fix: prevent modifying the original agent's model_settings

This fixes the issue where the original agent's model_settings was being directly modified during the tool choice reset process. The original implementation caused the agent's tool_choice to unintentionally reset to "auto" for subsequent runs, which could be unexpected behavior.

The fix creates new copies of the agent and model settings objects using dataclasses.replace() instead of modifying the original objects. This ensures that the tool choice reset is limited to the current run only, maintaining the expected behavior for sequential runs with the same agent.

Addresses feedback from @baderalfahad about the agent instance being modified when it should maintain its original state between runs.

Author: xianghuijin

src/agents/_run_impl.py

@@ -213,19 +213,25 @@ async def execute_tools_and_side_effects(
             tools = agent.tools
             # Only reset in the problematic scenarios where loops are likely unintentional
             if cls._should_reset_tool_choice(agent.model_settings, tools):
-                agent.model_settings = dataclasses.replace(
+                # Create a modified copy instead of modifying the original agent
+                new_model_settings = dataclasses.replace(
                     agent.model_settings,
                     tool_choice="auto"
                 )
+                # Create a new internal agent with updated settings
+                agent = dataclasses.replace(agent, model_settings=new_model_settings)
 
             if (
                 run_config.model_settings and
                 cls._should_reset_tool_choice(run_config.model_settings, tools)
             ):
-                run_config.model_settings = dataclasses.replace(
+                # Also update the run_config model settings with a copy
+                new_run_config_settings = dataclasses.replace(
                     run_config.model_settings,
                     tool_choice="auto"
                 )
+                # Create a new run_config with the new settings
+                run_config = dataclasses.replace(run_config, model_settings=new_run_config_settings)
 
         # Second, check if there are any handoffs
         if run_handoffs := processed_response.handoffs: