fix: resolve DeepSeek tool_calls args parsing validation error

Added ToolCallArgsParsingWrapper to handle AI providers (like DeepSeek)
that return tool_calls.args as JSON strings instead of dictionaries.

The wrapper monkey-patches ChatOpenAI's _create_chat_result method to
parse string arguments before AIMessage construction, preventing
Pydantic validation errors.

Changes:
- New: agent/chat_model_wrapper.py - Wrapper implementation
- Modified: agent/base_agent/base_agent.py - Wrap model during init
- Modified: CHANGELOG.md - Document fix as v0.4.1
- New: tests/unit/test_chat_model_wrapper.py - Unit tests

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/unit/test_chat_model_wrapper.py

@@ -0,0 +1,216 @@
+"""
+Unit tests for ChatModelWrapper - tool_calls args parsing fix
+"""
+
+import json
+import pytest
+from unittest.mock import Mock, AsyncMock
+from langchain_core.messages import AIMessage
+from langchain_core.outputs import ChatResult, ChatGeneration
+
+from agent.chat_model_wrapper import ToolCallArgsParsingWrapper
+
+
+class TestToolCallArgsParsingWrapper:
+    """Tests for ToolCallArgsParsingWrapper"""
+
+    @pytest.fixture
+    def mock_model(self):
+        """Create a mock chat model"""
+        model = Mock()
+        model._llm_type = "mock-model"
+        return model
+
+    @pytest.fixture
+    def wrapper(self, mock_model):
+        """Create a wrapper around mock model"""
+        return ToolCallArgsParsingWrapper(model=mock_model)
+
+    def test_fix_tool_calls_with_string_args(self, wrapper):
+        """Test that string args are parsed to dict"""
+        # Create message with tool_calls where args is a JSON string
+        message = AIMessage(
+            content="",
+            tool_calls=[
+                {
+                    "name": "buy",
+                    "args": '{"symbol": "AAPL", "amount": 10}',  # String, not dict
+                    "id": "call_123"
+                }
+            ]
+        )
+
+        fixed_message = wrapper._fix_tool_calls(message)
+
+        # Check that args is now a dict
+        assert isinstance(fixed_message.tool_calls[0]['args'], dict)
+        assert fixed_message.tool_calls[0]['args'] == {"symbol": "AAPL", "amount": 10}
+
+    def test_fix_tool_calls_with_dict_args(self, wrapper):
+        """Test that dict args are left unchanged"""
+        # Create message with tool_calls where args is already a dict
+        message = AIMessage(
+            content="",
+            tool_calls=[
+                {
+                    "name": "buy",
+                    "args": {"symbol": "AAPL", "amount": 10},  # Already a dict
+                    "id": "call_123"
+                }
+            ]
+        )
+
+        fixed_message = wrapper._fix_tool_calls(message)
+
+        # Check that args is still a dict
+        assert isinstance(fixed_message.tool_calls[0]['args'], dict)
+        assert fixed_message.tool_calls[0]['args'] == {"symbol": "AAPL", "amount": 10}
+
+    def test_fix_tool_calls_with_invalid_json(self, wrapper):
+        """Test that invalid JSON string is left unchanged"""
+        # Create message with tool_calls where args is an invalid JSON string
+        message = AIMessage(
+            content="",
+            tool_calls=[
+                {
+                    "name": "buy",
+                    "args": 'invalid json {',  # Invalid JSON
+                    "id": "call_123"
+                }
+            ]
+        )
+
+        fixed_message = wrapper._fix_tool_calls(message)
+
+        # Check that args is still a string (parsing failed)
+        assert isinstance(fixed_message.tool_calls[0]['args'], str)
+        assert fixed_message.tool_calls[0]['args'] == 'invalid json {'
+
+    def test_fix_tool_calls_no_tool_calls(self, wrapper):
+        """Test that messages without tool_calls are left unchanged"""
+        message = AIMessage(content="Hello, world!")
+        fixed_message = wrapper._fix_tool_calls(message)
+
+        assert fixed_message == message
+
+    def test_generate_with_string_args(self, wrapper, mock_model):
+        """Test _generate method with string args"""
+        # Create a response with string args
+        original_message = AIMessage(
+            content="",
