Network requests fail. Timeouts happen. Clients retry. And without idempotency keys, a single payment request can become three charges. Idempotency keys are one of those API patterns that's easy to overlook until something goes wrong in production. Here's how they work, when to use them, and how to implement them correctly. What Is Idempotency? An operation is idempotent if running it multiple times produces the same result as running it once. GET requests are naturally idempotent — fetching a resource 10 times changes nothing. But POST requests (creating a payment, sending an email, provisioning a server) are not. Each call creates a new thing. Idempotency keys give POST requests the safety of GET requests. You attach a unique key to the request, and the server uses it to deduplicate repeated calls.…