diff --git a/tests/unit/vertex_adk/test_agent_engine_templates_adk.py b/tests/unit/vertex_adk/test_agent_engine_templates_adk.py
index 6eb3be2d78..a0d36dc1ac 100644
--- a/tests/unit/vertex_adk/test_agent_engine_templates_adk.py
+++ b/tests/unit/vertex_adk/test_agent_engine_templates_adk.py
@@ -28,6 +28,7 @@
 from google.adk.events.event_actions import EventActions
 from google.adk.sessions.base_session_service import BaseSessionService
 from google.adk.sessions.in_memory_session_service import InMemorySessionService
+from google.api_core import exceptions as api_exceptions
 from google.api_core import operation as ga_operation
 from google.auth import credentials as auth_credentials
 from google.auth.transport import mtls
@@ -1195,6 +1196,37 @@ def test_project_id_without_project(
         assert app.project_id() is None
         get_project_id_mock.assert_not_called()
 
+    @pytest.mark.parametrize(
+        "error",
+        [
+            api_exceptions.PermissionDenied("denied"),
+            api_exceptions.Unauthenticated("unauthenticated"),
+            # b/561814776: gRPC UNKNOWN, seen when an Agent Gateway terminates
+            # the HTTP/2 stream to cloudresourcemanager.
+            api_exceptions.Unknown("Stream removed"),
+            api_exceptions.DeadlineExceeded("deadline exceeded"),
+            RuntimeError("something unexpected"),
+        ],
+    )
+    def test_project_id_fails_open_on_lookup_error(
+        self,
+        get_project_id_mock: mock.Mock,
+        error: Exception,
+    ):
+        """A failed lookup falls back to the project number instead of raising.
+
+        project_id() runs from set_up() on the cold-start path, so an escaping
+        exception kills the worker over a lookup nothing depends on.
+        """
+        app = agent_engines.AdkApp(agent=_TEST_AGENT)
+        app._tmpl_attrs["project"] = _TEST_PROJECT_NUMBER
+        # Discard the lookup that vertexai.init() made in setup_method.
+        get_project_id_mock.reset_mock()
+        get_project_id_mock.side_effect = error
+
+        assert app.project_id() == _TEST_PROJECT_NUMBER
+        get_project_id_mock.assert_called_once_with(_TEST_PROJECT_NUMBER)
+
     @pytest.mark.usefixtures("caplog")
     def test_enable_tracing(
         self,
diff --git a/tests/unit/vertex_adk/test_reasoning_engine_templates_adk.py b/tests/unit/vertex_adk/test_reasoning_engine_templates_adk.py
index c9daf249a2..a1eac3ce62 100644
--- a/tests/unit/vertex_adk/test_reasoning_engine_templates_adk.py
+++ b/tests/unit/vertex_adk/test_reasoning_engine_templates_adk.py
@@ -23,6 +23,7 @@
 from typing import Optional
 
 from google import auth
+from google.api_core import exceptions as api_exceptions
 import vertexai
 from google.cloud.aiplatform import initializer
 from vertexai.agent_engines import _utils
@@ -1152,6 +1153,37 @@ def test_project_id_without_project(
         assert app.project_id() is None
         get_project_id_mock.assert_not_called()
 
+    @pytest.mark.parametrize(
+        "error",
+        [
+            api_exceptions.PermissionDenied("denied"),
+            api_exceptions.Unauthenticated("unauthenticated"),
+            # b/561814776: gRPC UNKNOWN, seen when an Agent Gateway terminates
+            # the HTTP/2 stream to cloudresourcemanager.
+            api_exceptions.Unknown("Stream removed"),
+            api_exceptions.DeadlineExceeded("deadline exceeded"),
+            RuntimeError("something unexpected"),
+        ],
+    )
+    def test_project_id_fails_open_on_lookup_error(
+        self,
+        get_project_id_mock: mock.Mock,
+        error: Exception,
+    ):
+        """A failed lookup falls back to the project number instead of raising.
+
+        project_id() runs from set_up() on the cold-start path, so an escaping
+        exception kills the worker over a lookup nothing depends on.
+        """
+        app = reasoning_engines.AdkApp(agent=_TEST_AGENT)
+        app._tmpl_attrs["project"] = _TEST_PROJECT_NUMBER
+        # Discard the lookup that vertexai.init() made in setup_method.
+        get_project_id_mock.reset_mock()
+        get_project_id_mock.side_effect = error
+
+        assert app.project_id() == _TEST_PROJECT_NUMBER
+        get_project_id_mock.assert_called_once_with(_TEST_PROJECT_NUMBER)
+
     @mock.patch.dict(os.environ)
     def test_span_content_capture_disabled_by_default(self):
         app = reasoning_engines.AdkApp(agent=_TEST_AGENT)
diff --git a/vertexai/agent_engines/templates/adk.py b/vertexai/agent_engines/templates/adk.py
index 04f67bed7e..a2e700cdc2 100644
--- a/vertexai/agent_engines/templates/adk.py
+++ b/vertexai/agent_engines/templates/adk.py
@@ -1823,11 +1823,18 @@ def project_id(self) -> Optional[str]:
                 from google.cloud.aiplatform.utils import (
                     resource_manager_utils,
                 )
-                from google.api_core import exceptions
 
                 return resource_manager_utils.get_project_id(project)
-            # Fail open as temporary workaround for identity_type config parameter
-            except (exceptions.PermissionDenied, exceptions.Unauthenticated):
+            # This runs from set_up() on the Agent Engine cold-start path, so
+            # anything escaping here kills the uvicorn worker before it serves.
+            # The lookup is cosmetic -- every caller works with the project
+            # number -- so fail open on everything, which is what
+            # aiplatform.initializer already does for the identical call.
+            except Exception as e:  # pylint: disable=broad-exception-caught
+                _warn(
+                    "Failed to convert project number to project ID, proceeding"
+                    f" with the project number: {e}"
+                )
                 return project
 
         return project or None
diff --git a/vertexai/preview/reasoning_engines/templates/adk.py b/vertexai/preview/reasoning_engines/templates/adk.py
index 3e2fb00ab7..5e76221601 100644
--- a/vertexai/preview/reasoning_engines/templates/adk.py
+++ b/vertexai/preview/reasoning_engines/templates/adk.py
@@ -1730,11 +1730,18 @@ def project_id(self) -> Optional[str]:
                 from google.cloud.aiplatform.utils import (
                     resource_manager_utils,
                 )
-                from google.api_core import exceptions
 
                 return resource_manager_utils.get_project_id(project)
-            # Fail open as temporary workaround for identity_type config parameter
-            except (exceptions.PermissionDenied, exceptions.Unauthenticated):
+            # This runs from set_up() on the Agent Engine cold-start path, so
+            # anything escaping here kills the uvicorn worker before it serves.
+            # The lookup is cosmetic -- every caller works with the project
+            # number -- so fail open on everything, which is what
+            # aiplatform.initializer already does for the identical call.
+            except Exception as e:  # pylint: disable=broad-exception-caught
+                _warn(
+                    "Failed to convert project number to project ID, proceeding"
+                    f" with the project number: {e}"
+                )
                 return project
 
         return project or None