fix: preserve cache_metadata in VertexAiSessionService event round-trip

contributing/samples/gepa/experiment.py

@@ -44,6 +44,7 @@
 from tau_bench.types import EnvRunResult
 from tau_bench.types import RunConfig
 import tau_bench_agent as tau_bench_agent_lib
+import utils
 
 
 def run_tau_bench_rollouts(

contributing/samples/gepa/run_experiment.py

@@ -26,6 +26,7 @@
 import experiment
 import gepa_utils
 from google.genai import types
+import utils
 
 _OUTPUT_DIR = flags.DEFINE_string(
     'output_dir',

src/google/adk/sessions/vertex_ai_session_service.py

@@ -36,6 +36,7 @@
 from ..events.event import Event
 from ..events.event_actions import EventActions
 from ..events.event_actions import EventCompaction
+from ..models.cache_metadata import CacheMetadata
 from ..utils.vertex_ai_utils import get_express_mode_api_key
 from .base_session_service import BaseSessionService
 from .base_session_service import GetSessionConfig
@@ -311,6 +312,10 @@ async def append_event(self, session: Session, event: Event) -> Event:
             else None
         ),
     }
+    if event.cache_metadata:
+      metadata_dict['cache_metadata'] = event.cache_metadata.model_dump(
+          exclude_none=True, mode='json'
+      )
     if event.grounding_metadata:
       metadata_dict['grounding_metadata'] = event.grounding_metadata.model_dump(
           exclude_none=True, mode='json'
@@ -481,6 +486,10 @@ def _from_api_event(api_event_obj: vertexai.types.SessionEvent) -> Event:
         getattr(event_metadata, 'grounding_metadata', None),
         types.GroundingMetadata,
     )
+    cache_metadata = _session_util.decode_model(
+        getattr(event_metadata, 'cache_metadata', None),
+        CacheMetadata,
+    )
   else:
     long_running_tool_ids = None
     partial = None
@@ -491,6 +500,7 @@ def _from_api_event(api_event_obj: vertexai.types.SessionEvent) -> Event:
     compaction_data = None
     usage_metadata_data = None
     grounding_metadata = None
+    cache_metadata = None
 
   if actions:
     actions_dict = actions.model_dump(exclude_none=True, mode='python')
@@ -539,6 +549,7 @@ def _from_api_event(api_event_obj: vertexai.types.SessionEvent) -> Event:
       branch=branch,
       custom_metadata=custom_metadata,
       grounding_metadata=grounding_metadata,
+      cache_metadata=cache_metadata,
       long_running_tool_ids=long_running_tool_ids,
       usage_metadata=usage_metadata,
   )

tests/unittests/sessions/test_vertex_ai_session_service.py

@@ -344,6 +344,8 @@ def _convert_to_object(data):
           'requested_auth_configs',
           'rawEvent',
           'raw_event',
+          'cache_metadata',
+          'usage_metadata',
       ]:
         kwargs[key] = value
       else:
@@ -1306,3 +1308,52 @@ class DummyModel(pydantic.BaseModel):
 
   assert appended_event.actions.compaction is not None
   assert appended_event.actions.compaction.start_timestamp == 1000.0
+
+
+@pytest.mark.asyncio
+@pytest.mark.usefixtures('mock_get_api_client')
+async def test_append_event_with_cache_and_usage_metadata():
+  """cache_metadata and usage_metadata round-trip through append and get."""
+  session_service = mock_vertex_ai_session_service()
+  session = await session_service.get_session(
+      app_name='123', user_id='user', session_id='1'
+  )
+  assert session is not None
+
+  cache_meta = CacheMetadata(
+      cache_name='projects/123/locations/us-central1/cachedContents/456',
+      expire_time=9999999999.0,
+      fingerprint='abc123hash',
+      invocations_used=3,
+      contents_count=10,
+      created_at=1700000000.0,
+  )
+  usage_meta = genai_types.GenerateContentResponseUsageMetadata(
+      prompt_token_count=100,
+      candidates_token_count=50,
+      total_token_count=150,
+      cached_content_token_count=80,
+  )
+  event_to_append = Event(
+      invocation_id='cache_test_invocation',
+      author='model',
+      timestamp=1734005536.0,
+      content=genai_types.Content(
+          parts=[genai_types.Part(text='cached response')]
+      ),
+      cache_metadata=cache_meta,
+      usage_metadata=usage_meta,
+  )
+
+  await session_service.append_event(session, event_to_append)
+
+  retrieved_session = await session_service.get_session(
+      app_name='123', user_id='user', session_id='1'
+  )
+  assert retrieved_session is not None
+
+  appended_event = retrieved_session.events[-1]
+  # cache_metadata is preserved
+  assert appended_event.cache_metadata == cache_meta
+  # usage_metadata is preserved
+  assert appended_event.usage_metadata == usage_meta