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I tried it on my test wpcom site with upgrades and I didn't see the header, but it appears to be because Config::can_use_cache(); returned false. Investigating..

I have instead used the ATOMIC_CLIENT_ID constant, which is available on both Atomic and Pressable. However, I couldn't see the header even when I removed the condition and served the header all the time in atomic. Is the header filtered out by the edge? @thingalon

I also noticed that the files aren't cached in wpcom, which is a bummer. I think this will get resolved when we start serving from static files. boost-cache directory seems to be doing just fine. @thingalon previously recommended serving from inside the uploads folder. Do you see any downsides to serving from wp-content/boost-cache?

I think the boost-cache directory is probably a good place to put these files. We can be in complete control of them, and we'll be sure we're not interfering with files from either the blog or another plugin.
If we rely on the 404 handler to regenerate static files then wp-content/boost-cache/ and a sub directory like static will be easy to identify.
However, if we do this, then we have to be sure that the boost-cache directory itself isn't removed if the "Page Cache" module is toggled off.

Ah, sorry I missed this last week @haqadn. :)

I have instead used the ATOMIC_CLIENT_ID constant, which is available on both Atomic and Pressable. However, I couldn't see the header even when I removed the condition and served the header all the time in atomic. Is the header filtered out by the edge? @thingalon

Yes, we strip that header out on its way through the Edge. However, you can check if a given request was cached or not by looking at the x-ac header in the response.

If you request the same URL a few times and see HIT in there, you know that you've successfully hit the cache. :) Make sure you're not authenticated, as a login cookie causes a cache bypass. Using curl should do it.

I also noticed that the files aren't cached in wpcom, which is a bummer. I think this will get resolved when we start serving from static files. boost-cache directory seems to be doing just fine. @thingalon previously recommended serving from inside the uploads folder. Do you see any downsides to serving from wp-content/boost-cache?

I believe that wp-content/boost-cache should be fine.

I'm not sure which files aren't being cached on wpcom; if you can send me details, I can help out with that.