[bugfix] Handle empty args for ToolCallPart (#1515)

Author: arnaudstiegler

pydantic_ai_slim/pydantic_ai/messages.py

@@ -508,6 +508,8 @@ def args_as_dict(self) -> dict[str, Any]:
         """
         if isinstance(self.args, dict):
             return self.args
+        if isinstance(self.args, str) and not self.args:
+            return {}
         args = pydantic_core.from_json(self.args)
         assert isinstance(args, dict), 'args should be a dict'
         return cast(dict[str, Any], args)

tests/test_tools.py

@@ -549,6 +549,20 @@ def test_init_plain_tool_invalid():
         Tool(ctx_tool, takes_ctx=False)
 
 
+@pytest.mark.parametrize(
+    'args, expected',
+    [
+        ('', {}),
+        ({'x': 42, 'y': 'value'}, {'x': 42, 'y': 'value'}),
+        ('{"a": 1, "b": "c"}', {'a': 1, 'b': 'c'}),
+    ],
+)
+def test_tool_call_part_args_as_dict(args: Union[str, dict[str, Any]], expected: dict[str, Any]):
+    part = ToolCallPart(tool_name='foo', args=args)
+    result = part.args_as_dict()
+    assert result == expected
+
+
 def test_return_pydantic_model():
     agent = Agent('test')