diff --git a/tests/unittests/tools/test_forwarding_artifact_service.py b/tests/unittests/tools/test_forwarding_artifact_service.py
new file mode 100644
index 0000000000..9bdad13a2e
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+++ b/tests/unittests/tools/test_forwarding_artifact_service.py
@@ -0,0 +1,185 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from unittest.mock import AsyncMock
+
+from google.adk.tools._forwarding_artifact_service import ForwardingArtifactService
+from google.genai import types
+from pytest import mark
+from pytest import raises
+
+
+class _StubSession:
+
+  def __init__(self, session_id: str):
+    self.id = session_id
+
+
+class _StubInvocationContext:
+  """Minimal InvocationContext stub for ForwardingArtifactService tests."""
+
+  def __init__(self, artifact_service=None):
+    self.app_name = 'test_app'
+    self.user_id = 'test_user'
+    self.session = _StubSession('test_session')
+    self.artifact_service = artifact_service
+
+
+class _StubToolContext:
+  """Minimal ToolContext stub for ForwardingArtifactService tests."""
+
+  def __init__(self, invocation_context):
+    self._invocation_context = invocation_context
+    self.save_artifact = AsyncMock(return_value=1)
+    self.load_artifact = AsyncMock(return_value=None)
+    self.list_artifacts = AsyncMock(return_value=[])
+
+
+def _create_service(artifact_service=None):
+  invocation_context = _StubInvocationContext(artifact_service)
+  tool_context = _StubToolContext(invocation_context)
+  return (
+      ForwardingArtifactService(tool_context),
+      tool_context,
+      invocation_context,
+  )
+
+
+@mark.asyncio
+async def test_save_artifact_delegates_to_tool_context():
+  service, tool_context, _ = _create_service()
+  artifact = types.Part(text='hello')
+
+  result = await service.save_artifact(
+      app_name='ignored_app',
+      user_id='ignored_user',
+      filename='file.txt',
+      artifact=artifact,
+      custom_metadata={'k': 'v'},
+  )
+
+  assert result == 1
+  tool_context.save_artifact.assert_awaited_once_with(
+      filename='file.txt',
+      artifact=artifact,
+      custom_metadata={'k': 'v'},
+  )
+
+
+@mark.asyncio
+async def test_load_artifact_delegates_to_tool_context():
+  service, tool_context, _ = _create_service()
+  artifact = types.Part(text='hello')
+  tool_context.load_artifact.return_value = artifact
+
+  result = await service.load_artifact(
+      app_name='ignored_app',
+      user_id='ignored_user',
+      filename='file.txt',
+      version=2,
+  )
+
+  assert result is artifact
+  tool_context.load_artifact.assert_awaited_once_with(
+      filename='file.txt', version=2
+  )
+
+
+@mark.asyncio
+async def test_list_artifact_keys_delegates_to_tool_context():
+  service, tool_context, _ = _create_service()
+  tool_context.list_artifacts.return_value = ['a.txt', 'b.txt']
+
+  result = await service.list_artifact_keys(
+      app_name='ignored_app', user_id='ignored_user'
+  )
+
+  assert result == ['a.txt', 'b.txt']
+  tool_context.list_artifacts.assert_awaited_once_with()
+
+
+@mark.asyncio
+async def test_delete_artifact_delegates_to_invocation_context_service():
+  root_artifact_service = AsyncMock()
+  service, _, invocation_context = _create_service(root_artifact_service)
+
+  await service.delete_artifact(
+      app_name='ignored_app', user_id='ignored_user', filename='file.txt'
+  )
+
+  root_artifact_service.delete_artifact.assert_awaited_once_with(
+      app_name=invocation_context.app_name,
+      user_id=invocation_context.user_id,
+      session_id=invocation_context.session.id,
+      filename='file.txt',
+  )
+
+
+@mark.asyncio
+async def test_delete_artifact_raises_when_no_root_artifact_service():
+  service, _, _ = _create_service(artifact_service=None)
+
+  with raises(ValueError, match='Artifact service is not initialized.'):
+    await service.delete_artifact(
+        app_name='ignored_app', user_id='ignored_user', filename='file.txt'
+    )
+
+
+@mark.asyncio
+async def test_list_versions_delegates_to_invocation_context_service():
+  root_artifact_service = AsyncMock()
+  root_artifact_service.list_versions.return_value = [1, 2, 3]
+  service, _, invocation_context = _create_service(root_artifact_service)
+
+  result = await service.list_versions(
+      app_name='ignored_app', user_id='ignored_user', filename='file.txt'
+  )
+
+  assert result == [1, 2, 3]
+  root_artifact_service.list_versions.assert_awaited_once_with(
+      app_name=invocation_context.app_name,
+      user_id=invocation_context.user_id,
+      session_id=invocation_context.session.id,
+      filename='file.txt',
+  )
+
+
+@mark.asyncio
+async def test_list_versions_raises_when_no_root_artifact_service():
+  service, _, _ = _create_service(artifact_service=None)
+
+  with raises(ValueError, match='Artifact service is not initialized.'):
+    await service.list_versions(
+        app_name='ignored_app', user_id='ignored_user', filename='file.txt'
+    )
+
+
+@mark.asyncio
+async def test_list_artifact_versions_raises_not_implemented():
+  service, _, _ = _create_service()
+
+  with raises(NotImplementedError):
+    await service.list_artifact_versions(
+        app_name='ignored_app', user_id='ignored_user', filename='file.txt'
+    )
+
+
+@mark.asyncio
+async def test_get_artifact_version_raises_not_implemented():
+  service, _, _ = _create_service()
+
+  with raises(NotImplementedError):
+    await service.get_artifact_version(
+        app_name='ignored_app', user_id='ignored_user', filename='file.txt'
+    )