def get_price_change_percent(symbol: str, days_ago: int) -> dict:
    """
    Calculates the percentage change in a stock's price over a specified number of days.
    Args:
        symbol (str): The stock symbol (e.g., "AAPL").
        days_ago (int): The number of days to look back for the price change calculation. Must be positive.
    Returns:
        dict: Contains the symbol, percentage change, and period, or an error message.
    """
    if not isinstance(days_ago, int) or days_ago <= 0:
        return {"status": "error", "message": "Days ago must be a positive integer."}

    performance = calculate_performance(symbol, days_ago)
    if performance is not None:
        return {
            "status": "success",
            "symbol": symbol,
            "price_change_percent": performance,
            "period_days": days_ago,
        }
    else:
        return {
            "status": "error",
            "message": f"Could not calculate price change for {symbol} over {days_ago} days. Ensure symbol is valid and data is available for the period.",
        }

root_agent = Agent(
    name="stock_agent",
    model="gemini-2.0-flash",  # Or your preferred model
    description="Agent specialized in providing stock market information and analysis.",
    instruction="""
        You are an AI assistant specializing in stock market data.
        Users will ask for stock prices, price changes, or the best performing stock from a list.
        Use the provided tools to answer these questions accurately.
        - For current price, use `get_stock_price`.
        - For price change percentage over a period, use `get_price_change_percent`.
        - For finding the best performing stock in a list over a period, use `get_best_performing`.
        Always state the symbol and the period clearly in your response if applicable.
        If a stock symbol is invalid or data is unavailable, inform the user clearly.
    """,
    tools=[
        get_stock_price,
        get_price_change_percent,
        get_best_performing,
    ],
)

# Initialize InMemorySessionService
session_service = InMemorySessionService()
active_sessions = {}  # Cache for active session IDs per user

....

# Get or create a session for this user
session_id = await get_or_create_session(user_id)