Agentic commerce
Idempotency key
An idempotency key is a client-supplied identifier that lets a server recognise a retried request as the same logical operation, so submitting it twice takes effect once.
It matters most precisely where failure is ambiguous. A request that times out at the network layer may or may not have executed; without a key, the safe-looking choice — retry — is how one refund becomes two and one charge becomes two.
Agents make this failure mode routine rather than rare. They retry automatically, often more aggressively than a human would, and they do it against endpoints that move money. Any write operation exposed to an agent that does not accept an idempotency key is a double-execution waiting for a slow network.