[BUG] Agent permanent stuck in ValidationException with Bedrock Converse : The number of toolResult blocks at messages.{id}.content exceeds the number of toolUse blocks of previous turn

[guptabhishek]

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

1.20.0

Python Version

3.13.5

Operating System

macos 15.7.2

Installation Method

pip

Steps to Reproduce

1. Create an agent with any main tool and also add strands stop tool
2. Ask agent something that triggers the main tool,
3. As the tool is in progress, trigger stop (can be done via direct tool call): https://github.com/strands-agents/tools/blob/main/src/strands_tools/stop.py#L20

from strands import Agent, ToolContext, tool
from strands_tools import stop

@tool(context=True)
def the_tool(tool_context: ToolContext):
    tool_context.agent.tool.stop()
    tool_context.invocation_state.get("request_state", {})["stop_event_loop"] = True
    return f"Result stored in {id}"

agent = Agent(tools=[the_tool, stop])

result = agent("Say hi, then run the_tool")
agent("Continue on!")

Expected Behavior

Agent should stop gracefully when stop tool is triggered and then resume back and function normally when asked followup/message is sent.

Actual Behavior

Stop tool, as expected stops the agent but then causes agent to reach a permanent failure scenario (stuck in error) as conversation gets corrupted with following exception:

    raise error_class(parsed_response, operation_name)
botocore.errorfactory.ValidationException: An error occurred (ValidationException) when calling the ConverseStream operation: The number of toolResult blocks at messages.26.content exceeds the number of toolUse blocks of previous turn.
└ Bedrock region: us-west-2
└ Model id: us.anthropic.claude-sonnet-4-20250514-v1:0

Full Error

ERROR | strands.event_loop.event_loop | cycle failed
Traceback (most recent call last):
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/strands/event_loop/event_loop.py", line 192, in event_loop_cycle
    async for tool_event in tool_events:
        yield tool_event
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/strands/event_loop/event_loop.py", line 536, in _handle_tool_execution
    async for event in events:
        yield event
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/strands/event_loop/event_loop.py", line 280, in recurse_event_loop
    async for event in events:
        yield event
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/strands/event_loop/event_loop.py", line 155, in event_loop_cycle
    async for model_event in model_events:
        if not isinstance(model_event, ModelStopReason):
            yield model_event
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/strands/event_loop/event_loop.py", line 400, in _handle_model_execution
    raise e
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/strands/event_loop/event_loop.py", line 341, in _handle_model_execution
    async for event in stream_messages(
    ...<7 lines>...
        yield event
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/strands/event_loop/streaming.py", line 457, in stream_messages
    async for event in process_stream(chunks, start_time):
        yield event
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/strands/event_loop/streaming.py", line 391, in process_stream
    async for chunk in chunks:
    ...<22 lines>...
            handle_redact_content(chunk["redactContent"], state)
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/strands/models/bedrock.py", line 647, in stream
    await task
  File "/opt/homebrew/Cellar/python@3.13/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/threads.py", line 25, in to_thread
    return await loop.run_in_executor(None, func_call)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.13/3.13.5/Frameworks/Python.framework/Versions/3.13/lib/python3.13/concurrent/futures/thread.py", line 59, in run
    result = self.fn(*self.args, **self.kwargs)
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/opentelemetry/instrumentation/threading/__init__.py", line 171, in wrapped_func
    return original_func(*func_args, **func_kwargs)
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/strands/models/bedrock.py", line 768, in _stream
    raise e
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/strands/models/bedrock.py", line 683, in _stream
    response = self.client.converse_stream(**request)
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/botocore/client.py", line 602, in _api_call
    return self._make_api_call(operation_name, kwargs)
           ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/botocore/context.py", line 123, in wrapper
    return func(*args, **kwargs)
  File "{ProjectPath}/.venv/lib/python3.13/site-packages/botocore/client.py", line 1078, in _make_api_call
    raise error_class(parsed_response, operation_name)
botocore.errorfactory.ValidationException: An error occurred (ValidationException) when calling the ConverseStream operation: The number of toolResult blocks at messages.26.content exceeds the number of toolUse blocks of previous turn.
└ Bedrock region: us-west-2
└ Model id: us.anthropic.claude-sonnet-4-20250514-v1:0

Additional Context

Agent initialization:

agent = Agent(
    model=self.bedrock_model,
    agent_id=f"{self.agent_type}-strands-agent",
    tools=all_tools,
    system_prompt=self.get_system_prompt(),
    name=self.get_agent_name(),
    description=self.get_agent_description(),
    callback_handler=None,
)

Possible Solution

No response

Related Issues

No response

[zastrowm]

Can reproduce with:

from strands import Agent, ToolContext, tool
from strands_tools import stop

@tool(context=True)
def the_tool(tool_context: ToolContext):
    tool_context.agent.tool.stop()
    tool_context.invocation_state.get("request_state", {})["stop_event_loop"] = True
    return f"Result stored in {id}"

agent = Agent(tools=[the_tool, stop])

result = agent("Say hi, then run the_tool")
agent("Continue on!")

[zastrowm]

Direct method invocation should be used from within another tool. Instead of calling the stop tool, replicate what it's doing:

from strands import Agent, ToolContext, tool

@tool(context=True)
def the_tool(tool_context: ToolContext):
    tool_context.invocation_state.get("request_state", {})["stop_event_loop"] = True
    return f"Result stored in {id}"

agent = Agent(tools=[the_tool])

result = agent("Say hi, then run the_tool")

At least in the short term, Strands should block nested tool calls, as we don't properly handle this. I'll try to add this to the solution to #1176