fix(conversational_mixin): handle empty message index in route events

Updated the message index handling in the  class to return  when there are no messages. Added tests to ensure that route events do not reference index zero when the transcript is empty, and verified the correct emission of conversation message events during flow handling.

lib/crewai/src/crewai/experimental/conversational_mixin.py

@@ -479,7 +479,7 @@ class _ConversationalMixin:
                 session_id=state.id,
                 route=route,
                 user_message=state.current_user_message,
-                message_index=max(len(state.messages) - 1, 0),
+                message_index=(len(state.messages) - 1) if state.messages else None,
                 previous_intent=previous_intent,
             ),
         )

lib/crewai/tests/test_flow_conversation.py

@@ -1036,14 +1036,23 @@ class TestConversationalFlow:
                 return "worked"
 
         flow = DeferredFlow()
+        observed_events: list[str] = []
         started_events: list[FlowStartedEvent] = []
 
         with crewai_event_bus.scoped_handlers():
 
             @crewai_event_bus.on(FlowStartedEvent)
             def capture(_: Any, event: FlowStartedEvent) -> None:
+                observed_events.append(event.type)
                 started_events.append(event)
 
+            @crewai_event_bus.on(ConversationMessageAddedEvent)
+            def capture_message(
+                _: Any, event: ConversationMessageAddedEvent
+            ) -> None:
+                if event.role == "user":
+                    observed_events.append(event.type)
+
             flow.handle_turn("turn 1")
             flow.handle_turn("turn 2")
             flow.handle_turn("turn 3")
@@ -1053,6 +1062,30 @@ class TestConversationalFlow:
             "deferred conversational traces should emit one session-level "
             "flow_started event, not one per turn"
         )
+        assert observed_events[0] == "flow_started"
+        assert observed_events[1] == "conversation_message_added"
+
+    def test_route_event_uses_no_message_index_for_empty_transcript(self) -> None:
+        """Route events do not reference index zero when no message exists."""
+
+        @ConversationConfig()
+        class DeferredFlow(ConversationalFlow):
+            pass
+
+        flow = DeferredFlow()
+        route_events: list[ConversationRouteSelectedEvent] = []
+
+        with crewai_event_bus.scoped_handlers():
+
+            @crewai_event_bus.on(ConversationRouteSelectedEvent)
+            def capture(_: Any, event: ConversationRouteSelectedEvent) -> None:
+                route_events.append(event)
+
+            flow._emit_conversation_route_selected("converse")
+            crewai_event_bus.flush()
+
+        assert len(route_events) == 1
+        assert route_events[0].message_index is None
 
     def test_finalize_session_traces_emits_finished_and_finalizes_batch(self) -> None:
         """``finalize_session_traces()`` emits one ``FlowFinishedEvent`` + one ``finalize_batch``.