fix: use regional endpoint for non-global Discovery Engine data stores

DiscoveryEngineSearchTool always used the default global endpoint
(discoveryengine.googleapis.com), causing 400 errors when accessing
data stores in regional locations like "eu" or "us".

Parse the location from the data_store_id or search_engine_id resource
path and construct the appropriate regional endpoint (e.g.,
eu-discoveryengine.googleapis.com) via ClientOptions.

Fixes #4697

Author: weiguangli-io

src/google/adk/tools/discovery_engine_search_tool.py

@@ -36,6 +36,9 @@
     r'search_result_mode.*DOCUMENTS', re.IGNORECASE
 )
 
+_LOCATION_PATTERN = re.compile(r"/locations/([^/]+)/")
+_DEFAULT_ENDPOINT = "discoveryengine.googleapis.com"
+
 
 class SearchResultMode(enum.Enum):
   """Search result mode for discovery engine search."""
@@ -102,10 +105,19 @@ def __init__(
 
     credentials, _ = google.auth.default()
     quota_project_id = getattr(credentials, 'quota_project_id', None)
-    options = (
-        client_options.ClientOptions(quota_project_id=quota_project_id)
-        if quota_project_id
-        else None
+
+    resource_id = data_store_id or search_engine_id or ""
+    location_match = _LOCATION_PATTERN.search(resource_id)
+    location = location_match.group(1) if location_match else "global"
+    api_endpoint = (
+        f"{location}-{_DEFAULT_ENDPOINT}"
+        if location != "global"
+        else _DEFAULT_ENDPOINT
+    )
+
+    options = client_options.ClientOptions(
+        api_endpoint=api_endpoint,
+        quota_project_id=quota_project_id,
     )
     self._discovery_engine_client = discoveryengine.SearchServiceClient(
         credentials=credentials, client_options=options

tests/unittests/tools/test_discovery_engine_search_tool.py

@@ -122,7 +122,8 @@ def test_discovery_engine_search_success(
       assert result["results"][0]["content"] == "Test Content"
       mock_auth.assert_called_once()
       mock_client_options.ClientOptions.assert_called_once_with(
-          quota_project_id="test-quota-project"
+          api_endpoint="discoveryengine.googleapis.com",
+          quota_project_id="test-quota-project",
       )
       mock_search_client.assert_called_once_with(
           credentials=mock_credentials,
@@ -320,3 +321,71 @@ def test_auto_detect_does_not_retry_on_unrelated_error(
     assert result["status"] == "error"
     assert "Permission denied" in result["error_message"]
     assert mock_search_client.return_value.search.call_count == 1
+
+  @mock.patch.object(discovery_engine_search_tool, "client_options")
+  @mock.patch.object(discoveryengine, "SearchServiceClient")
+  def test_regional_endpoint_eu_data_store(
+      self, mock_search_client, mock_client_options
+  ):
+    """Test that an EU data store uses the EU regional endpoint."""
+    DiscoveryEngineSearchTool(
+        data_store_id="projects/my-project/locations/eu/collections/default_collection/dataStores/my-ds"
+    )
+    mock_client_options.ClientOptions.assert_called_once_with(
+        api_endpoint="eu-discoveryengine.googleapis.com",
+        quota_project_id=None,
+    )
+
+  @mock.patch.object(discovery_engine_search_tool, "client_options")
+  @mock.patch.object(discoveryengine, "SearchServiceClient")
+  def test_regional_endpoint_us_search_engine(
+      self, mock_search_client, mock_client_options
+  ):
+    """Test that a US search engine uses the US regional endpoint."""
+    DiscoveryEngineSearchTool(
+        search_engine_id="projects/my-project/locations/us/collections/default_collection/engines/my-engine"
+    )
+    mock_client_options.ClientOptions.assert_called_once_with(
+        api_endpoint="us-discoveryengine.googleapis.com",
+        quota_project_id=None,
+    )
+
+  @mock.patch.object(discovery_engine_search_tool, "client_options")
+  @mock.patch.object(discoveryengine, "SearchServiceClient")
+  def test_regional_endpoint_single_region(
+      self, mock_search_client, mock_client_options
+  ):
+    """Test that a single-region location uses the correct endpoint."""
+    DiscoveryEngineSearchTool(
+        data_store_id="projects/my-project/locations/europe-west1/collections/default_collection/dataStores/my-ds"
+    )
+    mock_client_options.ClientOptions.assert_called_once_with(
+        api_endpoint="europe-west1-discoveryengine.googleapis.com",
+        quota_project_id=None,
+    )
+
+  @mock.patch.object(discovery_engine_search_tool, "client_options")
+  @mock.patch.object(discoveryengine, "SearchServiceClient")
+  def test_global_endpoint_explicit(
+      self, mock_search_client, mock_client_options
+  ):
+    """Test that a global data store uses the default global endpoint."""
+    DiscoveryEngineSearchTool(
+        data_store_id="projects/my-project/locations/global/collections/default_collection/dataStores/my-ds"
+    )
+    mock_client_options.ClientOptions.assert_called_once_with(
+        api_endpoint="discoveryengine.googleapis.com",
+        quota_project_id=None,
+    )
+
+  @mock.patch.object(discovery_engine_search_tool, "client_options")
+  @mock.patch.object(discoveryengine, "SearchServiceClient")
+  def test_global_endpoint_no_location_in_id(
+      self, mock_search_client, mock_client_options
+  ):
+    """Test that a short ID without location falls back to global endpoint."""
+    DiscoveryEngineSearchTool(data_store_id="test_data_store")
+    mock_client_options.ClientOptions.assert_called_once_with(
+        api_endpoint="discoveryengine.googleapis.com",
+        quota_project_id=None,
+    )