diff --git a/tests/tools/test_homeassistant_tool.py b/tests/tools/test_homeassistant_tool.py
index e18dcb385..654424a0a 100644
--- a/tests/tools/test_homeassistant_tool.py
+++ b/tests/tools/test_homeassistant_tool.py
@@ -5,6 +5,7 @@ handler validation, and availability gating.
 """
 
 import json
+from unittest.mock import patch
 
 import pytest
 
@@ -304,6 +305,60 @@ class TestEntityIdValidation:
             assert "Invalid entity_id" not in result["error"]
 
 
+# ---------------------------------------------------------------------------
+# String-data deserialization (XML tool calling workaround)
+# ---------------------------------------------------------------------------
+
+
+class TestCallServiceStringData:
+    """data param may arrive as a JSON string (XML tool calling mode)."""
+
+    @patch("tools.homeassistant_tool._run_async", return_value={"success": True})
+    def test_string_data_deserialized(self, mock_run):
+        """JSON string data is parsed into a dict before dispatch."""
+        _handle_call_service({
+            "domain": "climate",
+            "service": "set_hvac_mode",
+            "entity_id": "climate.living_room",
+            "data": '{"hvac_mode": "heat"}',
+        })
+        call_args = mock_run.call_args[0][0]  # the coroutine arg
+        # _run_async was called, meaning we got past validation
+
+    @patch("tools.homeassistant_tool._run_async", return_value={"success": True})
+    def test_dict_data_passthrough(self, mock_run):
+        """Dict data (JSON tool calling mode) still works unchanged."""
+        _handle_call_service({
+            "domain": "light",
+            "service": "turn_on",
+            "entity_id": "light.bedroom",
+            "data": {"brightness": 255},
+        })
+        mock_run.assert_called_once()
+
+    def test_invalid_json_string_returns_error(self):
+        """Malformed JSON string in data returns a clear error."""
+        result = json.loads(_handle_call_service({
+            "domain": "light",
+            "service": "turn_on",
+            "entity_id": "light.bedroom",
+            "data": "{not valid json}",
+        }))
+        assert "error" in result
+        assert "Invalid JSON" in result["error"]
+
+    @patch("tools.homeassistant_tool._run_async", return_value={"success": True})
+    def test_empty_string_data_becomes_none(self, mock_run):
+        """Empty/whitespace string data is treated as None."""
+        _handle_call_service({
+            "domain": "light",
+            "service": "turn_on",
+            "entity_id": "light.bedroom",
+            "data": "   ",
+        })
+        mock_run.assert_called_once()
+
+
 # ---------------------------------------------------------------------------
 # Security: domain/service name format validation
 # ---------------------------------------------------------------------------
diff --git a/tools/homeassistant_tool.py b/tools/homeassistant_tool.py
index 757aeb088..2e698a459 100644
--- a/tools/homeassistant_tool.py
+++ b/tools/homeassistant_tool.py
@@ -458,10 +458,10 @@ HA_CALL_SERVICE_SCHEMA = {
             "data": {
                 "type": "string",
                 "description": (
-                    "Additional service data provided as a valid JSON string. Examples: "
-                    '\'{"brightness": 255, "color_name": "blue"}\' for lights, '
-                    '\'{"temperature": 22, "hvac_mode": "heat"}\' for climate, '
-                    '\'{"volume_level": 0.5}\' for media players.'
+                    "Additional service data as a JSON string. Examples: "
+                    '{"brightness": 255, "color_name": "blue"} for lights, '
+                    '{"temperature": 22, "hvac_mode": "heat"} for climate, '
+                    '{"volume_level": 0.5} for media players.'
                 ),
             },
         },