Your agent lost the response. Do not let it retry the refund.

A support agent issues a refund, the connection drops, no confirmation comes back. So it sends the refund again. Congratulations, you've paid the customer twice.

That scenario opens a useful piece at Unite.AI by Himanshu Goel, and the core point is one every backend engineer already knows: no response doesn't mean failure. The request may have failed, succeeded silently, or still be in flight.

Benchmarks vs. vibes

A September 2026 preprint, "Where Does Exactly-Once Live?", introduced a benchmark called LIMBO and ran 25,930 episodes across nine models against simulated services with lost acknowledgements, delays and duplicate deliveries. In its main test, there were 1,279 episodes where the agent caused a duplicate effect, and in 90 percent of those the agent reported that it had completed the task successfully. These are controlled simulations, not production failure rates, but the lesson stands: an agent's report of success is not evidence of what happened.

What to do

• Use idempotency keys, in code, not in the prompt. Keep the key stable across every retry. Know your provider's limits; Stripe, for instance, may prune keys after at least 24 hours, after which a reused key starts a new request.
• Record the operation before you send it: its ID, exact parameters and any provider reference. Reject a changed request disguised as the same one. A different amount is a new action that needs new approval.
• Treat unknown as a real state. If you can't confirm what happened and there's no duplicate-safe guarantee, stop writing and reconcile through a status query or a person with authority. A status check can miss a request that's still processing.
• Resume, don't restart. If the refund worked but the email failed, retry the email.
• Test the second attempt. Drop responses and delay execution on purpose in a test environment before shipping.

Approval to act once is not approval to act again.

Scoreboard update: Humans +1. Agents report success even after acting twice, so humans keep the recovery path.

Stay undead.