diff --git a/.changeset/tame-snakes-move.md b/.changeset/tame-snakes-move.md
new file mode 100644
index 00000000000..a845151cc84
--- /dev/null
+++ b/.changeset/tame-snakes-move.md
@@ -0,0 +1,2 @@
+---
+---
diff --git a/integration/presets/longRunningApps.ts b/integration/presets/longRunningApps.ts
index 876c3eaa9ca..0d2352b3e11 100644
--- a/integration/presets/longRunningApps.ts
+++ b/integration/presets/longRunningApps.ts
@@ -72,6 +72,8 @@ export const createLongRunningApps = () => {
      * Tanstack apps - basic flows
      */
     { id: 'tanstack.react-start', config: tanstack.reactStart, env: envs.withEmailCodes },
+    { id: 'tanstack.react-start.withCustomRoles', config: tanstack.reactStart, env: envs.withCustomRoles },
+    { id: 'tanstack.react-start.withEmailCodesProxy', config: tanstack.reactStart, env: envs.withEmailCodesProxy },
 
     /**
      * Various apps - basic flows
diff --git a/integration/templates/tanstack-react-start/src/routes/index.tsx b/integration/templates/tanstack-react-start/src/routes/index.tsx
index a179e88ed71..758cbb3b954 100644
--- a/integration/templates/tanstack-react-start/src/routes/index.tsx
+++ b/integration/templates/tanstack-react-start/src/routes/index.tsx
@@ -1,4 +1,4 @@
-import { Show, SignIn, SignOutButton, UserButton } from '@clerk/tanstack-react-start';
+import { OrganizationSwitcher, Show, SignIn, SignOutButton, UserButton } from '@clerk/tanstack-react-start';
 import { createFileRoute } from '@tanstack/react-router';
 
 export const Route = createFileRoute('/')({
@@ -15,6 +15,9 @@ function Home() {
           <p>View your profile here</p>
           <UserButton />
         </div>
+        <div>
+          <OrganizationSwitcher />
+        </div>
         <div>
           <SignOutButton />
         </div>
diff --git a/integration/templates/tanstack-react-start/src/routes/me.tsx b/integration/templates/tanstack-react-start/src/routes/me.tsx
new file mode 100644
index 00000000000..57fcfd375df
--- /dev/null
+++ b/integration/templates/tanstack-react-start/src/routes/me.tsx
@@ -0,0 +1,36 @@
+import { createFileRoute } from '@tanstack/react-router';
+import { createServerFn } from '@tanstack/react-start';
+import { auth } from '@clerk/tanstack-react-start/server';
+
+const fetchAuthData = createServerFn({ method: 'GET' }).handler(async () => {
+  const { userId, sessionId, orgId, orgRole, orgSlug } = await auth();
+  return { userId, sessionId, orgId, orgRole, orgSlug };
+});
+
+export const Route = createFileRoute('/me')({
+  component: MePage,
+  beforeLoad: async () => await fetchAuthData(),
+  loader: async ({ context }) => {
+    return {
+      userId: context.userId,
+      sessionId: context.sessionId,
+      orgId: context.orgId,
+      orgRole: context.orgRole,
+      orgSlug: context.orgSlug,
+    };
+  },
+});
+
+function MePage() {
+  const state = Route.useLoaderData();
+
+  return (
+    <div>
+      <p data-testid='userId'>{state.userId ?? ''}</p>
+      <p data-testid='sessionId'>{state.sessionId ?? ''}</p>
+      <p data-testid='orgId'>{state.orgId ?? ''}</p>
+      <p data-testid='orgRole'>{state.orgRole ?? ''}</p>
+      <p data-testid='orgSlug'>{state.orgSlug ?? ''}</p>
+    </div>
+  );
+}
diff --git a/integration/tests/tanstack-start/error-handling.test.ts b/integration/tests/tanstack-start/error-handling.test.ts
new file mode 100644
index 00000000000..1d3a6c3efaa
--- /dev/null
+++ b/integration/tests/tanstack-start/error-handling.test.ts
@@ -0,0 +1,35 @@
+import { expect, test } from '@playwright/test';
+
+import { appConfigs } from '../../presets';
+import { testAgainstRunningApps } from '../../testUtils';
+
+testAgainstRunningApps({ withEnv: [appConfigs.envs.withEmailCodes] })(
+  'error handling tests for @tanstack-react-start',
+  ({ app }) => {
+    test.describe.configure({ mode: 'parallel' });
+
+    test('request with invalid Authorization header is handled gracefully', async () => {
+      const url = new URL('/me', app.serverUrl);
+      const res = await fetch(url.toString(), {
+        headers: {
+          Authorization: 'Bearer invalid_token_here',
+        },
+      });
+
+      // Clerk middleware treats an invalid bearer token as unauthenticated (not a crash)
+      expect(res.status).toBe(200);
+    });
+
+    test('request with malformed cookie is handled gracefully', async () => {
+      const url = new URL('/me', app.serverUrl);
+      const res = await fetch(url.toString(), {
+        headers: {
+          Cookie: '__session=malformed_jwt_value; __client_uat=0',
+        },
+      });
+
+      // Clerk middleware handles malformed cookies gracefully, treating the request as unauthenticated
+      expect(res.status).toBe(200);
+    });
+  },
+);
diff --git a/integration/tests/tanstack-start/organizations.test.ts b/integration/tests/tanstack-start/organizations.test.ts
new file mode 100644
index 00000000000..c18e50ac0b9
--- /dev/null
+++ b/integration/tests/tanstack-start/organizations.test.ts
@@ -0,0 +1,117 @@
+import type { OrganizationMembershipRole } from '@clerk/backend';
+import { expect, test } from '@playwright/test';
+
+import { appConfigs } from '../../presets';
+import type { FakeOrganization, FakeUser } from '../../testUtils';
+import { createTestUtils, testAgainstRunningApps } from '../../testUtils';
+
