Fix mqtt json light allows to set brightness value >255 (#104220)

homeassistant/components/mqtt/light/schema_json.py

@@ -367,10 +367,13 @@ class MqttLightJson(MqttEntity, LightEntity, RestoreEntity):
             if brightness_supported(self.supported_color_modes):
                 try:
                     if brightness := values["brightness"]:
-                        self._attr_brightness = int(
+                        self._attr_brightness = min(
+                            int(
                                 brightness  # type: ignore[operator]
                                 / float(self._config[CONF_BRIGHTNESS_SCALE])
                                 * 255
+                            ),
+                            255,
                         )
                     else:
                         _LOGGER.debug(

tests/components/mqtt/test_light_json.py

@@ -1785,6 +1785,24 @@ async def test_brightness_scale(
     assert state.state == STATE_ON
     assert state.attributes.get("brightness") == 255
 
+    # Turn on the light with half brightness
+    async_fire_mqtt_message(
+        hass, "test_light_bright_scale", '{"state":"ON", "brightness": 50}'
+    )
+
+    state = hass.states.get("light.test")
+    assert state.state == STATE_ON
+    assert state.attributes.get("brightness") == 128
+
+    # Test limmiting max brightness
+    async_fire_mqtt_message(
+        hass, "test_light_bright_scale", '{"state":"ON", "brightness": 103}'
+    )
+
+    state = hass.states.get("light.test")
+    assert state.state == STATE_ON
+    assert state.attributes.get("brightness") == 255
+
 
 @pytest.mark.parametrize(
     "hass_config",