diff --git a/apps/abstraction/vendors/hottis_wago_modbus.py b/apps/abstraction/vendors/hottis_wago_modbus.py
new file mode 100644
index 0000000..7cb8392
--- /dev/null
+++ b/apps/abstraction/vendors/hottis_wago_modbus.py
@@ -0,0 +1,58 @@
+"""Hottis Wago Modbus vendor transformations."""
+
+import logging
+from typing import Any
+
+logger = logging.getLogger(__name__)
+
+def transform_relay_to_vendor(payload: dict[str, Any]) -> str:
+    """Transform abstract relay payload to Hottis Wago Modbus format.
+    
+    Hottis Wago Modbus expects plain text 'true' or 'false' (not JSON).
+    
+    Example:
+    - Abstract: {'power': 'on'}
+    - Hottis Wago Modbus: 'true' or 'false'
+    """
+    power = payload.get("power", "off")
+
+    # Map abstract "on"/"off" to vendor "true"/"false"
+    if isinstance(power, str):
+        power_lower = power.lower()
+        if power_lower in {"on", "true", "1"}:
+            return "true"
+        if power_lower in {"off", "false", "0"}:
+            return "false"
+
+    # Fallback: any truthy value -> "true", else "false"
+    return "true" if power else "false"
+
+
+def transform_relay_to_abstract(payload: str) -> dict[str, Any]:
+    """Transform Hottis Wago Modbus relay payload to abstract format.
+    
+    Hottis Wago Modbus sends plain text 'true' or 'false'.
+    
+    Example:
+    - Hottis Wago Modbus: 'true'
+    - Abstract: {'power': 'on'}
+    """
+    value = payload.strip().lower()
+
+    if value == "true":
+        power = "on"
+    elif value == "false":
+        power = "off"
+    else:
+        # Fallback for unexpected values: keep as-is
+        logger.warning("Unexpected relay payload from Hottis Wago Modbus: %r", payload)
+        power = value
+
+    return {"power": power}
+
+
+# Registry of handlers for this vendor
+HANDLERS = {
+    ("relay", "to_vendor"): transform_relay_to_vendor,
+    ("relay", "to_abstract"): transform_relay_to_abstract,
+}