+testAgainstRunningApps({ withEnv: [appConfigs.envs.withCustomRoles] })(
+  'organization auth tests for @tanstack-react-start',
+  ({ app }) => {
+    test.describe.configure({ mode: 'serial' });
+
+    let fakeAdmin: FakeUser;
+    let fakeViewer: FakeUser;
+    let fakeNonMember: FakeUser;
+    let fakeOrganization: FakeOrganization;
+
+    test.beforeAll(async () => {
+      const m = createTestUtils({ app });
+      fakeAdmin = m.services.users.createFakeUser();
+      const admin = await m.services.users.createBapiUser(fakeAdmin);
+      fakeOrganization = await m.services.users.createFakeOrganization(admin.id);
+      fakeViewer = m.services.users.createFakeUser();
+      const viewer = await m.services.users.createBapiUser(fakeViewer);
+      await m.services.clerk.organizations.createOrganizationMembership({
+        organizationId: fakeOrganization.organization.id,
+        role: 'org:viewer' as OrganizationMembershipRole,
+        userId: viewer.id,
+      });
+      fakeNonMember = m.services.users.createFakeUser();
+      await m.services.users.createBapiUser(fakeNonMember);
+    });
+
+    test.afterAll(async () => {
+      await fakeOrganization.delete();
+      await fakeNonMember.deleteIfExists();
+      await fakeViewer.deleteIfExists();
+      await fakeAdmin.deleteIfExists();
+      await app.teardown();
+    });
+
+    test('admin auth object includes orgId, orgRole, orgSlug after selecting org', async ({ page, context }) => {
+      const u = createTestUtils({ app, page, context });
+      await u.page.goToRelative('/');
+
+      await u.po.signIn.waitForMounted();
+      await u.po.signIn.signInWithEmailAndInstantPassword({
+        email: fakeAdmin.email,
+        password: fakeAdmin.password,
+      });
+
+      await u.po.userButton.waitForMounted();
+
+      await u.po.organizationSwitcher.waitForMounted();
+      await u.po.organizationSwitcher.waitForAnOrganizationToSelected();
+
+      await u.page.goToRelative('/me');
+
+      const userId = await u.page.getByTestId('userId').textContent();
+      const orgId = await u.page.getByTestId('orgId').textContent();
+      const orgRole = await u.page.getByTestId('orgRole').textContent();
+      const orgSlug = await u.page.getByTestId('orgSlug').textContent();
+
+      expect(userId).toBeTruthy();
+      expect(orgId).toBe(fakeOrganization.organization.id);
+      expect(orgRole).toBe('org:admin');
+      expect(orgSlug).toBeTruthy();
+    });
+
+    test('non-member auth object has null orgId', async ({ page, context }) => {
+      const u = createTestUtils({ app, page, context });
+      await u.page.goToRelative('/');
+
+      await u.po.signIn.waitForMounted();
+      await u.po.signIn.signInWithEmailAndInstantPassword({
+        email: fakeNonMember.email,
+        password: fakeNonMember.password,
+      });
+
+      await u.po.userButton.waitForMounted();
+
+      await u.page.goToRelative('/me');
+
+      const userId = await u.page.getByTestId('userId').textContent();
+      const orgId = await u.page.getByTestId('orgId').textContent();
+
+      expect(userId).toBeTruthy();
+      expect(orgId).toBe('');
+    });
+
+    test('viewer org role is correctly reflected in auth response', async ({ page, context }) => {
+      const u = createTestUtils({ app, page, context });
+      await u.page.goToRelative('/');
+
+      await u.po.signIn.waitForMounted();
+      await u.po.signIn.signInWithEmailAndInstantPassword({
+        email: fakeViewer.email,
+        password: fakeViewer.password,
+      });
+
+      await u.po.userButton.waitForMounted();
+
+      await u.po.organizationSwitcher.waitForMounted();
+      await u.po.organizationSwitcher.waitForAnOrganizationToSelected();
+
+      await u.page.goToRelative('/me');
+
+      const userId = await u.page.getByTestId('userId').textContent();
+      const orgId = await u.page.getByTestId('orgId').textContent();
+      const orgRole = await u.page.getByTestId('orgRole').textContent();
+
+      expect(userId).toBeTruthy();
+      expect(orgId).toBe(fakeOrganization.organization.id);
+      expect(orgRole).toBe('org:viewer');
+    });
+  },
+);
diff --git a/integration/tests/tanstack-start/proxy.test.ts b/integration/tests/tanstack-start/proxy.test.ts
new file mode 100644
index 00000000000..2b2134c91dc
--- /dev/null
+++ b/integration/tests/tanstack-start/proxy.test.ts
@@ -0,0 +1,74 @@
+import { expect, test } from '@playwright/test';
+
+import { appConfigs } from '../../presets';
+import type { FakeUser } from '../../testUtils';
+import { createTestUtils, testAgainstRunningApps } from '../../testUtils';
+
+testAgainstRunningApps({ withEnv: [appConfigs.envs.withEmailCodesProxy] })(
+  'frontend API proxy tests for @tanstack-react-start',
+  ({ app }) => {
+    test.describe.configure({ mode: 'parallel' });
+
+    let fakeUser: FakeUser;
+
+    test.beforeAll(async () => {
+      const u = createTestUtils({ app });
+      fakeUser = u.services.users.createFakeUser();
+      await u.services.users.createBapiUser(fakeUser);
+    });
+
+    test.afterAll(async () => {
+      await fakeUser.deleteIfExists();
+      await app.teardown();
+    });
+
+    test('handshake redirect uses forwarded headers for proxyUrl, not localhost', async () => {
+      // This test proves that the SDK must derive proxyUrl from x-forwarded-* headers.
