[Python] How can I create a tool for langgaph's agent to save data in db?

I created an agent using Langgraph in Python, and I developed a tool for them to save a todo in a database. But that tool doesn't work and raises an error!

I use SQLAlchemy to connect to the DB.

This is my DB code:

from datetime import datetime

from sqlalchemy import DateTime
from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker, AsyncSession
from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column

engine = create_async_engine(
settings.DATABASE_URL,
pool_size=20,
max_overflow=40,
pool_timeout=30,
pool_pre_ping=True,
)

AsyncSessionLocal = async_sessionmaker(
engine,
class_=AsyncSession,
expire_on_commit=False,
autoflush=False,
autocommit=False,
)


and this is my agent:

async def run_agent(thread_id:int, human_message:str) -> MessagesState:
"""
talk with your agent or execute agent

Args:
thread_id (int): thread id
human_message (str): message of human

Returns:
MessagesState: return MessagesState
"""

config = {"configurable": {"thread_id": thread_id}}

async with AsyncSqliteSaver.from_conn_string(settings.AI_MEMORY_DB) as checkpointer:

agent = create_agent(
model=chat_model,
tools=TOOLS,
system_prompt=settings.SYSTEM_PROMPT,
checkpointer=checkpointer
)

res = await agent.ainvoke({"messages": HumanMessage(human_message)}, config=config)

return res


and this is my tools:

@tool
async def create_todo(date:str, time:str, title:str) -> str:
"""
Use this tool IMMEDIATELY and WITHOUT EXCEPTION whenever the user wants to
create a new todo, reminder, appointment, plan, or schedule anything for the future.

This tool stores the user's schedule in a database so that it can be better recalled later and reports can be generated on the tasks.

You MUST call this tool when you see phrases like:
- Remind me to ...
- Set a reminder for ...
- Create a todo for ...
- I have a meeting/appointment/plan on ...
- Remember to call someone tomorrow
- Schedule something
- Don't let me forget to ...
- Put in my calendar ...

Even if the user speaks in Persian (Farsi) or gives the date in Persian calendar (Shamsi),
you MUST convert it to Gregorian (YYYY-MM-DD) yourself and call this tool.

Examples that MUST trigger this tool:
- "Remind me to go to the doctor next Monday at 3 PM"

- "Set a reminder for December 25, 2025 at 14:30 for team meeting"

Args:
date (str): Date in EXACT YYYY-MM-DD format (example: 2025-11-26, 2025-12-25)
time (str): Time in 24-hour HH:MM format with leading zeros (example: 09:30, 17:00, 08:15)
title (str): Full title or description of the task/reminder exactly as the user wants it saved

Returns:
str: Success or error message
"""
try:
async with AsyncSessionLocal() as session:
async with session.begin():
_datetime = f"{date} {time}"
datetime_to_do_it = await sync_to_async(datetime.strptime)(_datetime, "%Y-%m-%d %H:%M")
todo = Todo(title=title, datetime_to_do_it=datetime_to_do_it)
session.add(todo)


return "todo sucessfuly saved"
except Exception as ex:
return f"something wrong! {ex}"



TOOLS = [
create_todo,
]



But when I want my agent to save my todo, that doesn't work and raises an error:

Exception terminating connection <AdaptedConnection <asyncpg.connection.Connection object at 0x7f1f5017dc70>>
Traceback (most recent call last):
File "/home/user/Documents/python_projects/Diana/venv/lib/python3.12/site-packages/sqlalchemy/pool/base.py", line 372, in _close_connection
self._dialect.do_terminate(connection)
File "/home/user/Documents/python_projects/Diana/venv/lib/python3.12/site-packages/sqlalchemy/dialects/postgresql/asyncpg.py", line 1127, in do_terminate
dbapi_connection.terminate()
File "/home/user/Documents/python_projects/Diana/venv/lib/python3.12/site-packages/sqlalchemy/connectors/asyncio.py", line 402, in terminate
self.await_(asyncio.shield(self._terminate_graceful_close())) # type: ignore[attr-defined] # noqa: E501
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/user/Documents/python_projects/Diana/venv/lib/python3.12/site-packages/sqlalchemy/util/_concurrency_py3k.py", line 132, in await_only
return current.parent.switch(awaitable) # type: ignore[no-any-return,attr-defined] # noqa: E501
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/user/Documents/python_projects/Diana/venv/lib/python3.12/site-packages/sqlalchemy/util/_concurrency_py3k.py", line 196, in greenlet_spawn
value = await result
^^^^^^^^^^^^
File "/home/user/Documents/python_projects/Diana/venv/lib/python3.12/site-packages/sqlalchemy/dialects/postgresql/asyncpg.py", line 912, in _terminate_graceful_close
await self._connection.close(timeout=2)
File "/home/user/Documents/python_projects/Diana/venv/lib/python3.12/site-packages/asyncpg/connection.py", line 1504, in close
await self._protocol.close(timeout)
File "asyncpg/protocol/protocol.pyx", line 627, in close
File "asyncpg/protocol/protocol.pyx", line 660, in asyncpg.protocol.protocol.BaseProtocol._request_cancel
File "/home/user/Documents/python_projects/Diana/venv/lib/python3.12/site-packages/asyncpg/connection.py", line 1673, in _cancel_current_command
self._cancellations.add(self._loop.create_task(self._cancel(waiter)))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.12/asyncio/base_events.py", line 456, in create_task
self._check_closed()
File "/usr/lib/python3.12/asyncio/base_events.py", line 541, in _check_closed
raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed
^C^Csys:1: RuntimeWarning: coroutine 'Connection._cancel' was never awaited
RuntimeWarning: Enable tracemalloc to get the object allocation traceback


I know this error raises when my agent wants to use it because I tested it in IPython.

Continue reading...