What is idempotency and which HTTP methods are idempotent?

Idempotency means that making the same request multiple times produces the same result as making it once. This is critical for reliable communication over unreliable networks — if a request times out, the client can safely retry an idempotent request without fear of duplicating side effects. GET, HEAD, PUT, DELETE, and OPTIONS are idempotent. POST and PATCH are not inherently idempotent — sending the same POST twice typically creates two resources. However, you can make POST effectively idempotent by requiring an idempotency key header (e.g., Idempotency-Key: UUID), where the server deduplicates requests with the same key. Stripe's payment API is a well-known example that uses this pattern.