Pass agents as tools to the ToolCallingAgent

[Marc-Anderson]

This PR implements passing agents as tools to the ToolCallingAgent instead of exclusively relying on tool definitions in the prompt text.

• adds ManagedAgentTool class to the default tools
  • defines the structure for an agent-as-a-tool using the established tool schema
• adds assemble_final_tool_list to ToolCallingAgent
  • combines tools and managed-agents-as-tools into available_tools before passing to generate/generate_stream
• add a functional example showing how to use multi-ToolCallingAgent flows
  • the team often stops early with the default prompt template, but the example is fully functional

[albertvillanova]

Thanks a lot for pointing out this issue and for taking the time to propose a fix: really appreciated!

I've created a dedicated issue to track this problem and ensure it's handled consistently across agent types:

• #1455

Regarding the implementation, I am not sure about the introduction of a new Tool class for managed agents. I think there might be a simpler way to solve it. For example, we could just ensure that managed agents have the necessary attributes and are included in the list of tools passed to model.generate, alongside the other tools.

Additionally, some regression tests would be required.

And finally, the example could be added in a different PR.

[albertvillanova]

You are accessing self.agent.description if "description" attribute does not exist in self.agent!

[albertvillanova]

This just returns the task description.

[Copilot]

Pull Request Overview

This PR enables agents to be passed as tools into ToolCallingAgent, consolidating traditional tools and managed agents and providing a fully functional example.

• Introduces ManagedAgentTool to wrap agents in the tool schema
• Adds assemble_final_tool_list to combine ordinary tools and managed agents
• Updates agent execution paths to use the assembled tool list and includes a multi-agent example

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/smolagents/default_tools.py	Added ManagedAgentTool class for agent-as-tool functionality
src/smolagents/agents.py	Updated _step_stream/_step calls to use assemble_final_tool_list and implemented that method
examples/multi_agent_workflow.py	Added a full example showing how to coordinate multiple agents

Comments suppressed due to low confidence (2)

src/smolagents/default_tools.py:572

• Add a class-level docstring to ManagedAgentTool explaining its purpose and how it delegates tasks to an agent.

class ManagedAgentTool(Tool):

src/smolagents/agents.py:1404

• This new method unifies tools and managed agents but currently has no accompanying tests. Consider adding unit tests to verify that both regular tools and managed agents are correctly included in the returned list.

    def assemble_final_tool_list(self) -> list[Tool]:

[Copilot]

The fallback description string references self.agent.description, which may not exist and could raise an AttributeError. Consider defaulting to a safe placeholder or checking for the attribute before using it.

Suggested change

	f"Delegates a task to the '{self.agent.name}' agent who is responsible for: {self.agent.description}.",
	f"Delegates a task to the '{self.agent.name}' agent who is responsible for: {getattr(self.agent, 'description', 'no specific description provided')}.",

[Copilot]

The forward method currently just returns the input task. To actually delegate work, it should invoke the wrapped agent (e.g., self.agent.run(task)) and return its result.

Suggested change

	return task
	return self.agent.run(task)