SQLAlchemySession(create_tables=True) cannot create tables on MySQL due to unbounded String columns

The session guide lists MySQL as a supported database, and the API reference includes mysql+aiomysql:// and documents create_tables=True for development/testing. However, automatic table creation fails against a fresh MySQL database.

Reproduction

Reproduced with MySQL Community Server 8.0.46, Python 3.12.3 on Linux, openai-agents 0.22.0, SQLAlchemy 2.0.52, and aiomysql 0.3.2. The same failure occurs on main at 89c02c828ee8510fe9a84ee6675608193aa13b02.

Install the dependencies, then set MYSQL_URL to a mysql+aiomysql:// connection URL for an empty test database with permission to create tables. No model call or OpenAI API key is needed.

pip install 'openai-agents[sqlalchemy]==0.22.0' 'SQLAlchemy==2.0.52' 'aiomysql==0.3.2' cryptography

import asyncio
import os

from sqlalchemy import text
from sqlalchemy.ext.asyncio import create_async_engine

from agents.extensions.memory import SQLAlchemySession


async def main():
    engine = create_async_engine(os.environ["MYSQL_URL"])
    try:
        async with engine.connect() as conn:
            print("MySQL:", await conn.scalar(text("SELECT VERSION()")))

        session = SQLAlchemySession(
            "mysql-create-tables",
            engine=engine,
            create_tables=True,
        )
        await session.add_items([{"role": "user", "content": "hello"}])
    finally:
        await engine.dispose()


asyncio.run(main())

Actual result

The connection succeeds and reports MySQL 8.0.46. On the first add_items() call, automatic schema initialization fails with:

sqlalchemy.exc.CompileError: (in table 'agent_sessions', column 'session_id'): VARCHAR requires a length on dialect mysql

This happens while SQLAlchemy compiles the first CREATE TABLE, before any CREATE statement reaches MySQL. No tables are created.

Both session_id columns in the current implementation use String without a length.

Expected behavior / scope clarification

Is SQLAlchemySession(create_tables=True) intended to support MySQL?

If so, I would expect the example above to create the required tables and store the message, and I'd be happy to submit a focused fix with MySQL regression coverage.

If automatic schema creation is intentionally unsupported on MySQL, I can instead submit a documentation clarification.

One possible implementation direction is a MySQL-specific bounded VARCHAR via with_variant(), which would preserve the existing PostgreSQL/SQLite types. I have not chosen a length here because doing so would introduce a MySQL-specific maximum session ID length that should be agreed first.

This report is limited to automatic table creation on a fresh database; it does not assess create_tables=False with a manually provisioned schema.

Start in src/agents/extensions/memory/sqlalchemy_session.py, where both session_id columns use unbounded String, and reproduce the failure with the provided MySQL setup and script. Review SQLAlchemy with_variant() and the existing PostgreSQL/SQLite types before choosing the bounded MySQL type. Done means create_tables=True creates the required tables and stores the message, with MySQL regression coverage.