fix: #2940 emit RealtimeHistoryUpdated on transcript_delta

When RealtimeSession received a RealtimeModelTranscriptDeltaEvent, it
accumulated transcript text into self._history but only forwarded the
raw model event. Subscribers that follow the documented history_added /
history_updated pattern never observed the live transcript update.

Emit RealtimeHistoryUpdated after the history mutation, matching the
input_audio_transcription_completed branch, and update the session test
that asserted transcript_delta was an ignored event.

Author: Quratulain-bilal

src/agents/realtime/session.py

@@ -313,6 +313,9 @@ async def on_event(self, event: RealtimeModelEvent) -> None:
                     content=[AssistantAudio(transcript=self._item_transcripts[item_id])],
                 ),
             )
+            await self._put_event(
+                RealtimeHistoryUpdated(info=self._event_info, history=self._history)
+            )
 
             # Check if we should run guardrails based on debounce threshold
             current_length = len(self._item_transcripts[item_id])

tests/realtime/test_session.py

@@ -565,16 +565,9 @@ async def test_item_deleted_event_removes_item(self, mock_model, mock_agent):
 
     @pytest.mark.asyncio
     async def test_ignored_events_only_generate_raw_events(self, mock_model, mock_agent):
-        """Test that ignored events (transcript_delta, connection_status, other) only generate raw
-        events"""
+        """Test that ignored events (connection_status, other) only generate raw events."""
         session = RealtimeSession(mock_model, mock_agent, None)
 
-        # Test transcript delta (should be ignored per TODO comment)
-        transcript_event = RealtimeModelTranscriptDeltaEvent(
-            item_id="item_1", delta="hello", response_id="resp_1"
-        )
-        await session.on_event(transcript_event)
-
         # Test connection status (should be ignored)
         connection_event = RealtimeModelConnectionStatusEvent(status="connected")
         await session.on_event(connection_event)
@@ -583,13 +576,35 @@ async def test_ignored_events_only_generate_raw_events(self, mock_model, mock_ag
         other_event = RealtimeModelOtherEvent(data={"custom": "data"})
         await session.on_event(other_event)
 
-        # Should only have 3 raw events (no transformed events)
-        assert session._event_queue.qsize() == 3
+        # Should only have 2 raw events (no transformed events)
+        assert session._event_queue.qsize() == 2
 
-        for _ in range(3):
+        for _ in range(2):
             event = await session._event_queue.get()
             assert isinstance(event, RealtimeRawModelEvent)
 
+    @pytest.mark.asyncio
+    async def test_transcript_delta_emits_history_updated(self, mock_model, mock_agent):
+        """transcript_delta updates the live history and must emit RealtimeHistoryUpdated
+        so subscribers following history_added/history_updated stay in sync (issue #2940)."""
+        session = RealtimeSession(mock_model, mock_agent, None)
+
+        transcript_event = RealtimeModelTranscriptDeltaEvent(
+            item_id="item_1", delta="hello", response_id="resp_1"
+        )
+        await session.on_event(transcript_event)
+
+        # Expect a raw event plus a history_updated event.
+        assert session._event_queue.qsize() == 2
+
+        events = [await session._event_queue.get() for _ in range(2)]
+        assert any(isinstance(e, RealtimeRawModelEvent) for e in events)
+        history_updated = next(
+            (e for e in events if isinstance(e, RealtimeHistoryUpdated)), None
+        )
+        assert history_updated is not None
+        assert history_updated.history == session._history
+
     @pytest.mark.asyncio
     async def test_function_call_event_triggers_tool_handling(self, mock_model, mock_agent):
         """Test that function_call events trigger tool call handling synchronously when disabled"""