diff --git a/UAT_PLAYBOOK.md b/UAT_PLAYBOOK.md
index fb51993..f3daa58 100644
--- a/UAT_PLAYBOOK.md
+++ b/UAT_PLAYBOOK.md
@@ -377,6 +377,26 @@ These cases cover the `CustomerPortal` initialisation path that bridges an Authe
 | TC-WEB-5.25.10 | Unauthenticated user is sent to login (no infinite loop) | Without signing in, navigate directly to `/`. | `App.tsx` renders the LoginPage. `CustomerPortal` does not render. No `session-from-auth` request is made. |
 | TC-WEB-5.25.11 | Session persists across reload via Better Auth cookie | After TC-WEB-5.25.1 succeeds, reload the page. | Portal dashboard re-renders. A fresh `GET /api/auth/get-session` + `POST /api/portal/session-from-auth` pair runs and yields 200/201. Greeting still reads "Hi, <FirstName>". |
 
+### 5.27 Customer Portal — Authenticated HTML-route cold mount (GRO-2099)
+
+These cases guard against the regression where a customer who had just completed SSO sign-in was bounced back to `/login` (with a blank React root) when navigating directly to `/portal`, `/book`, `/schedule`, or even `/login` itself. Root cause: `Dashboard.tsx`'s `!sessionId && !isImpersonating && !getDevUser()` guard fired during the CustomerPortal's bootstrap — before the SSO bridge resolved `portalSessionId` — and redirected to `/login`. The fix: `CustomerPortal` now shows a loading state while the bootstrap is in flight, so the portal chrome and its `!sessionId` child guards do not mount prematurely. App.tsx additionally redirects an authenticated user at `/login` to `/` instead of rendering `null`.
+
+**Pre-conditions:**
+
+- TC-WEB-5.25.1 — TC-WEB-5.25.3 must pass on the build under test.
+- Clear cookies and localStorage between cases.
+
+| # | Scenario | Steps | Expected |
+|---|----------|-------|----------|
+| TC-WEB-5.27.1 | Authenticated customer lands on `/portal` after direct nav | 1. From clean state, complete TC-WEB-5.25.1 (SSO sign-in as a customer). 2. Land on `/`. 3. `browser_navigate` (full page load) directly to `/portal`. | Final URL stays at `/portal`. The React root is non-empty. The portal dashboard renders with the customer's name. No `Navigate to /login` fires. |
+| TC-WEB-5.27.2 | Authenticated customer lands on `/book` and `/schedule` after direct nav | From TC-WEB-5.27.1, `browser_navigate` to `/book` then `/schedule` (one fresh navigation each). | Each final URL stays at the navigated path. The portal chrome is visible. The page does not redirect to `/login`. |
+| TC-WEB-5.27.3 | Authenticated customer at `/login` is auto-redirected to `/` | From TC-WEB-5.27.1, `browser_navigate` to `/login`. | The browser ends at `/` (not at a blank `/login`). The portal dashboard renders. No blank React root at `/login`. |
+| TC-WEB-5.27.4 | Loading state is visible during the bootstrap, no portal chrome flash | 1. With the UAT build under test, open DevTools → Network and throttle to Slow 3G. 2. Sign in via SSO. 3. Land on `/`. | A "Loading…" element (`role="status"`) is briefly visible. The portal nav (Home / Appointments / etc.) is NOT visible during the loading window. No `Navigate to /login` fires during the bootstrap. |
+| TC-WEB-5.27.5 | SSO bridge still runs and yields 201 | From TC-WEB-5.27.4 (or TC-WEB-5.27.1), inspect Network. | The same `GET /api/auth/get-session` (200) → `POST /api/portal/session-from-auth` (201) sequence from TC-WEB-5.25.2 still runs. The customer name appears in the greeting. |
+| TC-WEB-5.27.6 | Unauthenticated direct nav to `/portal` still ends at `/login` (no regression) | Clear cookies. `browser_navigate` to `/portal`. | The portal briefly shows the loading state, then `CustomerPortal`'s `!session && !portalSessionId` guard redirects to `/login`. The login form renders. No infinite loop. |
+| TC-WEB-5.27.7 | Groomer SSO still works (no regression) | 1. From clean state, sign in via SSO as the groomer identity (uat-groomer). 2. Land on `/`. | `App.tsx`'s staff check redirects to `/admin`. The groomer nav renders. No `CustomerPortal` flash. No `/portal` redirect loop. |
+| TC-WEB-5.27.8 | Impersonation session still works (no regression) | 1. With an active impersonation session, open `/?sessionId=<id>`. | The amber "STAFF VIEW" chrome renders. The portal loads. No `/login` redirect. |
+
 ### 5.26 Customer Portal — RescheduleFlow under SSO Bridge (GRO-2012)
 
 These cases guard against the regression where an SSO-bridge customer (no `?sessionId=` URL param, no impersonation session) could trigger the RescheduleFlow and have `RescheduleFlow` receive `sessionId={null}`, which caused the internal `/api/book/availability` call to send `X-Impersonation-Session-Id: ` (empty) and return 401. The fix: `CustomerPortal` now passes `sessionId={session?.id ?? portalSessionId}` to `<RescheduleFlow>` (matching the fallback `renderSection()` already used).
diff --git a/src/App.tsx b/src/App.tsx
index 1bbb17f..b97da60 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -378,8 +378,12 @@ export function App() {
     return <Navigate to="/login" replace />;
   }
 
-  // Show login BEFORE checking needsSetup (needsSetup is never set for unauthenticated users)
-  if (!authDisabled && !session) {
+  // Show login BEFORE checking needsSetup (needsSetup is never set for unauthenticated users).
+  // At /login with a valid session, fall through so the staff redirect below can
+  // route staff to /admin and the final render can redirect customers to / (portal).
+  // Previously, an authenticated customer at /login would see a blank page because
+  // the final render returns null at /login (showCustomerPortal is false). See GRO-2099.
+  if (!authDisabled && !session && location.pathname === "/login") {
     return <LoginPage />;
   }
 
