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..0b11f5285a 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-creation is disabled.
     """
     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..1abd14e2ad 100755
--- a/tests/unittests/cli/test_fast_api.py
+++ b/tests/unittests/cli/test_fast_api.py
@@ -132,6 +132,12 @@ async def dummy_run_async(
     run_config: Optional[RunConfig] = None,
     invocation_id: Optional[str] = None,
 ):
+
+  if not invocation_id and not new_message:
+    if state_delta:
+      logger.warning("state_delta ignored in no-op resume")
+    return
+
   run_config = run_config or RunConfig()
   yield _event_1()
   await asyncio.sleep(0)
@@ -1411,5 +1417,23 @@ def test_builder_save_rejects_traversal(builder_test_client, tmp_path):
   assert not (tmp_path / "app" / "tmp" / "escape.yaml").exists()
 
 
+def test_agent_run_resume_without_message(test_app, create_test_session):
+  """Test that /run allows resuming a session without providing a new message."""
+  info = create_test_session
+  url = "/run"
+  payload = {
+      "app_name": info["app_name"],
+      "user_id": info["user_id"],
+      "session_id": info["session_id"],
+      "streaming": False,
+  }
+
+  response = test_app.post(url, json=payload)
+
+  # Verify the web server and dummy runner work together to return success
+  assert response.status_code == 200
+  assert response.json() == []
+
+
 if __name__ == "__main__":
   pytest.main(["-xvs", __file__])