerk.setActive.mockReturnValueOnce(Promise.resolve());
diff --git a/packages/ui/src/components/UserButton/useMultisessionActions.tsx b/packages/ui/src/components/UserButton/useMultisessionActions.tsx
index bb46235f382..48a0c736a7e 100644
--- a/packages/ui/src/components/UserButton/useMultisessionActions.tsx
+++ b/packages/ui/src/components/UserButton/useMultisessionActions.tsx
@@ -1,4 +1,5 @@
 import { navigateIfTaskExists } from '@clerk/shared/internal/clerk-js/sessionTasks';
+import { buildAddAccountUrl } from '@clerk/shared/internal/clerk-js/url';
 import { useClerk, usePortalRoot } from '@clerk/shared/react';
 import type { SignedInSessionResource, UserButtonProps, UserResource } from '@clerk/shared/types';
 
@@ -102,7 +103,7 @@ export const useMultisessionActions = (opts: UseMultisessionActionsParams) => {
   };
 
   const handleAddAccountClicked = () => {
-    clerkWindowNavigate(clerk, opts.signInUrl || window.location.href);
+    clerkWindowNavigate(clerk, buildAddAccountUrl(opts.signInUrl || window.location.href));
     return sleep(2000);
   };
 
diff --git a/packages/ui/src/contexts/components/SignIn.ts b/packages/ui/src/contexts/components/SignIn.ts
index 2d44b0bb838..b1e1058cb1b 100644
--- a/packages/ui/src/contexts/components/SignIn.ts
+++ b/packages/ui/src/contexts/components/SignIn.ts
@@ -1,4 +1,8 @@
-import { SIGN_IN_INITIAL_VALUE_KEYS, SIGN_UP_MODES } from '@clerk/shared/internal/clerk-js/constants';
+import {
+  CLERK_ADD_ACCOUNT,
+  SIGN_IN_INITIAL_VALUE_KEYS,
+  SIGN_UP_MODES,
+} from '@clerk/shared/internal/clerk-js/constants';
 import { RedirectUrls } from '@clerk/shared/internal/clerk-js/redirectUrls';
 import { getTaskEndpoint } from '@clerk/shared/internal/clerk-js/sessionTasks';
 import { buildURL } from '@clerk/shared/internal/clerk-js/url';
@@ -101,7 +105,12 @@ export const useSignInContext = (): SignInContextType => {
   signUpUrl = buildURL({ base: signUpUrl, hashSearchParams: [queryParams, preservedParams] }, { stringify: true });
   waitlistUrl = buildURL({ base: waitlistUrl, hashSearchParams: [queryParams, preservedParams] }, { stringify: true });
 
-  const authQueryString = redirectUrls.toSearchParams().toString();
+  const authSearchParams = redirectUrls.toSearchParams();
+  if (queryParams[CLERK_ADD_ACCOUNT]) {
+    // Survives the OAuth / email-link round trip so a failed add-account attempt lands back on the form.
+    authSearchParams.set(CLERK_ADD_ACCOUNT, queryParams[CLERK_ADD_ACCOUNT]);
+  }
+  const authQueryString = authSearchParams.toString();
 
   // Callback routes owned by the SignIn tree are always SignIn-rooted — including the combined-flow
   // branches mounted at `create/sso-callback` and `create/verify` under the SignIn component
diff --git a/packages/ui/src/mosaic/user-button/__tests__/user-button.model.test.tsx b/packages/ui/src/mosaic/user-button/__tests__/user-button.model.test.tsx
index 7f4ee1ea062..1e2e39d9bd0 100644
--- a/packages/ui/src/mosaic/user-button/__tests__/user-button.model.test.tsx
+++ b/packages/ui/src/mosaic/user-button/__tests__/user-button.model.test.tsx
@@ -789,7 +789,7 @@ describe('useUserButtonModel', () => {
     expect(navigate).not.toHaveBeenCalled();
 
     fireEvent.click(screen.getByText('add-account'));
-    expect(navigate).toHaveBeenCalledWith('/sign-in');
+    expect(navigate).toHaveBeenCalledWith('http://localhost:3000/sign-in#/?__clerk_add_account=true');
   });
 
   it('navigates to a create-organization URL when one is given', () => {
diff --git a/packages/ui/src/mosaic/user-button/user-button.model.tsx b/packages/ui/src/mosaic/user-button/user-button.model.tsx
index 20404039508..58360116214 100644
--- a/packages/ui/src/mosaic/user-button/user-button.model.tsx
+++ b/packages/ui/src/mosaic/user-button/user-button.model.tsx
@@ -1,4 +1,5 @@
 import { buildTaskUrl } from '@clerk/shared/internal/clerk-js/sessionTasks';
+import { buildAddAccountUrl } from '@clerk/shared/internal/clerk-js/url';
 import { getFullName, getIdentifier } from '@clerk/shared/internal/clerk-js/user';
 import { useClerk, useOrganization, usePortalRoot, useSession, useUser } from '@clerk/shared/react';
 import type { CustomPage, OrganizationResource, UserResource } from '@clerk/shared/types';
@@ -292,7 +293,9 @@ export function useUserButtonModel(
     onInviteMembers: canInviteMembers ? () => clerk.openInviteMembers({ getContainer }) : undefined,
     // Covers both restricted instances and users at their creation limit.
     onCreateOrganization: user.createOrganizationEnabled ? createOrganization : undefined,
-    onAddAccount: singleSessionMode ? undefined : () => void router.navigate(clerk.buildSignInUrl()),
+    onAddAccount: singleSessionMode
+      ? undefined
+      : () => void router.navigate(buildAddAccountUrl(clerk.buildSignInUrl())),
     onAcceptSuggestion: async suggestionId => {
       const suggestion = suggestionData.find(s => s.id === suggestionId);
       try {
diff --git a/packages/ui/src/router/HashRouter.tsx b/packages/ui/src/router/HashRouter.tsx
index 1d1e2e8b452..0de46158a18 100644
--- a/packages/ui/src/router/HashRouter.tsx
+++ b/packages/ui/src/router/HashRouter.tsx
@@ -18,7 +18,9 @@ export const HashRouter = ({ preservedParams, children }: HashRouterProps): JSX.
     if (!toURL) {
       return;
     }
-    window.location.hash = stripOrigin(toURL).substring(1 + hashRouterBase.length);
+    const hash = stripOrigin(toURL).substring(1 + hashRouterBase.length);
+    // The index route with a query would otherwise become `#?x`, which is not read as a fragment URL.
+    window.location.hash = hash.startsWith('?') ? '/' + hash : hash;
     return Promise.resolve();
   };
 
