diff --git a/backend/app/routers/auth.py b/backend/app/routers/auth.py
index b856287..74f8692 100644
--- a/backend/app/routers/auth.py
+++ b/backend/app/routers/auth.py
@@ -121,9 +121,21 @@ def signup_policy(db: Session = Depends(get_db)):
     # door that is locked. It was offering one: registration can be turned off
     # site-wide, and the only way to find out was to fill the form in and be
     # refused.
+    # Single sign-on closes the password door, and this is the one question
+    # the sign-up form already asks. Saying so here means the form can decline
+    # to draw itself rather than collect a name, an email, a password twice and
+    # an invite code, and then be refused by the POST.
+    from app.config import settings as cfg
+    sso_only = _get_sso_settings()["sso_only"]
     return {"invite_required": site_settings.get_flag("invite_only"),
             "first_user": False,
-            "registration_open": site_settings.get_flag("registration_enabled")}
+            "sso_only": sso_only,
+            # So the page can name the provider rather than say "single
+            # sign-on" at somebody who only knows it as PedsHub SSO.
+            "provider_name": (cfg.OIDC_PROVIDER_NAME
+                              if cfg.OIDC_PROVIDER_URL and cfg.OIDC_CLIENT_ID else None),
+            "registration_open": (not sso_only
+                                  and site_settings.get_flag("registration_enabled"))}
 
 
 @router.post("/register")
diff --git a/backend/tests/test_sso_hardening.py b/backend/tests/test_sso_hardening.py
index c8395f5..8f9c8db 100644
--- a/backend/tests/test_sso_hardening.py
+++ b/backend/tests/test_sso_hardening.py
@@ -119,3 +119,43 @@ class OneTimeCodeTests(unittest.TestCase):
 
 if __name__ == "__main__":
     unittest.main()
+
+
+class SignupPolicyTests(unittest.TestCase):
+    """The form asks this before it draws itself.
+
+    Under SSO-only it used to draw the whole password sign-up — name, email,
+    password twice, invite code — and only the POST refused it. The page now
+    has what it needs to decline.
+    """
+
+    def setUp(self):
+        self.engine = create_engine("sqlite://", connect_args={"check_same_thread": False},
+                                    poolclass=StaticPool)
+        Base.metadata.create_all(self.engine)
+        self.db = Session(self.engine)
+        self.db.add(User(id=1, name="Someone", email="someone@example.com",
+                         hashed_password=None))
+        self.db.commit()
+        app = FastAPI()
+        app.include_router(auth.router, prefix="/auth")
+        app.dependency_overrides[get_db] = lambda: self.db
+        self.client = TestClient(app)
+
+    def tearDown(self):
+        self.client.close()
+        self.db.close()
+        self.engine.dispose()
+
+    def test_sso_only_closes_registration_in_the_policy(self):
+        with patch.object(auth, "_get_sso_settings", return_value={"sso_only": True}):
+            body = self.client.get("/auth/signup-policy").json()
+        self.assertTrue(body["sso_only"])
+        self.assertFalse(body["registration_open"])
+
+    def test_otherwise_the_switch_decides(self):
+        with patch.object(auth, "_get_sso_settings", return_value={"sso_only": False}), \
+             patch.object(auth.site_settings, "get_flag", return_value=True):
+            body = self.client.get("/auth/signup-policy").json()
+        self.assertFalse(body["sso_only"])
+        self.assertTrue(body["registration_open"])
diff --git a/frontend/src/components/Navbar.jsx b/frontend/src/components/Navbar.jsx
index 75ca652..4466411 100644
--- a/frontend/src/components/Navbar.jsx
+++ b/frontend/src/components/Navbar.jsx
@@ -161,6 +161,17 @@ export default function Navbar({ onSignIn, onRegister, onSearch }) {
   const [peek, setPeek] = useState(false)
   // Registered by the quiz player while it is on screen without a rail.
   const sessionDrawer = useSessionDrawer()
+  //: Whether anybody may sign themselves up. Asked only when logged out —
+  //: there is nothing to draw otherwise, and it is one request per visit.
+  const [signupOpen, setSignupOpen] = useState(true)
+  useEffect(() => {
+    if (user) return undefined
+    let live = true
+    api.get('/auth/signup-policy')
+      .then(res => { if (live) setSignupOpen(res.data?.registration_open !== false) })
+      .catch(() => {})
+    return () => { live = false }
+  }, [user])
   //: Is the thing this button opens currently open? On a page that has taken
   //: the button over, that is the page's drawer; otherwise it is the site
   //: menu. Either way the bars fold into a cross.
@@ -266,10 +277,13 @@ export default function Navbar({ onSignIn, onRegister, onSearch }) {
                 ? <button onClick={onSignIn} style={{ fontSize: '0.875rem', color: 'var(--navbar-fg)', opacity: 0.8, background: 'none', border: 'none', cursor: 'pointer', padding: '6px 10px' }}>Sign In</button>
                 : <Link to="/login" style={{ fontSize: '0.875rem', color: 'var(--navbar-fg)', opacity: 0.8, textDecoration: 'none', padding: '6px 10px' }}>Sign In</Link>
               }
-              {onRegister
+              {/* Only where there is a door. On a site that signs in through
+                  a provider — or with registration simply switched off —
+                  this led to a form whose only outcome was a refusal. */}
+              {signupOpen && (onRegister
                 ? <button onClick={onRegister} className="btn btn-primary" style={{ fontSize: '0.85rem', padding: '6px 14px', borderRadius: 8 }}>Register</button>
                 : <Link to="/register" className="btn btn-primary" style={{ fontSize: '0.85rem', padding: '6px 14px', textDecoration: 'none', borderRadius: 8 }}>Register</Link>
-              }
+              )}
             </div>
           )}
         </div>
diff --git a/frontend/src/pages/LandingPage.jsx b/frontend/src/pages/LandingPage.jsx
index c5f1c2e..6fb86d5 100644
--- a/frontend/src/pages/LandingPage.jsx
+++ b/frontend/src/pages/LandingPage.jsx
@@ -253,15 +253,30 @@ function AuthModal({ mode, onClose, onSwitch }) {
   // required" and nowhere to put one.
   const [inviteRequired, setInviteRequired] = useState(false)
   const [inviteCode, setInviteCode] = useState('')
+  const [signupOpen, setSignupOpen] = useState(true)
 
