diff --git a/src/google/adk/cli/adk_web_server.py b/src/google/adk/cli/adk_web_server.py
index b97932d042..570da8a97d 100644
--- a/src/google/adk/cli/adk_web_server.py
+++ b/src/google/adk/cli/adk_web_server.py
@@ -202,7 +202,7 @@ class RunAgentRequest(common.BaseModel):
   app_name: str
   user_id: str
   session_id: str
-  new_message: types.Content
+  new_message: Optional[types.Content] = None
   streaming: bool = False
   state_delta: Optional[dict[str, Any]] = None
   # for resume long-running functions
diff --git a/src/google/adk/runners.py b/src/google/adk/runners.py
index 3aaa54e257..057df62563 100644
--- a/src/google/adk/runners.py
+++ b/src/google/adk/runners.py
@@ -481,8 +481,7 @@ async def run_async(
       The events generated by the agent.
 
     Raises:
-      ValueError: If the session is not found; If both invocation_id and
-        new_message are None.
+      ValueError: If the session is not found and `auto_create_session` is False.
     """
     run_config = run_config or RunConfig()
 
@@ -497,12 +496,22 @@ async def _run_with_trace(
         session = await self._get_or_create_session(
             user_id=user_id, session_id=session_id
         )
+
         if not invocation_id and not new_message:
-          raise ValueError(
-              'Running an agent requires either a new_message or an '
-              'invocation_id to resume a previous invocation. '
-              f'Session: {session_id}, User: {user_id}'
+          if state_delta:
+            logger.warning(
+                'state_delta provided without new_message or invocation_id for '
+                'session %s. The state_delta will be ignored.',
+                session_id,
+            )
+          logger.info(
+              'Performing no-op resume for session %s: no new_message or '
+              'invocation_id.',
+              session_id,
           )
+          # If nothing is provided, this is a no-op resume. We return early
+          # without yielding any events.
+          return
 
         if invocation_id:
           if (
diff --git a/tests/unittests/cli/test_fast_api.py b/tests/unittests/cli/test_fast_api.py
index 0c69605349..58e4307c13 100755
--- a/tests/unittests/cli/test_fast_api.py
+++ b/tests/unittests/cli/test_fast_api.py
@@ -132,6 +132,7 @@ async def dummy_run_async(
     run_config: Optional[RunConfig] = None,
     invocation_id: Optional[str] = None,
 ):
+
   run_config = run_config or RunConfig()
   yield _event_1()
   await asyncio.sleep(0)
@@ -1411,5 +1412,43 @@ def test_builder_save_rejects_traversal(builder_test_client, tmp_path):
   assert not (tmp_path / "app" / "tmp" / "escape.yaml").exists()
 
 
+async def _noop_run_async(*args, **kwargs):
+  """A mock that does nothing and yields no events for no-op resume tests."""
+  for item in []:
+    yield item
+
+
+@pytest.mark.parametrize(
+    "extra_payload",
+    [
+        {},
+        {"state_delta": {"some_key": "some_value"}},
+    ],
+    ids=["no_state_delta", "with_state_delta"],
+)
+def test_agent_run_resume_without_message(
+    test_app, create_test_session, monkeypatch, extra_payload
+):
+  """Test that /run allows resuming a session without providing a new message."""
+  # Override the global mock with a specific no-op mock for this test
+  monkeypatch.setattr("google.adk.runners.Runner.run_async", _noop_run_async)
+
+  info = create_test_session
+  url = "/run"
+  payload = {
+      "app_name": info["app_name"],
+      "user_id": info["user_id"],
+      "session_id": info["session_id"],
+      "streaming": False,
+      **extra_payload,
+  }
+
+  response = test_app.post(url, json=payload)
+
+  # Verify the web server handles the request and returns success
+  assert response.status_code == 200
+  assert response.json() == []
+
+
 if __name__ == "__main__":
   pytest.main(["-xvs", __file__])
diff --git a/tests/unittests/test_runners.py b/tests/unittests/test_runners.py
index 62b8d7334b..a9a0a4bce8 100644
--- a/tests/unittests/test_runners.py
+++ b/tests/unittests/test_runners.py
@@ -11,13 +11,14 @@
 # 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.
-
 import importlib
+import logging
 from pathlib import Path
 import sys
 import textwrap
 from typing import AsyncGenerator
 from typing import Optional
+from unittest import mock
 from unittest.mock import AsyncMock
 
 from google.adk.agents.base_agent import BaseAgent
@@ -1321,5 +1322,39 @@ def test_infer_agent_origin_detects_mismatch_for_user_agent(
     assert "actual_name" in runner._app_name_alignment_hint
 
 
+@pytest.mark.asyncio
+async def test_run_async_no_op_resume_logging(caplog):
+  """Verifies that the actual Runner logic logs a warning when state_delta is ignored."""
+  from google.adk.runners import Runner
+
+  # 1. Setup dependencies
+  mock_agent = mock.MagicMock()
+  mock_agent.name = "test_agent"
+
+  # Added app_name="test_app" to satisfy validation
+  runner = Runner(
+      app_name="test_app",
+      agent=mock_agent,
+      session_service=mock.AsyncMock(),
+  )
+
+  # 2. Capture logs while running the actual logic in runners.py
+  with caplog.at_level(logging.WARNING):
+    # Call run_async without message or invocation_id, but WITH state_delta
+    async for _ in runner.run_async(
+        user_id="user",
+        session_id="session",
+        new_message=None,
+        state_delta={"test": 1},
+    ):
+      pass
+
+  # 3. This verifies the REAL warning logic in src/google/adk/runners.py
+  assert any(
+      "state_delta provided without new_message" in r.message
+      for r in caplog.records
+  )
+
+
 if __name__ == "__main__":
   pytest.main([__file__])