Have `print_streaming_chunk` by default in Agents

[bilgeyucel]

Is your feature request related to a problem? Please describe.
Agents might take some time to resolve a user query, and the reasoning process is not transparent. We need to enable streaming by default to provide more visibility into tool calls and results

Describe the solution you'd like
Currently, there's no default value for streaming_callback of the Agent component and we have a print_streaming_chunk util function for generators https://github.com/bilgeyucel/haystack-personal-fork/blob/main/haystack/components/generators/utils.py

We can set it as the default value for streaming_callback parameter

Describe alternatives you've considered
Leave it as it is.

[haystack-issue-resolver-agent]

Adding Default Streaming for Agent Transparency

I've analyzed the codebase and this is a great feature request! The Agent component currently requires users to explicitly provide a streaming_callback to get visibility into the tool-calling process, which can make the Agent feel like a "black box" during execution.

Current State

The Agent component in haystack/components/agents/agent.py currently has:

streaming_callback: Optional[StreamingCallbackT] = None,

And there's already a perfect utility function print_streaming_chunk in haystack/components/generators/utils.py that:

• Displays tool calls with function names and arguments as they arrive
• Shows tool call results when available
• Prints the main content (text tokens) as they stream
• Formats everything nicely with clear labels like [TOOL CALL], [TOOL RESULT], [ASSISTANT]

Proposed Implementation

The solution is straightforward - import print_streaming_chunk and set it as the default value:

# In haystack/components/agents/agent.py

from haystack.components.generators.utils import print_streaming_chunk

@component
class Agent:
    def __init__(
        self,
        *,
        chat_generator: ChatGenerator,
        tools: Optional[Union[List[Tool], Toolset]] = None,
        system_prompt: Optional[str] = None,
        exit_conditions: Optional[List[str]] = None,
        state_schema: Optional[Dict[str, Any]] = None,
        max_agent_steps: int = 100,
        raise_on_tool_invocation_failure: bool = False,
        streaming_callback: Optional[StreamingCallbackT] = print_streaming_chunk,  # Set as default
    ):
        # ... rest of implementation stays the same

Benefits

1. Transparency by default - Users will immediately see what tools are being called and their results
2. Better debugging - Easier to understand why an Agent is taking time or making certain decisions
3. Backward compatible - Users can still override with streaming_callback=None if they prefer silent operation
4. Consistent with existing patterns - Uses the well-designed print_streaming_chunk function already in the codebase

Considerations

The only potential consideration is whether some users might find the default output verbose in production scenarios. However, since this improves the development experience significantly and can be easily disabled by passing streaming_callback=None, the benefits outweigh this concern.

Would you like me to create a pull request with this implementation?