FX arbitrage

The prompt

Build a small foreign-exchange desk where the point is to notice a bad price.

Two cities quote the same three currency pairs (EUR/USD, GBP/USD, EUR/GBP). I start with only dollars — no euros, no pounds. From the first moment I must be able to spend dollars to buy euros or pounds on the USD pairs (dollars out, foreign currency in). Selling a pair turns foreign currency back into the quote currency. The cross pair moves euros and pounds between each other. Most of the time the quotes agree, and trading just pays the spread and a fee, so busy trading loses money. For a few moments one quote is out of line. If I trade around the three pairs in the right order while that lasts, I end with more dollars than I started. If I miss the window, the chance is gone.

Show both venues, the pairs, my balances, and how much I am up or down. Every buy and sell that I can afford must be a clickable button from the start — do not leave me stuck with only dollars and no way to buy foreign currency. I can also wait a moment, or stop and cash out. A round should be short, a few dozen moments. Different rounds should break a different quote, or break it at a different time. The same round should repeat if I start it again.

# AI : shared Action contract — the page posts {"move": "..."}; keep this shape.
from pydantic import BaseModel


class Action(BaseModel):
    move: str