Fix LLMStreamChunkEvent chronological ordering issue

- Modified BaseEvent to set timestamp as Optional[datetime] = None instead of at creation time
- Updated crewai_event_bus.emit() to set timestamp at emission time if not already set
- Added comprehensive tests for chronological ordering of stream chunk events
- Added test for preserving manually set timestamps
- Added test for BaseEvent timestamp setting behavior

Fixes GitHub issue #3008 where streaming chunk events were emitted out of chronological order due to variable processing delays between event creation and emission.

Co-Authored-By: João <joao@crewai.com>

src/crewai/utilities/events/base_events.py

@@ -9,7 +9,7 @@ from crewai.utilities.serialization import to_serializable
 class BaseEvent(BaseModel):
     """Base class for all events"""
 
-    timestamp: datetime = Field(default_factory=datetime.now)
+    timestamp: Optional[datetime] = None
     type: str
     source_fingerprint: Optional[str] = None  # UUID string of the source entity
     source_type: Optional[str] = None  # "agent", "task", "crew"

src/crewai/utilities/events/crewai_event_bus.py

@@ -1,5 +1,6 @@
 import threading
 from contextlib import contextmanager
+from datetime import datetime
 from typing import Any, Callable, Dict, List, Type, TypeVar, cast
 
 from blinker import Signal
@@ -67,6 +68,9 @@ class CrewAIEventsBus:
             source: The object emitting the event
             event: The event instance to emit
         """
+        if event.timestamp is None:
+            event.timestamp = datetime.now()
+            
         for event_type, handlers in self._handlers.items():
             if isinstance(event, event_type):
                 for handler in handlers:

tests/utilities/test_events.py

@@ -736,6 +736,84 @@ def test_streaming_fallback_to_non_streaming():
             llm.call = original_call
 
 
+def test_llm_stream_chunk_events_chronological_order():
+    """Test that LLM stream chunk events are emitted in chronological order."""
+    import time
+    
+    events_received = []
+    
+    with crewai_event_bus.scoped_handlers():
+        @crewai_event_bus.on(LLMStreamChunkEvent)
+        def handle_stream_chunk(source, event):
+            events_received.append((event.timestamp, event.chunk))
+        
+        llm = LLM(model="gpt-4o", stream=True)
+        
+        event1 = LLMStreamChunkEvent(chunk="first")
+        time.sleep(0.002)
+        event2 = LLMStreamChunkEvent(chunk="second") 
+        time.sleep(0.001)
+        event3 = LLMStreamChunkEvent(chunk="third")
+        
+        crewai_event_bus.emit(llm, event=event3)
+        crewai_event_bus.emit(llm, event=event1)
+        crewai_event_bus.emit(llm, event=event2)
+        
+        assert len(events_received) == 3
+        timestamps = [event[0] for event in events_received]
+        assert timestamps == sorted(timestamps), "Events should be emitted in chronological order"
+
+
+def test_llm_stream_chunk_events_preserve_manual_timestamps():
+    """Test that manually set timestamps are preserved."""
+    from datetime import datetime, timedelta
+    
+    events_received = []
+    
+    with crewai_event_bus.scoped_handlers():
+        @crewai_event_bus.on(LLMStreamChunkEvent)
+        def handle_stream_chunk(source, event):
+            events_received.append((event.timestamp, event.chunk))
+        
+        llm = LLM(model="gpt-4o", stream=True)
+        
+        base_time = datetime.now()
+        event1 = LLMStreamChunkEvent(chunk="first", timestamp=base_time)
+        event2 = LLMStreamChunkEvent(chunk="second", timestamp=base_time + timedelta(seconds=1))
+        
+        crewai_event_bus.emit(llm, event=event1)
+        crewai_event_bus.emit(llm, event=event2)
+        
+        assert len(events_received) == 2
+        assert events_received[0][0] == base_time
+        assert events_received[1][0] == base_time + timedelta(seconds=1)
+
+
+def test_base_event_timestamp_set_on_emission():
+    """Test that BaseEvent timestamp is set when event is emitted if not already set."""
+    from crewai.utilities.events.base_events import BaseEvent
+    
+    events_received = []
+    
+    class TestEvent(BaseEvent):
+        type: str = "test_event"
+        message: str
+    
+    with crewai_event_bus.scoped_handlers():
+        @crewai_event_bus.on(TestEvent)
+        def handle_test_event(source, event):
+            events_received.append(event)
+        
+        event_without_timestamp = TestEvent(message="test")
+        assert event_without_timestamp.timestamp is None
+        
+        crewai_event_bus.emit("test_source", event=event_without_timestamp)
+        
+        assert len(events_received) == 1
+        assert events_received[0].timestamp is not None
+        assert isinstance(events_received[0].timestamp, datetime)
+
+
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_streaming_empty_response_handling():
     """Test that streaming handles empty responses correctly."""