[BUG] @tool with no input arguments fails

[liorheber]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.2.0

Python Version

=3.12

Operating System

MacOS 15.3

Installation Method

other

Steps to Reproduce

1. Create a strands tool without an input argument

@tool
async def fetch_schemas() -> str:
 """Get all schemas including data sources, tables, and column information.
Returns comprehensive schema information
"""
...do something...

Expected Behavior

I expect the agent to execute the code without issues.

Actual Behavior

I get the following error -

Tool #1: fetch_schemas
cycle failed
Traceback (most recent call last):
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/event_loop/event_loop.py", line 215, in event_loop_cycle
    async for event in events:
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/event_loop/event_loop.py", line 433, in _handle_tool_execution
    validate_and_prepare_tools(message, tool_uses, tool_results, invalid_tool_use_ids)
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/tools/executor.py", line 124, in validate_and_prepare_tools
    validate_tool_use(tool)
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/tools/tools.py", line 32, in validate_tool_use
    validate_tool_use_name(tool)
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/tools/tools.py", line 53, in validate_tool_use_name
    valid_name_pattern = bool(re.match(tool_name_pattern, tool_name))
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/liorheber/.local/share/uv/python/cpython-3.12.9-macos-aarch64-none/lib/python3.12/re/__init__.py", line 167, in match
    return _compile(pattern, flags).match(string)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: expected string or bytes-like object, got 'NoneType'
Traceback (most recent call last):
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/event_loop/event_loop.py", line 215, in event_loop_cycle
    async for event in events:
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/event_loop/event_loop.py", line 433, in _handle_tool_execution
    validate_and_prepare_tools(message, tool_uses, tool_results, invalid_tool_use_ids)
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/tools/executor.py", line 124, in validate_and_prepare_tools
    validate_tool_use(tool)
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/tools/tools.py", line 32, in validate_tool_use
    validate_tool_use_name(tool)
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/tools/tools.py", line 53, in validate_tool_use_name
    valid_name_pattern = bool(re.match(tool_name_pattern, tool_name))
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/liorheber/.local/share/uv/python/cpython-3.12.9-macos-aarch64-none/lib/python3.12/re/__init__.py", line 167, in match
    return _compile(pattern, flags).match(string)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: expected string or bytes-like object, got 'NoneType'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/liorheber/WebstormProjects/ai/agent/main.py", line 288, in <module>
    main()
  File "/Users/liorheber/WebstormProjects/ai/agent/main.py", line 284, in main
    agent("What schemas are available?")
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/agent/agent.py", line 377, in __call__
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/Users/liorheber/.local/share/uv/python/cpython-3.12.9-macos-aarch64-none/lib/python3.12/concurrent/futures/_base.py", line 456, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/Users/liorheber/.local/share/uv/python/cpython-3.12.9-macos-aarch64-none/lib/python3.12/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "/Users/liorheber/.local/share/uv/python/cpython-3.12.9-macos-aarch64-none/lib/python3.12/concurrent/futures/thread.py", line 59, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/opentelemetry/instrumentation/threading/__init__.py", line 171, in wrapped_func
    return original_func(*func_args, **func_kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/agent/agent.py", line 373, in execute
    return asyncio.run(self.invoke_async(prompt, **kwargs))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/liorheber/.local/share/uv/python/cpython-3.12.9-macos-aarch64-none/lib/python3.12/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/Users/liorheber/.local/share/uv/python/cpython-3.12.9-macos-aarch64-none/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/liorheber/.local/share/uv/python/cpython-3.12.9-macos-aarch64-none/lib/python3.12/asyncio/base_events.py", line 691, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/agent/agent.py", line 398, in invoke_async
    async for event in events:
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/agent/agent.py", line 508, in stream_async
    async for event in events:
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/agent/agent.py", line 544, in _run_loop
    async for event in events:
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/agent/agent.py", line 583, in _execute_event_loop_cycle
    async for event in events:
  File "/Users/liorheber/WebstormProjects/ai/agent/.venv/lib/python3.12/site-packages/strands/event_loop/event_loop.py", line 245, in event_loop_cycle
    raise EventLoopException(e, invocation_state["request_state"]) from e
strands.types.exceptions.EventLoopException: expected string or bytes-like object, got 'NoneType'

Additional Context

No response

Possible Solution

No response

Related Issues

No response

[tetiana-myronivska]

+1 to this bug, our team is also experiencing it with the same strands and python version. downgrading strands version doesn't resolve this. the most bizarre thing is that this is the second bug that doesn't reproduce for all users (confirmed using the same version of strands, strands-tools, pydantic, and other packages that might cause this).

@liorheber in case it can be helpful, our team member came up with a workaround where we add a "dummy" parameter to the tool with no args, "force" the llm to generate that param by describing it in the docstring and never use that param in the tool.

[pgrayy]

Hello @liorheber, can you share an example script that reproduces the issue? No argument tools should be supported. As an example, I was able to run the following script successfully:

import strands
from strands import Agent


@strands.tool
def weather_tool() -> str:
    return "sunny"


agent = Agent(tools=[weather_tool])
agent("What is the weather?")

The output was:

I'll check the weather for you using the weather tool.
Tool #1: weather_tool
The weather is currently sunny! It looks like a beautiful day with clear skies and sunshine.

[pgrayy]

Oh wait, looking at your stack trace, this seems to be an issue in validate_tool_use_name. Let me take a closer look. If you could still provide a sample script that would be helpful.

[pgrayy]

Would you be able to provide a sample script that reproduces the issue? I was not able to on my end.

The error you see is being raised here because tool name is None. These come straight from the messages array though and so it seems. A few questions:

• What model are you using?
• Are you able to print out the messages array? Do you see some malformed tool use requests?
• Are you by chance modifying the messages array in a hook on the AfterModelInvocationEvent or MessageAddedEvent?

[github-actions]

This issue has been automatically closed as it was marked for auto-closure by the team and no additional responses was received within 7 days.