Problem
I found two issues in the _run_stream method of the MultiStepAgent class (src/smolagents/agents.py, lines 537-606):

1. Duplicate yield when max_steps is reached: When an agent reaches max_steps, the final action_step gets yielded twice - once in the finally block (line 601) and again on line 605. This causes consumers of the stream to receive duplicate step events.

2. NameError when max_steps=0: If max_steps is set to 0, the while loop never executes, so action_step is never defined. However, line 605 tries to yield this undefined variable, causing a NameError.

Steps to reproduce

For the duplicate yield issue:

1. Create an agent with max_steps=2
2. Run the agent with stream=True
3. The agent will execute 2 steps
4. On the final step (step 2), the while loop executes normally
5. The finally block on line 601 yields the action_step for step 2
6. step_number is incremented to 3, causing the while loop to exit
7. The condition on line 604 evaluates to True (returned_final_answer is False and step_number is 3)
8. Line 605 tries to yield action_step again, causing a duplicate

For the NameError issue:

1. Create an agent with max_steps=0
2. Run the agent with stream=True
3. The while loop condition (step_number <= 0) is False from the start, so the loop never executes
4. action_step is never defined because it's only created inside the while loop
5. The condition on line 604 evaluates to True (returned_final_answer is False and step_number is 1)
6. Line 605 tries to yield action_step, but it was never defined, causing a NameError

Actual behavior and error logs

This analysis is based on code review rather than runtime errors. Based on the code logic:

For the duplicate yield case (max_steps > 0):

• When the agent reaches max_steps, the execution flow leads to the same action_step being yielded twice
• First yield happens in the finally block (line 601) as part of normal step execution
• Second yield happens on line 605 when the condition on line 604 is True
• This means stream consumers would receive duplicate action_step events for the final step

For the NameError case (max_steps = 0):

• The while loop never executes because step_number (1) is not <= max_steps (0)
• Since action_step is only created inside the while loop (line 569), it never gets defined
• When line 604's condition evaluates to True, line 605 attempts to yield action_step
• Since action_step was never defined, this would raise a NameError: local variable 'action_step' referenced before assignment

The root cause is that line 605 assumes action_step is always defined, but this is only true when the while loop has executed at least once.

Expected behavior

1. Each action_step should only be yielded once
2. The code should handle edge cases like max_steps=0 gracefully without errors
3. Line 605 should be removed since action_step is already yielded in the finally block

Environment:

• OS: Windows 11
• Python version: 3.13.3
• Package version: latest (main branch)

Checklist

• I have searched the existing issues and have not found a similar bug report.
• I have provided a minimal, reproducible example.
• I have provided the full traceback of the error.
• I have provided my environment details.
• I am willing to work on this issue and submit a pull request.