@@ -401,6 +405,14 @@ export function App() {
   // Don't render portal chrome at /login — DevLoginSelector is shown instead
   const showCustomerPortal = !location.pathname.startsWith("/admin") && location.pathname !== "/login";
 
+  // At /login with a valid session, redirect to the portal root. Without this,
+  // the final render returns null at /login (showCustomerPortal is false) and
+  // the user sees a blank page after a successful sign-in. Staff are routed
+  // to /admin by the earlier staff check. See GRO-2099.
+  if (!authDisabled && session && location.pathname === "/login") {
+    return <Navigate to="/" replace />;
+  }
+
   return (
     <BrandingProvider>
       {location.pathname.startsWith("/admin") ? (
diff --git a/src/__tests__/portal.test.tsx b/src/__tests__/portal.test.tsx
index 96f1adf..73e0a92 100644
--- a/src/__tests__/portal.test.tsx
+++ b/src/__tests__/portal.test.tsx
@@ -558,4 +558,58 @@ describe("CustomerPortal SSO bridge", () => {
     expect(lastProps!.sessionId).toBe("sso-sess-1");
     expect(lastProps!.appointment.id).toBe("appt-1");
   });
+
+  // GRO-2099 regression: the portal chrome (and Dashboard's `!sessionId` guard)
+  // must NOT render before the SSO bridge resolves. A loading state must be
+  // shown instead. Previously, the Dashboard's redirect-to-/login guard fired
+  // mid-bootstrap, leaving the user with a blank page after sign-in.
+  it("renders a loading state during the SSO bridge (does not flash portal chrome)", async () => {
+    // Slow bridge: resolve get-session and session-from-auth after a tick so
+    // we can observe the loading state mid-bootstrap.
+    let resolveBridge!: (value: Response) => void;
+    const bridgePromise = new Promise<Response>((resolve) => {
+      resolveBridge = resolve;
+    });
+
+    global.fetch = vi.fn((input: RequestInfo, init?: RequestInit) => {
+      const url = typeof input === "string" ? input : input.toString();
+      if (url === "/api/branding") return Promise.resolve(brandingResponse);
+      if (url === "/api/auth/get-session") {
+        return Promise.resolve({
+          ok: true,
+          json: async () => ({ user: { email: "customer@example.com", role: "customer" } }),
+        } as Response);
+      }
+      if (url === "/api/portal/session-from-auth" && init?.method === "POST") {
+        return bridgePromise;
+      }
+      return Promise.resolve({ ok: true, json: async () => ({}) } as Response);
+    }) as unknown as typeof fetch;
+
+    const { CustomerPortal } = await import("../portal/CustomerPortal.js");
+    render(
+      <MemoryRouter initialEntries={["/"]}>
+        <CustomerPortal />
+      </MemoryRouter>
+    );
+
+    // Loading state is visible while the bridge is in flight. The portal nav
+    // (Home / Appointments / etc.) must NOT be present — its presence would
+    // indicate the chrome is rendering with a null session, which is the
+    // pre-GRO-2099 bug.
+    expect(await screen.findByRole("status")).toHaveTextContent(/Loading/i);
+    expect(screen.queryByText("Home")).not.toBeInTheDocument();
+    expect(screen.queryByText("Appointments")).not.toBeInTheDocument();
+
+    // Resolve the bridge and confirm the portal renders normally.
+    resolveBridge({
+      ok: true,
+      status: 201,
+      json: async () => ({ sessionId: "sso-sess-1", clientId: "client-1", clientName: "Jane Doe" }),
+    } as Response);
+
+    await waitFor(() => {
+      expect(screen.getByText(/Hi, Jane/)).toBeInTheDocument();
+    });
+  });
 });
diff --git a/src/portal/CustomerPortal.tsx b/src/portal/CustomerPortal.tsx
index 3b284b7..acb2f44 100644
--- a/src/portal/CustomerPortal.tsx
+++ b/src/portal/CustomerPortal.tsx
@@ -241,13 +241,31 @@ export function CustomerPortal() {
 
   const avatarInitials = (clientName.split(" ")[0] || "G").charAt(0).toUpperCase();
 
+  // Show a loading state while the SSO bridge is in progress. The portal chrome
+  // and its sections (e.g. Dashboard) assume a session is established and run
+  // their own auth guards — rendering them before the bridge resolves triggers
+  // a redirect to /login from `Dashboard.tsx`'s `!sessionId` check, breaking the
+  // post-sign-in flow. Once `initComplete` is true we know whether a session was
+  // established and can render the correct branch. See GRO-2099.
+  if (!initComplete) {
+    return (
+      <div
+        className="min-h-screen flex items-center justify-center bg-[#faf8f5]"
+        role="status"
+        aria-live="polite"
+      >
+        <div className="text-stone-500 text-sm">Loading…</div>
+      </div>
+    );
+  }
+
   // After init completes, redirect unauthenticated users to /login and staff to /admin.
   // The portal chrome must NEVER be visible to users without a valid client session.
   // For client dev users, we stay on the portal even if session is null — the dev-session
   // response may not have id set immediately, or there may be timing issues with the
   // session state. Dev users are verified via localStorage and the dev-session flow.
   // SSO customers are recognised by portalSessionId (set by the Better Auth bridge).
-  if (initComplete && !session && !portalSessionId) {
+  if (!session && !portalSessionId) {
     if (authError) {
       // GRO-1867: graceful 404 fallback — authenticated user has no client row.
       return (