fix: Allow empty tool call args (#1407)

Author: Wh1isper

pydantic_ai_slim/pydantic_ai/tools.py

@@ -333,7 +333,7 @@ async def _run(
     ) -> _messages.ToolReturnPart | _messages.RetryPromptPart:
         try:
             if isinstance(message.args, str):
-                args_dict = self._validator.validate_json(message.args)
+                args_dict = self._validator.validate_json(message.args or '{}')
             else:
                 args_dict = self._validator.validate_python(message.args)
         except ValidationError as e:

tests/test_tools.py

@@ -829,13 +829,18 @@ async def call_tools_first(messages: list[ModelMessage], info: AgentInfo) -> Mod
                     ToolCallPart(tool_name='my_tool', args={'a': 13, 'b': 4}),
                     ToolCallPart(tool_name='my_tool_plain', args={'b': 17}),
                     ToolCallPart(tool_name='my_tool_plain', args={'a': 4, 'b': 17}),
+                    ToolCallPart(tool_name='no_args_tool', args=''),
                 ]
             )
         else:
             return ModelResponse(parts=[TextPart('finished')])
 
     agent = Agent(FunctionModel(call_tools_first))
 
+    @agent.tool_plain
+    def no_args_tool() -> None:
+        return None
+
     @agent.tool
     def my_tool(ctx: RunContext[None], a: int, b: int = 2) -> int:
         return a + b
@@ -858,9 +863,10 @@ def my_tool_plain(*, a: int = 3, b: int) -> int:
             {'a': 13, 'b': 4},
             {'b': 17},
             {'a': 4, 'b': 17},
+            '',
         ]
     )
-    assert tool_returns == snapshot([15, 17, 51, 68])
+    assert tool_returns == snapshot([15, 17, 51, 68, None])
 
 
 def test_schema_generator():