   useEffect(() => {
     let live = true
     api.get('/auth/signup-policy')
-      .then(res => { if (live) setInviteRequired(!!res.data?.invite_required) })
+      .then(res => {
+        if (!live) return
+        setInviteRequired(!!res.data?.invite_required)
+        // Registration closed — by the switch, or because the site signs in
+        // through a provider. Either way there is no password sign-up to
+        // offer, and a Register tab that leads to a refusal is worse than no
+        // tab at all.
+        setSignupOpen(res.data?.registration_open !== false)
+      })
       .catch(() => {})
     return () => { live = false }
   }, [])
 
+  // Opened straight into Register — from the header button, or a stale tab —
+  // on a site that has since closed it. The tab is gone; the form must go too.
+  useEffect(() => {
+    if (!signupOpen && mode === 'register') onSwitch('login')
+  }, [signupOpen, mode, onSwitch])
+
   const reset = () => { setError(''); setUnverified(false); setResendSent(false); setRegistered(false) }
   const switchMode = (m) => { reset(); setName(''); setEmail(''); setPassword(''); setConfirm(''); onSwitch(m) }
 
@@ -321,7 +336,7 @@ function AuthModal({ mode, onClose, onSwitch }) {
 
         {/* Tabs */}
         <div className="lp-tabs">
-          {[['login', 'Sign In'], ['register', 'Register']].map(([m, label]) => (
+          {[['login', 'Sign In'], ...(signupOpen ? [['register', 'Register']] : [])].map(([m, label]) => (
             <button key={m} onClick={() => switchMode(m)} aria-pressed={mode === m}>{label}</button>
           ))}
         </div>
diff --git a/frontend/src/pages/RegisterPage.jsx b/frontend/src/pages/RegisterPage.jsx
index 1f8361a..355d81b 100644
--- a/frontend/src/pages/RegisterPage.jsx
+++ b/frontend/src/pages/RegisterPage.jsx
@@ -19,13 +19,22 @@ export default function RegisterPage() {
   // with, so the form knows whether to want a code.
   const [inviteRequired, setInviteRequired] = useState(false)
   const [inviteCode, setInviteCode] = useState('')
+  //: Null until the policy is known. A site on single sign-on has no password
+  //: sign-up, and drawing the form and refusing the POST is asking somebody
+  //: for a name, an email, a password twice and an invite code before telling
+  //: them the door does not exist.
+  const [policy, setPolicy] = useState(null)
   const { loginWithToken } = useAuth()
 
   useEffect(() => {
     let live = true
     api.get('/auth/signup-policy')
-      .then(res => { if (live) setInviteRequired(!!res.data?.invite_required) })
-      .catch(() => {})
+      .then(res => {
+        if (!live) return
+        setInviteRequired(!!res.data?.invite_required)
+        setPolicy(res.data || {})
+      })
+      .catch(() => { if (live) setPolicy({}) })
     return () => { live = false }
   }, [])
 
@@ -59,6 +68,28 @@ export default function RegisterPage() {
     }
   }
 
+  if (policy === null) {
+    return <div className="auth-page"><div className="auth-card"><div className="loading"><div className="spinner" /></div></div></div>
+  }
+
+  // Nothing to fill in on a site where accounts come from the provider.
+  if (policy.sso_only) {
+    return (
+      <div className="auth-page">
+        <div className="auth-card" style={{ textAlign: 'center' }}>
+          <h1>Accounts come by invitation</h1>
+          <p style={{ color: 'var(--text-muted)', marginBottom: 20 }}>
+            This site signs in through {policy.provider_name || 'single sign-on'}.
+            Ask an administrator for a sign-up link.
+          </p>
+          <Link to="/login" className="btn btn-primary" style={{ display: 'inline-block' }}>
+            Go to sign in
+          </Link>
+        </div>
+      </div>
+    )
+  }
+
   if (done) {
     return (
       <div className="auth-page">