diff --git a/packages/ui/src/router/__tests__/BaseRouter.test.tsx b/packages/ui/src/router/__tests__/BaseRouter.test.tsx
index 901ca2ea078..e48ff2c2b07 100644
--- a/packages/ui/src/router/__tests__/BaseRouter.test.tsx
+++ b/packages/ui/src/router/__tests__/BaseRouter.test.tsx
@@ -1,5 +1,6 @@
+import { PRESERVED_QUERYSTRING_PARAMS } from '@clerk/shared/internal/clerk-js/constants';
 import type { Clerk } from '@clerk/shared/types';
-import { act, render, screen } from '@testing-library/react';
+import { act, render, screen, waitFor } from '@testing-library/react';
 import React from 'react';
 import { afterAll, afterEach, beforeAll, describe, expect, it, vi } from 'vitest';
 
@@ -195,4 +196,44 @@ describe('BaseRouter basePath guard', () => {
       expect(screen.getByTestId('factor-one')).toBeInTheDocument();
     });
   });
+
+  describe('preserved query params', () => {
+    it('carries __clerk_add_account across an internal navigation', async () => {
+      setWindowLocation('https://www.example.com/sign-in?__clerk_add_account=true');
+
+      const NavigateTrigger = () => {
+        const router = useRouter();
+        return (
+          <button
+            type='button'
+            data-testid='go'
+            onClick={() => void router.navigate('factor-one')}
+          >
+            Go
+          </button>
+        );
+      };
+
+      render(
+        <PathRouter
+          basePath='/sign-in'
+          preservedParams={PRESERVED_QUERYSTRING_PARAMS}
+        >
+          <Route path='factor-one'>
+            <div data-testid='factor-one'>Factor One</div>
+          </Route>
+          <Route index>
+            <NavigateTrigger />
+          </Route>
+        </PathRouter>,
+      );
+
+      act(() => {
+        screen.getByTestId('go').click();
+      });
+
+      await waitFor(() => expect(screen.getByTestId('factor-one')).toBeInTheDocument());
+      expect(mockNavigate).toHaveBeenCalledWith(expect.stringContaining('__clerk_add_account=true'));
+    });
+  });
 });
diff --git a/packages/ui/src/router/__tests__/HashRouter.test.tsx b/packages/ui/src/router/__tests__/HashRouter.test.tsx
index ab6ee1c17a8..b0720013e09 100644
--- a/packages/ui/src/router/__tests__/HashRouter.test.tsx
+++ b/packages/ui/src/router/__tests__/HashRouter.test.tsx
@@ -25,12 +25,16 @@ vi.mock('@clerk/shared/react', () => {
   };
 });
 
-const Button = ({ to, children }: React.PropsWithChildren<{ to: string }>) => {
+const Button = ({
+  to,
+  searchParams,
+  children,
+}: React.PropsWithChildren<{ to: string; searchParams?: URLSearchParams }>) => {
   const router = useRouter();
   return (
     <button
       onClick={() => {
-        void router.navigate(to);
+        void router.navigate(to, { searchParams });
       }}
     >
       {children}
@@ -47,6 +51,12 @@ const Tester = () => (
     </Route>
     <Route path='foo'>
       <div id='bar'>Bar</div>
+      <Button
+        to='..'
+        searchParams={new URLSearchParams({ flag: '1' })}
+      >
+        Index with query
+      </Button>
     </Route>
   </HashRouter>
 );
@@ -106,4 +116,21 @@ describe('HashRouter', () => {
       expect(mockNavigate).toHaveBeenNthCalledWith(1, 'https://www.example.com/external');
     });
   });
+
+  describe('when navigating to the index route with a query', () => {
+    beforeEach(() => {
+      // @ts-ignore
+      window.location = new URL('https://www.example.com/hash#/foo?preserved=1');
+    });
+
+    it('keeps the fragment readable as a URL', async () => {
+      render(<Tester />);
+
+      const button = screen.getByRole('button', { name: /Index with query/i });
+      await userEvent.click(button);
+
+      expect(window.location.hash).toBe('#/?flag=1&preserved=1');
+      expect(screen.queryByText('Index')).toBeInTheDocument();
+    });
+  });
 });