fix: Make compactor optional in EventsCompactionConfig and add a default

If `EventsCompactionConfig` is provided without a `compactor`, a `SlidingWindowCompactor` is now automatically instantiated using the `root_agent`'s LLM. This simplifies configuration by providing a sensible default.

PiperOrigin-RevId: 816038579

src/google/adk/apps/app.py

@@ -21,7 +21,7 @@ from pydantic import Field
 
 from ..agents.base_agent import BaseAgent
 from ..agents.context_cache_config import ContextCacheConfig
-from ..apps.base_events_compactor import BaseEventsCompactor
+from ..apps.base_events_summarizer import BaseEventsSummarizer
 from ..plugins.base_plugin import BasePlugin
 from ..utils.feature_decorator import experimental
 
@@ -56,11 +56,13 @@ class EventsCompactionConfig(BaseModel):
       extra="forbid",
   )
 
-  compactor: BaseEventsCompactor
-  """The event compactor strategy for the application."""
+  summarizer: Optional[BaseEventsSummarizer] = None
+  """The event summarizer to use for compaction."""
+
   compaction_interval: int
   """The number of *new* user-initiated invocations that, once
   fully represented in the session's events, will trigger a compaction."""
+
   overlap_size: int
   """The number of preceding invocations to include from the
   end of the last compacted range. This creates an overlap between consecutive

src/google/adk/apps/base_events_summarizer.py

@@ -23,11 +23,11 @@ from ..utils.feature_decorator import experimental
 
 
 @experimental
-class BaseEventsCompactor(abc.ABC):
+class BaseEventsSummarizer(abc.ABC):
   """Base interface for compacting events."""
 
   @abc.abstractmethod
-  async def maybe_compact_events(
+  async def maybe_summarize_events(
       self, *, events: list[Event]
   ) -> Optional[Event]:
     """Compact a list of events into a single event.

src/google/adk/apps/compaction.py

@@ -17,6 +17,7 @@ from __future__ import annotations
 import logging
 
 from google.adk.apps.app import App
+from google.adk.apps.llm_event_summarizer import LlmEventSummarizer
 from google.adk.sessions.base_session_service import BaseSessionService
 from google.adk.sessions.session import Session
 
@@ -180,8 +181,13 @@ async def _run_compaction_for_sliding_window(
   if not events_to_compact:
     return None
 
+  if not app.events_compaction_config.summarizer:
+    app.events_compaction_config.summarizer = LlmEventSummarizer(
+        llm=app.root_agent.canonical_model
+    )
+
   compaction_event = (
-      await app.events_compaction_config.compactor.maybe_compact_events(
+      await app.events_compaction_config.summarizer.maybe_summarize_events(
           events=events_to_compact
       )
   )

src/google/adk/apps/llm_event_summarizer.py

@@ -19,15 +19,15 @@ from google.genai import types
 from google.genai.types import Content
 from google.genai.types import Part
 
+from ..apps.base_events_summarizer import BaseEventsSummarizer
 from ..events.event import Event
 from ..events.event_actions import EventActions
 from ..events.event_actions import EventCompaction
 from ..models.base_llm import BaseLlm
 from ..models.llm_request import LlmRequest
-from .base_events_compactor import BaseEventsCompactor
 
 
-class LlmEventSummarizer(BaseEventsCompactor):
+class LlmEventSummarizer(BaseEventsSummarizer):
   """An LLM-based event summarizer for sliding window compaction.
 
   This class is responsible for summarizing a provided list of events into a
@@ -81,7 +81,7 @@ class LlmEventSummarizer(BaseEventsCompactor):
             formatted_history.append(f'{event.author}: {part.text}')
     return '\\n'.join(formatted_history)
 
-  async def maybe_compact_events(
+  async def maybe_summarize_events(
       self, *, events: list[Event]
   ) -> Optional[Event]:
     """Compacts given events and returns the compacted content.