Comment by kentonv

We can't implement automatic caching of POST requests because there is no standard for computing cache keys for POST requests; it's different for every application.

E.g. presumably the body of the request matters for cache matching, but the body can be any arbitrary format the application chooses. The platform has no idea how to normalize it to compute a consistent cache key -- except perhaps to match the whole body byte-for-byte, but for many apps that would not produce the desired behavior. For example, if you had a trace ID in your requests, now none of your requests would hit cache because each one has a unique trace ID, but of course a trace ID is not intended to be considered for caching.

The Cache API can only implement the semantics that the HTTP standard specifies for caching, and the HTTP standard does not specify any semantics for caching POST requests.

That said, what we really should have done was left it up to the application to compute cache keys however they want, and only implemented the lookup from string cache key -> Response object. That's not what the standard says, though.