diff --git a/hermes_cli/dashboard_auth/middleware.py b/hermes_cli/dashboard_auth/middleware.py
index 2c5f5b4f7b95..abb653e7424e 100644
--- a/hermes_cli/dashboard_auth/middleware.py
+++ b/hermes_cli/dashboard_auth/middleware.py
@@ -185,6 +185,16 @@ def _auto_sso_response(request: Request) -> Response | None:
     from hermes_cli.dashboard_auth.prefix import prefix_from_request
 
     provider = providers[0]
+
+    # Auto-SSO is only meaningful for OAuth/redirect providers.  A
+    # pure-password provider (supports_password=True, e.g. basic) has no
+    # IDP to silently bounce to — its start_login() raises
+    # NotImplementedError.  Return None so the caller falls through to
+    # _unauth_response, which renders the /login interstitial with the
+    # password form.
+    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/tests/hermes_cli/test_dashboard_auth_middleware.py b/tests/hermes_cli/test_dashboard_auth_middleware.py
index 7c1d6a9c2b21..4398edc5cd74 100644
--- a/tests/hermes_cli/test_dashboard_auth_middleware.py
+++ b/tests/hermes_cli/test_dashboard_auth_middleware.py
@@ -593,3 +593,50 @@ def test_unverifiable_token_with_reachable_providers_redirects(_gated_state):
     r = client.get("/api/auth/me")
     assert r.status_code == 401
     assert "unreachable" not in r.text.lower()
+
+
+def test_auto_sso_skips_password_only_provider():
+    """When the sole provider is password-only (e.g. basic), auto-SSO must NOT
+    redirect to /auth/login (which would hit start_login() and crash with
+    NotImplementedError).  Instead the middleware should fall through to
+    _unauth_response → /login, where the password form renders.
+
+    Regression guard for #56067 / #57211 / #58166.
+    """
+    from plugins.dashboard_auth.basic import BasicAuthProvider, hash_password
+
+    clear_providers()
+    register_provider(
+        BasicAuthProvider(
+            username="admin",
+            password_hash=hash_password("test-password"),
+            secret=b"test-secret-at-least-16-bytes!!",
+        )
+    )
+    prev_host = getattr(web_server.app.state, "bound_host", None)
+    prev_port = getattr(web_server.app.state, "bound_port", None)
+    prev_required = getattr(web_server.app.state, "auth_required", None)
+    web_server.app.state.bound_host = "fly-app.fly.dev"
+    web_server.app.state.bound_port = 443
+    web_server.app.state.auth_required = True
+    try:
+        client = TestClient(
+            web_server.app, base_url="https://fly-app.fly.dev"
+        )
+        r = client.get("/", follow_redirects=False)
+        assert r.status_code == 302, (
+            f"Expected 302, got {r.status_code}: {r.text}"
+        )
+        # Must redirect to /login (password form), NOT /auth/login (OAuth).
+        location = r.headers["location"]
+        assert "/login" in location, f"Expected redirect to /login, got {location}"
+        assert "/auth/login" not in location, (
+            f"auto-SSO must NOT redirect to /auth/login for a password-only "
+            f"provider — that would crash with NotImplementedError. "
+            f"Got: {location}"
+        )
+    finally:
+        clear_providers()
+        web_server.app.state.bound_host = prev_host
+        web_server.app.state.bound_port = prev_port
+        web_server.app.state.auth_required = prev_required