How do you stop an AI agent from double-paying?

Tyson's agent Nigel reached for a supplier payment, and the network hiccuped before the reply came back. Nigel had no way to know whether the first call had landed, so it did what software does under uncertainty: it tried again. The question is what the second call meets. In Tyson's setup, every pay call Nigel issues carries an idempotency_key — a single value the agent generates once per intended payment. Nigel signs as authorized signer under a capped authority; a named human is Member and Manager of the Wyoming series that holds the rail. When the retry arrives bearing the same idempotency_key, the rail recognizes the key it has already seen, returns the original result, and posts nothing new. Idempotency means a repeated call with the same key has the same effect as making it once. So the retry resolves to the first payment's record rather than a fresh charge.