diff --git a/hermes_cli/dashboard_auth/middleware.py b/hermes_cli/dashboard_auth/middleware.py
index 2c5f5b4f7b95..899235a66a17 100644
--- a/hermes_cli/dashboard_auth/middleware.py
+++ b/hermes_cli/dashboard_auth/middleware.py
@@ -179,12 +179,15 @@ def _auto_sso_response(request: Request) -> Response | None:
     # token-only credentials (drain/service providers) are never candidates.
     providers = list_session_providers()
     if len(providers) != 1:
-        # Zero → nothing to redirect to. Two+ → user must choose at /login.
+        # Zero -> nothing to redirect to. Two+ -> user must choose at /login.
         return None
 
     from hermes_cli.dashboard_auth.prefix import prefix_from_request
 
     provider = providers[0]
+    # Password-only providers have no OAuth redirect flow; skip auto-redirect.
+    if getattr(provider, "supports_password", False):
+        return None
     prefix = prefix_from_request(request)
     next_param = _safe_next_target(request)
     from urllib.parse import quote
diff --git a/hermes_cli/dashboard_auth/routes.py b/hermes_cli/dashboard_auth/routes.py
index 568a11957be4..ca9c589d881f 100644
--- a/hermes_cli/dashboard_auth/routes.py
+++ b/hermes_cli/dashboard_auth/routes.py
@@ -195,6 +195,11 @@ async def auth_login(request: Request, provider: str, next: str = ""):
 
     try:
         ls = p.start_login(redirect_uri=_redirect_uri(request))
+    except NotImplementedError as e:
+        raise HTTPException(
+            status_code=400,
+            detail=str(e),
+        )
     except ProviderError as e:
         audit_log(
             AuditEvent.LOGIN_FAILURE,
diff --git a/tests/hermes_cli/test_dashboard_auth_middleware.py b/tests/hermes_cli/test_dashboard_auth_middleware.py
index 7c1d6a9c2b21..97be9e883431 100644
--- a/tests/hermes_cli/test_dashboard_auth_middleware.py
+++ b/tests/hermes_cli/test_dashboard_auth_middleware.py
@@ -325,6 +325,59 @@ def test_login_non_interactive_provider_returns_404_not_500(gated_app):
     assert "stub" in names
 
 
+def test_password_only_provider_prevents_auto_oauth_redirect_and_gates_login(gated_app):
+    """Regression: when the only session provider is password-only,
+    the auto-SSO redirect must NOT 500 on start_login().
+
+    Previously _auto_sso_response() picked the sole provider and called
+    start_login(), which raised NotImplementedError, surfacing as 500.
+    After the fix the middleware skips auto-redirect and falls through
+    to the /login interstitial, and /auth/login returns 400 instead of 500.
+    """
+    clear_providers()
+    from hermes_cli.dashboard_auth.base import DashboardAuthProvider
+    class PasswordOnlyProvider(DashboardAuthProvider):
+        name = "basic-like"
+        display_name = "Password Only"
+        supports_password = True
+        # supports_session defaults to True, so list_session_providers includes us
+        def start_login(self, *, redirect_uri):
+            raise NotImplementedError("password-only")
+        def complete_login(self, *, code, state, code_verifier, redirect_uri):
+            raise NotImplementedError
+        def verify_session(self, *, access_token):
+            return None
+        def refresh_session(self, *, refresh_token):
+            raise NotImplementedError
+        def revoke_session(self, *, refresh_token):
+            pass
+
+    register_provider(PasswordOnlyProvider())
+
+    try:
+        r = gated_app.get("/", follow_redirects=False)
+        assert r.status_code == 302, f"Expected 302, got {r.status_code}: {r.text}"
+        # Must land on /login, NOT /auth/login?provider=basic-like (which would 500)
+        assert "/login" in r.headers["location"], r.headers["location"]
+        assert "auth/login" not in r.headers["location"], r.headers["location"]
+
+        login = gated_app.get(r.headers["location"], follow_redirects=False)
+        assert login.status_code == 200, login.text
+        # The login interstitial should contain the password form, not an error page
+        assert "Sign in" in login.text or "password" in login.text.lower(), login.text[:200]
+
+        # Direct access to /auth/login?provider=basic-like should 400, not 500
+        auth_login = gated_app.get(
+            "/auth/login?provider=basic-like", follow_redirects=False
+        )
+        assert auth_login.status_code == 400, (
+            f"Expected 400 on start_login for password-only provider, got {auth_login.status_code}: {auth_login.text}"
+        )
+    finally:
+        clear_providers()
+        register_provider(StubAuthProvider())
+
+
 def test_callback_without_pkce_cookie_returns_400(gated_app):
     # No prior /auth/login → no PKCE cookie.
     r = gated_app.get(