If a client wants to check if a resource it already has is still fresh without downloading the whole thing again, what specific HTTP header would the API provide to allow for this efficient check?

The specific HTTP headers an API would provide to allow a client to check if a resource it already has is still fresh without downloading the whole thing again are `ETag` and `Last-Modified`. These headers are included in the API's response when it initially sends a resource to the client, enabling subsequent efficient freshness checks. The `ETag` header, which stands for Entity Tag, is an opaque identifier that uniquely represents a specific version of a resource. When an API provides a resource, it includes an `ETag` in its response, for example, `ETag: "W/"abcdef123456""`. The `W/` prefix indicates a weak ETag, which means two resources might be semantically equivalent even if their ETags are not byte-for-byte identical, but strong ETags without the `W/` prefix require byte-for-byte identity. A client stores this `ETag` along with the cac....