Fix - Allow brightness only light MQTT json light to be set up using the brightness flag or via supported_color_modes (#139585)

* Fix - Allow brightness only light MQTT json light to be set up using the `brightness` flag or via  `supported_color_modes`

* Improve comment

homeassistant/components/mqtt/light/schema_json.py

@@ -217,6 +217,10 @@ class MqttLightJson(MqttEntity, LightEntity, RestoreEntity):
                 self._attr_color_mode = next(iter(self.supported_color_modes))
             else:
                 self._attr_color_mode = ColorMode.UNKNOWN
+        elif config.get(CONF_BRIGHTNESS):
+            # Brightness is supported and no supported_color_modes are set,
+            # so set brightness as the supported color mode.
+            self._attr_supported_color_modes = {ColorMode.BRIGHTNESS}
 
     def _update_color(self, values: dict[str, Any]) -> None:
         color_mode: str = values["color_mode"]

tests/components/mqtt/test_light_json.py

@@ -361,6 +361,77 @@ async def test_no_color_brightness_color_temp_if_no_topics(
     assert state.state == STATE_UNKNOWN
 
 
+@pytest.mark.parametrize(
+    "hass_config",
+    [
+        {
+            mqtt.DOMAIN: {
+                light.DOMAIN: {
+                    "schema": "json",
+                    "name": "test",
+                    "state_topic": "test_light_rgb",
+                    "command_topic": "test_light_rgb/set",
+                    "supported_color_modes": ["brightness"],
+                }
+            }
+        },
+        {
+            mqtt.DOMAIN: {
+                light.DOMAIN: {
+                    "schema": "json",
+                    "name": "test",
+                    "state_topic": "test_light_rgb",
+                    "command_topic": "test_light_rgb/set",
+                    "brightness": True,
+                }
+            }
+        },
+    ],
+)
+async def test_brightness_only(
+    hass: HomeAssistant, mqtt_mock_entry: MqttMockHAClientGenerator
+) -> None:
+    """Test brightness only light.
+
+    There are two possible configurations for brightness only light:
+    1) Set up "brightness" as supported color mode.
+    2) Set "brightness" flag to true.
+    """
+    await mqtt_mock_entry()
+
+    state = hass.states.get("light.test")
+    assert state.state == STATE_UNKNOWN
+    assert state.attributes.get(light.ATTR_SUPPORTED_COLOR_MODES) == [
+        light.ColorMode.BRIGHTNESS
+    ]
+    expected_features = (
+        light.LightEntityFeature.FLASH | light.LightEntityFeature.TRANSITION
+    )
+    assert state.attributes.get(ATTR_SUPPORTED_FEATURES) == expected_features
+    assert state.attributes.get("rgb_color") is None
+    assert state.attributes.get("brightness") is None
+    assert state.attributes.get("color_temp_kelvin") is None
+    assert state.attributes.get("effect") is None
+    assert state.attributes.get("xy_color") is None
+    assert state.attributes.get("hs_color") is None
+
+    async_fire_mqtt_message(hass, "test_light_rgb", '{"state":"ON", "brightness": 50}')
+
+    state = hass.states.get("light.test")
+    assert state.state == STATE_ON
+    assert state.attributes.get("rgb_color") is None
+    assert state.attributes.get("brightness") == 50
+    assert state.attributes.get("color_temp_kelvin") is None
+    assert state.attributes.get("effect") is None
+    assert state.attributes.get("xy_color") is None
+    assert state.attributes.get("hs_color") is None
+
+    async_fire_mqtt_message(hass, "test_light_rgb", '{"state":"OFF"}')
+
+    state = hass.states.get("light.test")
+    assert state.state == STATE_OFF
+
+
 @pytest.mark.parametrize(
     "hass_config",
     [