fix: add trace field serializer for agent on tool usage events

lib/crewai/src/crewai/events/types/tool_usage_events.py

@@ -2,9 +2,9 @@ from collections.abc import Callable
 from datetime import datetime
 from typing import Any
 
-from pydantic import ConfigDict
+from pydantic import ConfigDict, SerializationInfo, field_serializer
 
-from crewai.events.base_events import BaseEvent
+from crewai.events.base_events import BaseEvent, _is_trace_context, _trace_agent_ref
 
 
 class ToolUsageEvent(BaseEvent):
@@ -26,6 +26,11 @@ class ToolUsageEvent(BaseEvent):
 
     model_config = ConfigDict(arbitrary_types_allowed=True)
 
+    @field_serializer("agent")
+    @classmethod
+    def _serialize_agent(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_agent_ref(v) if _is_trace_context(info) else v
+
     def __init__(self, **data: Any) -> None:
         if data.get("from_task"):
             task = data["from_task"]
@@ -99,6 +104,11 @@ class ToolExecutionErrorEvent(BaseEvent):
     tool_class: Callable[..., Any]
     agent: Any | None = None
 
+    @field_serializer("agent")
+    @classmethod
+    def _serialize_agent(cls, v: Any, info: SerializationInfo) -> Any:
+        return _trace_agent_ref(v) if _is_trace_context(info) else v
+
     def __init__(self, **data: Any) -> None:
         super().__init__(**data)
         # Set fingerprint data from the agent