diff --git a/apps/abstraction/main.py b/apps/abstraction/main.py
index 9d6439b..8d86b5b 100644
--- a/apps/abstraction/main.py
+++ b/apps/abstraction/main.py
@@ -180,18 +180,10 @@ async def handle_abstract_set(
     
     # Transform abstract payload to vendor-specific format
     vendor_payload = transform_abstract_to_vendor(device_type, device_technology, abstract_payload)
-    
-    # For MAX! thermostats and Shelly relays, vendor_payload is a plain string
-    # For other devices, it's a dict that needs JSON encoding
-    if (device_technology == "max" and device_type == "thermostat") or \
-       (device_technology == "shelly" and device_type == "relay"):
-        vendor_message = vendor_payload  # Already a string
-    else:
-        vendor_message = json.dumps(vendor_payload)
-    
-    logger.info(f"→ vendor SET {device_id}: {vendor_topic} ← {vendor_message}")
-    logger.debug(f"MQTT message published on {vendor_topic}: {vendor_message}")
-    await mqtt_client.publish(vendor_topic, vendor_message, qos=1)
+        
+    logger.info(f"→ vendor SET {device_id}: {vendor_topic} ← {vendor_payload}")
+    logger.debug(f"MQTT message published on {vendor_topic}: {vendor_payload}")
+    await mqtt_client.publish(vendor_topic, vendor_payload, qos=1)
 
 
 async def handle_vendor_state(
diff --git a/apps/abstraction/transformation.py b/apps/abstraction/transformation.py
index 755ae36..740080e 100644
--- a/apps/abstraction/transformation.py
+++ b/apps/abstraction/transformation.py
@@ -17,12 +17,12 @@ logger = logging.getLogger(__name__)
 # HANDLER FUNCTIONS: simulator technology
 # ============================================================================
 
-def _transform_light_simulator_to_vendor(payload: dict[str, Any]) -> dict[str, Any]:
+def _transform_light_simulator_to_vendor(payload: dict[str, Any]) -> str:
     """Transform abstract light payload to simulator format.
     
     Simulator uses same format as abstract protocol (no transformation needed).
     """
-    return payload
+    return json.dumps(payload)
 
 
 def _transform_light_simulator_to_abstract(payload: str) -> dict[str, Any]:
@@ -36,12 +36,12 @@ def _transform_light_simulator_to_abstract(payload: str) -> dict[str, Any]:
     return payload
 
 
-def _transform_thermostat_simulator_to_vendor(payload: dict[str, Any]) -> dict[str, Any]:
+def _transform_thermostat_simulator_to_vendor(payload: dict[str, Any]) -> str:
     """Transform abstract thermostat payload to simulator format.
     
     Simulator uses same format as abstract protocol (no transformation needed).
     """
-    return payload
+    return json.dumps(payload)
 
 
 def _transform_thermostat_simulator_to_abstract(payload: str) -> dict[str, Any]:
@@ -59,7 +59,7 @@ def _transform_thermostat_simulator_to_abstract(payload: str) -> dict[str, Any]:
 # HANDLER FUNCTIONS: zigbee2mqtt technology
 # ============================================================================
 
-def _transform_light_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> dict[str, Any]:
+def _transform_light_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> str:
     """Transform abstract light payload to zigbee2mqtt format.
     
     Transformations:
@@ -84,7 +84,7 @@ def _transform_light_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> dict[str,
             # Convert percentage (0-100) to zigbee2mqtt range (0-254)
             vendor_payload["brightness"] = round(abstract_brightness * 254 / 100)
     
-    return vendor_payload
+    return json.dumps(vendor_payload)
 
 
 def _transform_light_zigbee2mqtt_to_abstract(payload: str) -> dict[str, Any]:
@@ -115,7 +115,7 @@ def _transform_light_zigbee2mqtt_to_abstract(payload: str) -> dict[str, Any]:
     return abstract_payload
 
 
-def _transform_thermostat_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> dict[str, Any]:
+def _transform_thermostat_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> str:
     """Transform abstract thermostat payload to zigbee2mqtt format.
     
     Transformations:
@@ -132,7 +132,7 @@ def _transform_thermostat_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> dict
         # zigbee2mqtt expects current_heating_setpoint as string
         vendor_payload["current_heating_setpoint"] = str(payload["target"])
     
-    return vendor_payload
+    return json.dumps(vendor_payload)
 
 
 def _transform_thermostat_zigbee2mqtt_to_abstract(payload: str) -> dict[str, Any]:
@@ -171,14 +171,14 @@ def _transform_thermostat_zigbee2mqtt_to_abstract(payload: str) -> dict[str, Any
 # HANDLER FUNCTIONS: contact_sensor - zigbee2mqtt technology
 # ============================================================================
 
-def _transform_contact_sensor_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> dict[str, Any]:
+def _transform_contact_sensor_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> str:
     """Transform abstract contact sensor payload to zigbee2mqtt format.
     
     Contact sensors are read-only, so this should not be called for SET commands.
     Returns payload as-is for compatibility.
     """
     logger.warning("Contact sensors are read-only - SET commands should not be used")
-    return payload
+    return json.dumps(payload)
 
 
 def _transform_contact_sensor_zigbee2mqtt_to_abstract(payload: str) -> dict[str, Any]:
@@ -225,14 +225,14 @@ def _transform_contact_sensor_zigbee2mqtt_to_abstract(payload: str) -> dict[str,
 # HANDLER FUNCTIONS: contact_sensor - max technology (Homegear MAX!)
 # ============================================================================
 
-def _transform_contact_sensor_max_to_vendor(payload: dict[str, Any]) -> dict[str, Any]:
+def _transform_contact_sensor_max_to_vendor(payload: dict[str, Any]) -> str:
     """Transform abstract contact sensor payload to MAX! format.
     
