docs: realtime guardrail fallback behavior (#3157)

Author: mindbomber

docs/realtime/guide.md

@@ -261,6 +261,12 @@ agent = RealtimeAgent(
 )
 ```
 
+When a realtime output guardrail trips, the session interrupts the active response, forces
+`response.cancel`, emits `guardrail_tripped`, and sends a follow-up user message that names the
+triggered guardrail so the model can produce a replacement response. Your audio player should still
+listen for `audio_interrupted` and stop local playback immediately, because guardrails run on
+debounced transcript text and some audio may already be buffered when the tripwire fires.
+
 ## SIP and telephony
 
 The Python SDK includes a first-class SIP attach flow via [`OpenAIRealtimeSIPModel`][agents.realtime.openai_realtime.OpenAIRealtimeSIPModel].

tests/realtime/test_session.py

@@ -1764,8 +1764,14 @@ async def test_transcript_delta_triggers_guardrail_at_threshold(
 
         # Should have triggered guardrail and interrupted
         assert mock_model.interrupts_called == 1
+        interrupt_event = next(
+            event
+            for event in mock_model.sent_events
+            if isinstance(event, RealtimeModelSendInterrupt)
+        )
+        assert interrupt_event.force_response_cancel is True
         assert len(mock_model.sent_messages) == 1
-        assert "triggered_guardrail" in mock_model.sent_messages[0]
+        assert mock_model.sent_messages[0] == "guardrail triggered: triggered_guardrail"
 
         # Should have emitted guardrail_tripped event
         events = []