fix(python): preserve empty tool results

python/semantic_kernel/contents/chat_history.py

@@ -215,11 +215,11 @@ def add_message(
 
     def _prepare_for_add(
         self, role: AuthorRole, content: str | None = None, items: list[KernelContent] | None = None, **kwargs: Any
-    ) -> dict[str, str]:
+    ) -> dict[str, Any]:
         """Prepare a message to be added to the history."""
         kwargs["role"] = role
 
-        if role == AuthorRole.TOOL and content and not items:
+        if role == AuthorRole.TOOL and content is not None and not items:
             tool_call_id = kwargs.pop("tool_call_id", None)
             function_name = kwargs.pop("function_name", "unknown")
             function_result_content = FunctionResultContent(

python/tests/unit/contents/test_chat_history.py

@@ -133,6 +133,22 @@ def test_add_tool_message_to_dict_succeeds(chat_history: ChatHistory):
     assert result["tool_call_id"] == "call_123"
 
 
+def test_add_empty_tool_message_to_dict_succeeds(chat_history: ChatHistory):
+    chat_history.add_tool_message("", tool_call_id="call_123", function_name="test_function")
+
+    msg = chat_history.messages[-1]
+    assert isinstance(msg.items[0], FunctionResultContent)
+    assert msg.items[0].result == ""
+    assert msg.items[0].function_name == "test_function"
+    assert msg.items[0].id == "call_123"
+    assert msg.items[0].call_id == "call_123"
+
+    result = msg.to_dict()
+    assert result["content"] == ""
+    assert result["role"] == AuthorRole.TOOL
+    assert result["tool_call_id"] == "call_123"
+
+
 def test_add_tool_message_list(chat_history: ChatHistory):
     content = [FunctionResultContent(id="test", result="Tool message")]
     chat_history.add_tool_message(content)