Add on_summary_applied event for observability

Emits a SummaryAppliedEvent after context summarization completes,
  providing message counts so applications can track compression
  metrics.

src/pipecat/processors/aggregators/llm_context_summarizer.py

@@ -7,6 +7,7 @@
 """This module defines a summarizer for managing LLM context summarization."""
 
 import uuid
+from dataclasses import dataclass
 from typing import Optional
 
 from loguru import logger
@@ -27,6 +28,25 @@ from pipecat.utils.context.llm_context_summarization import (
 )
 
 
+@dataclass
+class SummaryAppliedEvent:
+    """Event data emitted when context summarization completes successfully.
+
+    Parameters:
+        original_message_count: Number of messages before summarization.
+        new_message_count: Number of messages after summarization.
+        summarized_message_count: Number of messages that were compressed
+            into the summary.
+        preserved_message_count: Number of recent messages preserved
+            uncompressed.
+    """
+
+    original_message_count: int
+    new_message_count: int
+    summarized_message_count: int
+    preserved_message_count: int
+
+
 class LLMContextSummarizer(BaseObject):
     """Summarizer for managing LLM context summarization.
 
@@ -39,6 +59,10 @@ class LLMContextSummarizer(BaseObject):
     - on_request_summarization: Emitted when summarization should be triggered.
         The aggregator should broadcast this frame to the LLM service.
 
+    - on_summary_applied: Emitted after a summary has been successfully applied
+        to the context. Receives a SummaryAppliedEvent with metrics about the
+        compression.
+
     Example::
 
         @summarizer.event_handler("on_request_summarization")
@@ -49,6 +73,10 @@ class LLMContextSummarizer(BaseObject):
                 context=frame.context,
                 ...
             )
+
+        @summarizer.event_handler("on_summary_applied")
+        async def on_summary_applied(summarizer, event: SummaryAppliedEvent):
+            logger.info(f"Compressed {event.original_message_count} -> {event.new_message_count} messages")
     """
 
     def __init__(
@@ -74,6 +102,7 @@ class LLMContextSummarizer(BaseObject):
         self._pending_summary_request_id: Optional[str] = None
 
         self._register_event_handler("on_request_summarization", sync=True)
+        self._register_event_handler("on_summary_applied")
 
     @property
     def task_manager(self) -> BaseTaskManager:
@@ -320,9 +349,23 @@ class LLMContextSummarizer(BaseObject):
         new_messages.extend(recent_messages)
 
         # Update context
+        original_message_count = len(messages)
+        num_system_preserved = 1 if first_system_msg else 0
         self._context.set_messages(new_messages)
 
+        # Messages actually summarized = index range minus the preserved system message
+        summarized_count = last_summarized_index + 1 - num_system_preserved
+
         logger.info(
-            f"{self}: Applied context summary, compressed {last_summarized_index + 1} messages "
+            f"{self}: Applied context summary, compressed {summarized_count} messages "
-            f"into summary. Context now has {len(new_messages)} messages (was {len(messages)})"
+            f"into summary. Context now has {len(new_messages)} messages (was {original_message_count})"
         )
+
+        # Emit event for observability
+        event = SummaryAppliedEvent(
+            original_message_count=original_message_count,
+            new_message_count=len(new_messages),
+            summarized_message_count=summarized_count,
+            preserved_message_count=len(recent_messages) + num_system_preserved,
+        )
+        await self._call_event_handler("on_summary_applied", event)