fix: Fix Tool and ComponentTool serialization when specifying outputs_to_string (#9524)

* Fix serialization of outputs_to_string in Tool and ComponentTool

* Add reno

* Fix mypy, simplify logic

* fix pylint

* Fix test

---------

Co-authored-by: David S. Batista <dsbatista@gmail.com>

haystack/tools/component_tool.py

@@ -206,14 +206,14 @@ class ComponentTool(Tool):
         """
         serialized_component = component_to_dict(obj=self._component, name=self.name)
 
-        serialized = {
+        serialized: Dict[str, Any] = {
             "component": serialized_component,
             "name": self.name,
             "description": self.description,
             "parameters": self._unresolved_parameters,
-            "outputs_to_string": self.outputs_to_string,
             "inputs_from_state": self.inputs_from_state,
-            "outputs_to_state": self.outputs_to_state,
+            # This is soft-copied as to not modify the attributes in place
+            "outputs_to_state": self.outputs_to_state.copy() if self.outputs_to_state else None,
         }
 
         if self.outputs_to_state is not None:
@@ -226,7 +226,11 @@ class ComponentTool(Tool):
             serialized["outputs_to_state"] = serialized_outputs
 
         if self.outputs_to_string is not None and self.outputs_to_string.get("handler") is not None:
-            serialized["outputs_to_string"] = serialize_callable(self.outputs_to_string["handler"])
+            # This is soft-copied as to not modify the attributes in place
+            serialized["outputs_to_string"] = self.outputs_to_string.copy()
+            serialized["outputs_to_string"]["handler"] = serialize_callable(self.outputs_to_string["handler"])
+        else:
+            serialized["outputs_to_string"] = None
 
         return {"type": generate_qualified_class_name(type(self)), "data": serialized}
 

haystack/tools/tool.py

@@ -122,7 +122,7 @@ class Tool:
             data["outputs_to_state"] = serialized_outputs
 
         if self.outputs_to_string is not None and self.outputs_to_string.get("handler") is not None:
-            data["outputs_to_string"] = serialize_callable(self.outputs_to_string["handler"])
+            data["outputs_to_string"]["handler"] = serialize_callable(self.outputs_to_string["handler"])
 
         return {"type": generate_qualified_class_name(type(self)), "data": data}
 

releasenotes/notes/fix-serialization-tool-and-comp-tool-017caac6bb56e744.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fix the serialization of ComponentTool and Tool when specifying outputs_to_string. Previously an error occurred on deserialization right after serializing if outputs_to_string is not None.

test/tools/test_component_tool.py

@@ -60,6 +60,10 @@ class SimpleComponent:
         return {"reply": f"Hello, {text}!"}
 
 
+def reply_formatter(input_text: str) -> str:
+    return f"Formatted reply: {input_text}"
+
+
 @dataclass
 class User:
     """A simple user dataclass."""
@@ -593,24 +597,33 @@ class TestToolComponentInPipelineWithOpenAI:
             component=SimpleComponent(),
             name="simple_tool",
             description="A simple tool",
+            outputs_to_string={"source": "reply", "handler": reply_formatter},
             inputs_from_state={"test": "input"},
             outputs_to_state={"output": {"source": "out", "handler": output_handler}},
         )
 
         # Test serialization
+        expected_tool_dict = {
+            "type": "haystack.tools.component_tool.ComponentTool",
+            "data": {
+                "component": {"type": "test_component_tool.SimpleComponent", "init_parameters": {}},
+                "name": "simple_tool",
+                "description": "A simple tool",
+                "parameters": None,
+                "outputs_to_string": {"source": "reply", "handler": "test_component_tool.reply_formatter"},
+                "inputs_from_state": {"test": "input"},
+                "outputs_to_state": {"output": {"source": "out", "handler": "test_component_tool.output_handler"}},
+            },
+        }
         tool_dict = tool.to_dict()
-        assert tool_dict["type"] == "haystack.tools.component_tool.ComponentTool"
-        assert tool_dict["data"]["name"] == "simple_tool"
-        assert tool_dict["data"]["description"] == "A simple tool"
-        assert "component" in tool_dict["data"]
-        assert tool_dict["data"]["inputs_from_state"] == {"test": "input"}
-        assert tool_dict["data"]["outputs_to_state"]["output"]["handler"] == "test_component_tool.output_handler"
+        assert tool_dict == expected_tool_dict
 
         # Test deserialization
-        new_tool = ComponentTool.from_dict(tool_dict)
+        new_tool = ComponentTool.from_dict(expected_tool_dict)
         assert new_tool.name == tool.name
         assert new_tool.description == tool.description
         assert new_tool.parameters == tool.parameters
+        assert new_tool.outputs_to_string == tool.outputs_to_string
         assert new_tool.inputs_from_state == tool.inputs_from_state
         assert new_tool.outputs_to_state == tool.outputs_to_state
         assert isinstance(new_tool._component, SimpleComponent)

test/tools/test_tool.py

@@ -13,6 +13,10 @@ def get_weather_report(city: str) -> str:
     return f"Weather report for {city}: 20°C, sunny"
 
 
+def format_string(text: str) -> str:
+    return f"Formatted: {text}"
+
+
 parameters = {"type": "object", "properties": {"city": {"type": "string"}}, "required": ["city"]}
 
 
@@ -84,6 +88,8 @@ class TestTool:
             description="Get weather report",
             parameters=parameters,
             function=get_weather_report,
+            outputs_to_string={"handler": format_string},
+            inputs_from_state={"state_key": "tool_input_key"},
             outputs_to_state={"documents": {"handler": get_weather_report, "source": "docs"}},
         )
 
@@ -94,8 +100,8 @@ class TestTool:
                 "description": "Get weather report",
                 "parameters": parameters,
                 "function": "test_tool.get_weather_report",
-                "outputs_to_string": None,
-                "inputs_from_state": None,
+                "outputs_to_string": {"handler": "test_tool.format_string"},
+                "inputs_from_state": {"state_key": "tool_input_key"},
                 "outputs_to_state": {"documents": {"source": "docs", "handler": "test_tool.get_weather_report"}},
             },
         }
@@ -108,6 +114,8 @@ class TestTool:
                 "description": "Get weather report",
                 "parameters": parameters,
                 "function": "test_tool.get_weather_report",
+                "outputs_to_string": {"handler": "test_tool.format_string"},
+                "inputs_from_state": {"state_key": "tool_input_key"},
                 "outputs_to_state": {"documents": {"source": "docs", "handler": "test_tool.get_weather_report"}},
             },
         }
@@ -118,8 +126,9 @@ class TestTool:
         assert tool.description == "Get weather report"
         assert tool.parameters == parameters
         assert tool.function == get_weather_report
-        assert tool.outputs_to_state["documents"]["source"] == "docs"
-        assert tool.outputs_to_state["documents"]["handler"] == get_weather_report
+        assert tool.outputs_to_string == {"handler": format_string}
+        assert tool.inputs_from_state == {"state_key": "tool_input_key"}
+        assert tool.outputs_to_state == {"documents": {"source": "docs", "handler": get_weather_report}}
 
 
 def test_check_duplicate_tool_names():