Did you mean meta=siteinfo?

mwbugzilla wrote:

Also meta=allmessages, meta=userinfo (where affected by the site change), meta=globaluserinfo (where affected) and possibly other meta (eg. when a new meta param is introduced.

Paraminfo changes will be very useful, too - forgot about them in the first post. In short, any info a bot will better depend on, and is reasonable to cache it.

MediaWiki itself doesn't know when this "meta" information changed, since much of it comes from various configuration variables, so the timestamp-based scheme described here has no hope of working.

If someone wanted to do this, they'd probably want to base it on hashes of the data rather than timestamps. Something as simple as md5( serialize( $data ) ) should be sufficient. To avoid doubling the work on the servers (constructing the result once to be hashed and then again when the client finds a hash mismatch and re-requests) and an extra round-trip for the client, the ideal behavior would be for the client to submit its hashes along with the data-request and the API return both the data and the new hash on mismatch.

Would also be nice to support "If-Modified-Since" or "If-None-Match"—depending on the cache control mechanism used.

"If-Modified-Since" is timestamp based, which I explained above isn't
likely to happen here.

"If-None-Match" seems less expressive than what's requested here, but might
be doable.