I built a small library so my LLM agent stops double-charging people

Agents that call tools over a network eventually retry a call they shouldn't have. If the tool isn't idempotent, that retry becomes a duplicate charge, a duplicate email, a duplicate order — quietly, with nothing in the logs to flag it. It's a decades-old distributed-systems problem wearing a new agent costume, and as far as I could tell, nobody had shipped a small, pip-installable fix for the agent-shaped version of it. So I built one. It's called latch . Worth saying up front: this is not a novel idea, and I'd rather say so myself than have someone point it out in the comments. The bug, live Here's the whole thing in one file, no library involved: def charge_card ( order_id , amount ): time . sleep ( 0.4 ) # the payment API is a little slow today ledger [ order_id ] += 1 return { " order_id " : order_id , " status " : " charged " } def agent_charge_with_naive_retry ( order_id , amount , max_retries = 2 ): for attempt in range ( max_retries ): thread = threading . Thread ( target = lambda : charge_card ( order_id , amount )) thread . start () thread . join ( timeout = 0.2 ) # the agent's own client-side timeout if thread . is_alive (): continue # "no response, let's retry" return # got a response The payment call takes 0.4s. The agent gives up waiting after 0.2s and retries. The first attempt is still running in the background — it wasn't cancelled, it can't be safely cancelled, Python threads don't work that way. So it finishes on its own time and charges the card. Then the retry charges it again. The agent's own view of the world is "everything's fine, got a response eventually." The ledger says otherwise. This is examples/naive_agent_example.py in the repo, if you want to run it and watch it happen rather than take my word for it. None of this is exotic. It's the same class of problem as "what happens when a payment gateway's webhook fires twice," which every backend engineer who's touched Stripe has dealt with. The fix has a name — idempotency keys — and it's b