+            tool_calls=[
+                {
+                    "name": "buy",
+                    "args": '{"symbol": "MSFT", "amount": 5}',
+                    "id": "call_456"
+                }
+            ]
+        )
+
+        mock_result = ChatResult(
+            generations=[ChatGeneration(message=original_message)]
+        )
+        mock_model._generate.return_value = mock_result
+
+        # Call wrapper's _generate
+        result = wrapper._generate(messages=[], stop=None, run_manager=None)
+
+        # Check that args is now a dict
+        fixed_message = result.generations[0].message
+        assert isinstance(fixed_message.tool_calls[0]['args'], dict)
+        assert fixed_message.tool_calls[0]['args'] == {"symbol": "MSFT", "amount": 5}
+
+    @pytest.mark.asyncio
+    async def test_agenerate_with_string_args(self, wrapper, mock_model):
+        """Test _agenerate method with string args"""
+        # Create a response with string args
+        original_message = AIMessage(
+            content="",
+            tool_calls=[
+                {
+                    "name": "sell",
+                    "args": '{"symbol": "GOOGL", "amount": 3}',
+                    "id": "call_789"
+                }
+            ]
+        )
+
+        mock_result = ChatResult(
+            generations=[ChatGeneration(message=original_message)]
+        )
+        mock_model._agenerate = AsyncMock(return_value=mock_result)
+
+        # Call wrapper's _agenerate
+        result = await wrapper._agenerate(messages=[], stop=None, run_manager=None)
+
+        # Check that args is now a dict
+        fixed_message = result.generations[0].message
+        assert isinstance(fixed_message.tool_calls[0]['args'], dict)
+        assert fixed_message.tool_calls[0]['args'] == {"symbol": "GOOGL", "amount": 3}
+
+    def test_invoke_with_string_args(self, wrapper, mock_model):
+        """Test invoke method with string args"""
+        original_message = AIMessage(
+            content="",
+            tool_calls=[
+                {
+                    "name": "buy",
+                    "args": '{"symbol": "NVDA", "amount": 20}',
+                    "id": "call_999"
+                }
+            ]
+        )
+
+        mock_model.invoke.return_value = original_message
+
+        # Call wrapper's invoke
+        result = wrapper.invoke(input=[])
+
+        # Check that args is now a dict
+        assert isinstance(result.tool_calls[0]['args'], dict)
+        assert result.tool_calls[0]['args'] == {"symbol": "NVDA", "amount": 20}
+
+    @pytest.mark.asyncio
+    async def test_ainvoke_with_string_args(self, wrapper, mock_model):
+        """Test ainvoke method with string args"""
+        original_message = AIMessage(
+            content="",
+            tool_calls=[
+                {
+                    "name": "sell",
+                    "args": '{"symbol": "TSLA", "amount": 15}',
+                    "id": "call_111"
+                }
+            ]
+        )
+
+        mock_model.ainvoke = AsyncMock(return_value=original_message)
+
+        # Call wrapper's ainvoke
+        result = await wrapper.ainvoke(input=[])
+
+        # Check that args is now a dict
+        assert isinstance(result.tool_calls[0]['args'], dict)
+        assert result.tool_calls[0]['args'] == {"symbol": "TSLA", "amount": 15}
+
+    def test_bind_tools_returns_wrapper(self, wrapper, mock_model):
+        """Test that bind_tools returns a new wrapper"""
+        mock_bound = Mock()
+        mock_model.bind_tools.return_value = mock_bound
+
+        result = wrapper.bind_tools(tools=[], strict=True)
+
+        # Check that result is a wrapper around the bound model
+        assert isinstance(result, ToolCallArgsParsingWrapper)
+        assert result.wrapped_model == mock_bound
+
+    def test_bind_returns_wrapper(self, wrapper, mock_model):
+        """Test that bind returns a new wrapper"""
+        mock_bound = Mock()
+        mock_model.bind.return_value = mock_bound
+
+        result = wrapper.bind(max_tokens=100)
+
+        # Check that result is a wrapper around the bound model
+        assert isinstance(result, ToolCallArgsParsingWrapper)
+        assert result.wrapped_model == mock_bound