Errors are not edge cases in API development. They are a primary output. Every API call that can fail will fail — due to invalid input, authentication problems, resource conflicts, rate limits, or infrastructure issues — and how an API communicates those failures determines whether integrators can handle them gracefully or are left guessing. An API that returns clear, consistent, actionable errors is a well-designed API. Everything else is guesswork at scale.

Networks fail. Connections drop. Load balancers time out. Servers restart mid-request. In a distributed system, any request that travels over a network can be sent but not confirmed, leaving the caller uncertain whether the operation completed. This is not a rare edge case — it is a routine condition that well-designed APIs must handle. Idempotency is the mechanism that makes retrying safe when certainty is unavailable.

What Idempotency Means

An operation is idempotent if performing it multiple times produces the same result as performing it once. The outcome is identical whether you call it one time or ten times; subsequent calls do not change anything that the first call already changed.