diff --git a/src/langbot/pkg/api/http/controller/groups/user.py b/src/langbot/pkg/api/http/controller/groups/user.py
index ee2ee7dc9..4ad52c7d4 100644
--- a/src/langbot/pkg/api/http/controller/groups/user.py
+++ b/src/langbot/pkg/api/http/controller/groups/user.py
@@ -111,7 +111,11 @@ class UserRouterGroup(group.RouterGroup):
         async def _() -> str:
             if getattr(getattr(self.ap, 'deployment', None), 'mode', 'oss') == 'cloud':
                 return self.http_status(403, 'password_login_disabled', 'Password login is disabled on LangBot Cloud')
-            json_data = await quart.request.json
+            json_data = await quart.request.get_json(silent=True)
+            if not isinstance(json_data, dict) or not all(
+                isinstance(json_data.get(field), str) for field in ('user', 'password')
+            ):
+                return self.http_status(400, -1, 'Invalid request parameters')
 
             user_email = json_data['user']
             password = json_data['password']
diff --git a/tests/integration/api/test_smoke.py b/tests/integration/api/test_smoke.py
index 2b717937d..14ca4579f 100644
--- a/tests/integration/api/test_smoke.py
+++ b/tests/integration/api/test_smoke.py
@@ -231,33 +231,42 @@ class TestInvalidPayload:
     """Tests for error handling with invalid payloads."""
 
     @pytest.mark.asyncio
-    async def test_missing_json_body(self, quart_test_client):
-        """
-        POST endpoint without JSON body handles gracefully.
-        """
-        # /api/v1/user/auth expects JSON with 'user' and 'password'
+    async def test_missing_json_body(self, quart_test_client, fake_api_app):
+        """A missing login body is rejected before authentication."""
         response = await quart_test_client.post('/api/v1/user/auth')
 
-        # Should return error (500, 400, or 401) with stable JSON structure
-        assert response.status_code in (400, 500, 401)
-        data = await response.get_json()
-
-        # Verify error response has expected structure
-        assert 'code' in data
-        assert 'msg' in data
+        assert response.status_code == 400
+        assert await response.get_json() == {'code': -1, 'msg': 'Invalid request parameters'}
+        fake_api_app.user_service.authenticate.assert_not_awaited()
 
     @pytest.mark.asyncio
-    async def test_invalid_json_structure(self, quart_test_client):
-        """
-        POST with wrong JSON structure returns stable error.
-        """
-        response = await quart_test_client.post('/api/v1/user/auth', json={'wrong_field': 'value'})
+    @pytest.mark.parametrize(
+        'payload',
+        [
+            {'wrong_field': 'value'},
+            [],
+            {'user': None, 'password': 'secret'},
+            {'user': 'owner@example.com', 'password': 42},
+        ],
+    )
+    async def test_invalid_json_structure(self, quart_test_client, fake_api_app, payload):
+        """Malformed credential fields never reach the authentication service."""
+        response = await quart_test_client.post('/api/v1/user/auth', json=payload)
 
-        # Should return error with stable JSON structure
-        assert response.status_code in (400, 500, 401)
-        data = await response.get_json()
-        assert 'code' in data
-        assert 'msg' in data
+        assert response.status_code == 400
+        assert await response.get_json() == {'code': -1, 'msg': 'Invalid request parameters'}
+        fake_api_app.user_service.authenticate.assert_not_awaited()
+
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize('password', ['', 'existing-password'])
+    async def test_string_credentials_reach_authentication(self, quart_test_client, fake_api_app, password):
+        response = await quart_test_client.post(
+            '/api/v1/user/auth', json={'user': 'owner@example.com', 'password': password}
+        )
+
+        assert response.status_code == 200
+        assert (await response.get_json())['data']['token'] == 'fake_token'
+        fake_api_app.user_service.authenticate.assert_awaited_once_with('owner@example.com', password, None)
 
 
 @pytest.mark.usefixtures('mock_circular_import_chain')