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 cached resource. To check for freshness later, the client sends a subsequent request to the API for the same resource, including the stored `ETag` in an `If-None-Match` request header. For instance, the client sends `If-None-Match: "W/"abcdef123456""`. The API then compares this received `ETag` with the `ETag` of the current version of the resource on its server. If the `ETag`s match, meaning the resource has not changed, the API responds with a `304 Not Modified` status code. This status code tells the client that its cached version is still fresh and can be used, without the API sending the resource body again, thus saving bandwidth. If the `ETag`s do not match, the resource has been updated, and the API sends the complete, new resource with a `200 OK` status code and an updated `ETag` header.

Similarly, the `Last-Modified` header specifies the date and time at which the resource was last changed by the API. When an API provides a resource, it includes this date and time in the `Last-Modified` response header, for example, `Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT`. A client stores this modification date with the cached resource. To check for freshness, the client sends a subsequent request for the resource including the stored date in an `If-Modified-Since` request header. For example, the client sends `If-Modified-Since: Tue, 15 Nov 1994 12:45:26 GMT`. The API compares this date with the actual last modification date of the resource on its server. If the resource has not been modified since the date provided by the client, the API responds with a `304 Not Modified` status code, indicating that the client's cached version is still valid. If the resource has been modified more recently than the date specified by the client, the API sends the complete, new resource with a `200 OK` status code and an updated `Last-Modified` header. Both `ETag` and `Last-Modified` enable efficient conditional requests, reducing unnecessary data transfer and improving performance.