diff --git a/src/google/adk/sessions/vertex_ai_session_service.py b/src/google/adk/sessions/vertex_ai_session_service.py
index d8bbab1a95..3d5b382f5b 100644
--- a/src/google/adk/sessions/vertex_ai_session_service.py
+++ b/src/google/adk/sessions/vertex_ai_session_service.py
@@ -48,6 +48,11 @@
 _USAGE_METADATA_CUSTOM_METADATA_KEY = '_usage_metadata'
 
 _SESSION_ID_PATTERN = re.compile(r'^[A-Za-z0-9_-]+$')
+_SESSION_RESOURCE_NAME_PATTERN = re.compile(
+    r'^projects/[^/]+/locations/[^/]+/'
+    r'(?:collections/[^/]+/engines/[^/]+|reasoningEngines/[^/]+)/'
+    r'sessions/([^/]+)$'
+)
 
 
 def _validate_session_id(session_id: str) -> None:
@@ -61,6 +66,19 @@ def _validate_session_id(session_id: str) -> None:
     )
 
 
+def _normalize_session_id(session_id: str) -> str:
+  """Returns the plain session ID from a session ID or Vertex resource name."""
+  match = (
+      _SESSION_RESOURCE_NAME_PATTERN.fullmatch(session_id)
+      if isinstance(session_id, str)
+      else None
+  )
+  if match:
+    session_id = match.group(1)
+  _validate_session_id(session_id)
+  return session_id
+
+
 def _quote_filter_literal(value: str) -> str:
   """Quotes filter values so embedded metacharacters stay inside the literal."""
   escaped_value = value.replace('\\', '\\\\').replace('"', '\\"')
@@ -177,7 +195,7 @@ async def get_session(
       session_id: str,
       config: Optional[GetSessionConfig] = None,
   ) -> Optional[Session]:
-    _validate_session_id(session_id)
+    session_id = _normalize_session_id(session_id)
     reasoning_engine_id = self._get_reasoning_engine_id(app_name)
     session_resource_name = (
         f'reasoningEngines/{reasoning_engine_id}/sessions/{session_id}'
@@ -277,7 +295,7 @@ async def list_sessions(
   async def delete_session(
       self, *, app_name: str, user_id: str, session_id: str
   ) -> None:
-    _validate_session_id(session_id)
+    session_id = _normalize_session_id(session_id)
     reasoning_engine_id = self._get_reasoning_engine_id(app_name)
     session_resource_name = (
         f'reasoningEngines/{reasoning_engine_id}/sessions/{session_id}'
diff --git a/tests/unittests/sessions/test_vertex_ai_session_service.py b/tests/unittests/sessions/test_vertex_ai_session_service.py
index b8c71701dc..5ff10cdfbc 100644
--- a/tests/unittests/sessions/test_vertex_ai_session_service.py
+++ b/tests/unittests/sessions/test_vertex_ai_session_service.py
@@ -725,6 +725,53 @@ async def test_get_and_delete_session():
   assert str(excinfo.value) == '404 Session not found: 1'
 
 
+@pytest.mark.asyncio
+@pytest.mark.usefixtures('mock_get_api_client')
+@pytest.mark.parametrize(
+    'resource_name',
+    [
+        (
+            'projects/my-project/locations/global/collections/'
+            'default_collection/engines/my-app/sessions/1'
+        ),
+        'projects/my-project/locations/us-central1/reasoningEngines/123/sessions/1',
+    ],
+)
+async def test_get_session_accepts_fully_qualified_resource_name(
+    resource_name: str,
+):
+  session_service = mock_vertex_ai_session_service()
+
+  session = await session_service.get_session(
+      app_name='123',
+      user_id='user',
+      session_id=resource_name,
+  )
+
+  assert session == MOCK_SESSION
+
+
+@pytest.mark.asyncio
+@pytest.mark.usefixtures('mock_get_api_client')
+async def test_delete_session_accepts_fully_qualified_resource_name():
+  session_service = mock_vertex_ai_session_service()
+
+  await session_service.delete_session(
+      app_name='123',
+      user_id='user',
+      session_id=(
+          'projects/my-project/locations/global/collections/default_collection/'
+          'engines/my-app/sessions/1'
+      ),
+  )
+
+  with pytest.raises(api_core_exceptions.NotFound) as excinfo:
+    await session_service.get_session(
+        app_name='123', user_id='user', session_id='1'
+    )
+  assert str(excinfo.value) == '404 Session not found: 1'
+
+
 @pytest.mark.asyncio
 @pytest.mark.usefixtures('mock_get_api_client')
 async def test_delete_session_rejects_other_users_session():
@@ -753,7 +800,15 @@ async def test_session_id_path_traversal_rejected():
   """Session IDs containing path-traversal characters must be rejected."""
   session_service = mock_vertex_ai_session_service()
 
-  for bad_id in ['..', '../foo', '..?force=true', 'a/b', '']:
+  for bad_id in [
+      '..',
+      '../foo',
+      '..?force=true',
+      'a/b',
+      '',
+      'projects/my-project/locations/global/sessions/../foo',
+      'reasoningEngines/123/sessions/1',
+  ]:
     with pytest.raises(ValueError):
       await session_service.delete_session(
           app_name='123', user_id='user', session_id=bad_id