     Contact sensors are read-only, so this should not be called for SET commands.
     Returns payload as-is for compatibility.
     """
     logger.warning("Contact sensors are read-only - SET commands should not be used")
-    return payload
+    return json.dumps(payload)
 
 
 def _transform_contact_sensor_max_to_abstract(payload: str) -> dict[str, Any]:
@@ -266,13 +266,13 @@ def _transform_contact_sensor_max_to_abstract(payload: str) -> dict[str, Any]:
 # HANDLER FUNCTIONS: temp_humidity_sensor - zigbee2mqtt technology
 # ============================================================================
 
-def _transform_temp_humidity_sensor_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> dict[str, Any]:
+def _transform_temp_humidity_sensor_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> str:
     """Transform abstract temp/humidity sensor payload to zigbee2mqtt format.
     
     Temp/humidity sensors are read-only, so this should not be called for SET commands.
     Returns payload as-is for compatibility.
     """
-    return payload
+    return json.dumps(payload)
 
 
 def _transform_temp_humidity_sensor_zigbee2mqtt_to_abstract(payload: str) -> dict[str, Any]:
@@ -284,36 +284,13 @@ def _transform_temp_humidity_sensor_zigbee2mqtt_to_abstract(payload: str) -> dic
     return payload
 
 
-# ============================================================================
-# HANDLER FUNCTIONS: temp_humidity_sensor - MAX! technology
-# ============================================================================
-
-def _transform_temp_humidity_sensor_max_to_vendor(payload: str) -> dict[str, Any]:
-    """Transform abstract temp/humidity sensor payload to MAX! format.
-    
-    Temp/humidity sensors are read-only, so this should not be called for SET commands.
-    Returns payload as-is for compatibility.
-    """
-
-    payload = json.loads(payload)
-
-    return payload
-
-
-def _transform_temp_humidity_sensor_max_to_abstract(payload: str) -> dict[str, Any]:
-    """Transform MAX! temp/humidity sensor payload to abstract format.
-    
-    Passthrough - MAX! provides temperature, humidity, battery directly.
-    """
-    payload = json.loads(payload)
-    return payload
 
 
 # ============================================================================
 # HANDLER FUNCTIONS: relay - zigbee2mqtt technology
 # ============================================================================
 
-def _transform_relay_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> dict[str, Any]:
+def _transform_relay_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> str:
     """Transform abstract relay payload to zigbee2mqtt format.
     
     Relay only has power on/off, same transformation as light.
@@ -325,7 +302,7 @@ def _transform_relay_zigbee2mqtt_to_vendor(payload: dict[str, Any]) -> dict[str,
         power_value = vendor_payload.pop("power")
         vendor_payload["state"] = power_value.upper() if isinstance(power_value, str) else power_value
     
-    return vendor_payload
+    return json.dumps(vendor_payload)
 
 
 def _transform_relay_zigbee2mqtt_to_abstract(payload: str) -> dict[str, Any]:
@@ -416,7 +393,7 @@ def _transform_relay_hottis_pv_modbus_to_abstract(payload: str) -> dict[str, Any
 # HANDLER FUNCTIONS: three_phase_powermeter - hottis_pv_modbus technology
 # ============================================================================
 
-def _transform_three_phase_powermeter_hottis_pv_modbus_to_vendor(payload: dict[str, Any]) -> dict[str, Any]:
+def _transform_three_phase_powermeter_hottis_pv_modbus_to_vendor(payload: dict[str, Any]) -> str:
     """Transform abstract three_phase_powermeter payload to hottis_pv_modbus format.
     energy: float = Field(..., description="Total energy in kWh")
     total_power: float = Field(..., description="Total power in W")
@@ -447,7 +424,7 @@ def _transform_three_phase_powermeter_hottis_pv_modbus_to_vendor(payload: dict[s
         "phase3_current": payload.get("phase3_current", 0.0),
     }
 
-    return vendor_payload
+    return json.dumps(vendor_payload)
 
 
 def _transform_three_phase_powermeter_hottis_pv_modbus_to_abstract(payload: str) -> dict[str, Any]:
@@ -573,12 +550,8 @@ TRANSFORM_HANDLERS: dict[tuple[str, str, str], TransformHandler] = {
     # Temperature & humidity sensor transformations (support both type aliases)
     ("temp_humidity_sensor", "zigbee2mqtt", "to_vendor"): _transform_temp_humidity_sensor_zigbee2mqtt_to_vendor,
     ("temp_humidity_sensor", "zigbee2mqtt", "to_abstract"): _transform_temp_humidity_sensor_zigbee2mqtt_to_abstract,
-    ("temp_humidity_sensor", "max", "to_vendor"): _transform_temp_humidity_sensor_max_to_vendor,
-    ("temp_humidity_sensor", "max", "to_abstract"): _transform_temp_humidity_sensor_max_to_abstract,
     ("temp_humidity", "zigbee2mqtt", "to_vendor"): _transform_temp_humidity_sensor_zigbee2mqtt_to_vendor,
     ("temp_humidity", "zigbee2mqtt", "to_abstract"): _transform_temp_humidity_sensor_zigbee2mqtt_to_abstract,
-    ("temp_humidity", "max", "to_vendor"): _transform_temp_humidity_sensor_max_to_vendor,
-    ("temp_humidity", "max", "to_abstract"): _transform_temp_humidity_sensor_max_to_abstract,
     
     # Relay transformations
     ("relay", "zigbee2mqtt", "to_vendor"): _transform_relay_zigbee2mqtt_to_vendor,