When designing an API, which HTTP method must guarantee that calling it many times will have the same final effect as calling it once, without any unwanted changes?

The HTTP method that must guarantee calling it many times will have the same final effect as calling it once, without any unwanted changes, is PUT. This guarantee is known as idempotence. An operation is idempotent if executing it multiple times produces the same result as executing it once, meaning the final state of the server's resources remains unchanged after one or more identical requests. When an API uses the PUT method, it is designed to create a resource at a specified Uniform Resource Identifier (URI) if it does not already exist, or to completely replace an existing resource at that URI with the new data provided in the request body. Therefore, sending the same PUT request multiple times will always result in the same final state for that specific resource on the server. There are no additional, unintended side effects or changes beyond the single, definitive creation or replacement of the target resource, regardless of how many times the request is sent.