+      // When a reverse proxy sits in front of the app, the raw request URL is localhost,
+      // but the handshake redirect must point to the public origin.
+      //
+      // We simulate a behind-proxy scenario by sending x-forwarded-proto and x-forwarded-host
+      // headers, with a __client_uat cookie (non-zero) but no session cookie, which forces
+      // a handshake. The handshake redirect Location should use the forwarded origin.
+      const url = new URL('/me', app.serverUrl);
+      const res = await fetch(url.toString(), {
+        headers: {
+          'x-forwarded-proto': 'https',
+          'x-forwarded-host': 'myapp.example.com',
+          'sec-fetch-dest': 'document',
+          Accept: 'text/html',
+          Cookie: '__clerk_db_jwt=needstobeset; __client_uat=1',
+        },
+        redirect: 'manual',
+      });
+
+      // The server should respond with a 307 handshake redirect
+      expect(res.status).toBe(307);
+      const location = res.headers.get('location') ?? '';
+      // The redirect must point to the public origin (from forwarded headers),
+      // NOT to http://localhost:PORT. If the SDK uses requestUrl.origin instead
+      // of forwarded headers, this assertion will fail.
+      const decoded = decodeURIComponent(location);
+      expect(decoded).toContain('https://myapp.example.com');
+      expect(decoded).not.toContain('localhost');
+    });
+
+    test('auth works correctly with proxy enabled', async ({ page, context }) => {
+      const u = createTestUtils({ app, page, context });
+      await u.page.goToRelative('/');
+
+      await u.po.signIn.waitForMounted();
+      await u.po.signIn.signInWithEmailAndInstantPassword({
+        email: fakeUser.email,
+        password: fakeUser.password,
+      });
+
+      await u.po.userButton.waitForMounted();
+
+      await u.page.goToRelative('/me');
+
+      const userId = await u.page.getByTestId('userId').textContent();
+      expect(userId).toBeTruthy();
+    });
+  },
+);
diff --git a/package.json b/package.json
index 920a4b4eae3..57229d041af 100644
--- a/package.json
+++ b/package.json
@@ -57,7 +57,7 @@
     "test:integration:react-router": "E2E_APP_ID=react-router.* pnpm test:integration:base --grep @react-router",
     "test:integration:sessions": "DISABLE_WEB_SECURITY=true E2E_SESSIONS_APP_1_ENV_KEY=sessions-prod-1 E2E_SESSIONS_APP_2_ENV_KEY=sessions-prod-2 E2E_SESSIONS_APP_1_HOST=multiple-apps-e2e.clerk.app pnpm test:integration:base --grep @sessions",
     "test:integration:sessions:staging": "DISABLE_WEB_SECURITY=true E2E_SESSIONS_APP_1_ENV_KEY=clerkstage-sessions-prod-1 E2E_SESSIONS_APP_2_ENV_KEY=clerkstage-sessions-prod-2 E2E_SESSIONS_APP_1_HOST=clerkstage-sessions-prod-1-e2e.clerk.app pnpm test:integration:base --grep @sessions",
-    "test:integration:tanstack-react-start": "E2E_APP_ID=tanstack.react-start pnpm test:integration:base --grep @tanstack-react-start",
+    "test:integration:tanstack-react-start": "E2E_APP_ID=tanstack.react-start* pnpm test:integration:base --grep @tanstack-react-start",
     "test:integration:vue": "E2E_APP_ID=vue.vite pnpm test:integration:base --grep @vue",
     "test:typedoc": "pnpm typedoc:generate && cd ./.typedoc && vitest run",
     "turbo:clean": "turbo daemon clean",