diff --git a/src/google/adk/cli/adk_web_server.py b/src/google/adk/cli/adk_web_server.py
index 57cf2c50a7..c3b81b287b 100644
--- a/src/google/adk/cli/adk_web_server.py
+++ b/src/google/adk/cli/adk_web_server.py
@@ -494,6 +494,7 @@ def __init__(
       logo_text: Optional[str] = None,
       logo_image_url: Optional[str] = None,
       url_prefix: Optional[str] = None,
+      auto_create_session: bool = False,
   ):
     self.agent_loader = agent_loader
     self.session_service = session_service
@@ -511,6 +512,7 @@ def __init__(
     self.current_app_name_ref: SharedValue[str] = SharedValue(value="")
     self.runner_dict = {}
     self.url_prefix = url_prefix
+    self.auto_create_session = auto_create_session
 
   async def get_runner_async(self, app_name: str) -> Runner:
     """Returns the cached runner for the given app."""
@@ -560,6 +562,7 @@ def _create_runner(self, agentic_app: App) -> Runner:
         session_service=self.session_service,
         memory_service=self.memory_service,
         credential_service=self.credential_service,
+        auto_create_session=self.auto_create_session,
     )
 
   def _instantiate_extra_plugins(self) -> list[BasePlugin]:
diff --git a/src/google/adk/cli/cli_tools_click.py b/src/google/adk/cli/cli_tools_click.py
index 0263c0e9f6..5b5d3e5c82 100644
--- a/src/google/adk/cli/cli_tools_click.py
+++ b/src/google/adk/cli/cli_tools_click.py
@@ -1419,6 +1419,14 @@ async def _lifespan(app: FastAPI):
 @fast_api_common_options()
 @adk_services_options(default_use_local_storage=True)
 @deprecated_adk_services_options()
+@click.option(
+    "--auto_create_session",
+    is_flag=True,
+    default=False,
+    help=(
+        "Automatically create a session if it doesn't exist when calling /run."
+    ),
+)
 def cli_api_server(
     agents_dir: str,
     eval_storage_uri: Optional[str] = None,
@@ -1439,6 +1447,7 @@ def cli_api_server(
     a2a: bool = False,
     reload_agents: bool = False,
     extra_plugins: Optional[list[str]] = None,
+    auto_create_session: bool = False,
 ):
   """Starts a FastAPI server for agents.
 
@@ -1471,6 +1480,7 @@ def cli_api_server(
           url_prefix=url_prefix,
           reload_agents=reload_agents,
           extra_plugins=extra_plugins,
+          auto_create_session=auto_create_session,
       ),
       host=host,
       port=port,
diff --git a/src/google/adk/cli/fast_api.py b/src/google/adk/cli/fast_api.py
index 287fce6796..9956fc5587 100644
--- a/src/google/adk/cli/fast_api.py
+++ b/src/google/adk/cli/fast_api.py
@@ -91,6 +91,7 @@ def get_fast_api_app(
     extra_plugins: Optional[list[str]] = None,
     logo_text: Optional[str] = None,
     logo_image_url: Optional[str] = None,
+    auto_create_session: bool = False,
 ) -> FastAPI:
 
   # Set up eval managers.
@@ -153,6 +154,7 @@ def get_fast_api_app(
       logo_text=logo_text,
       logo_image_url=logo_image_url,
       url_prefix=url_prefix,
+      auto_create_session=auto_create_session,
   )
 
   # Callbacks & other optional args for when constructing the FastAPI instance