Since Fusion use SSR, it would be nice to be able to retrieve "user agent" in server side. Since all CDNs are able to transfer this knowledge from the user to the server, it seems trivial to enable this feature for us, developers.
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Thanks the explanation @Sean
Thanks for this comment - unfortunately this capability does not make sense for our architecture because requests to Fusion do not map 1-1 with responses from our server. Because we cache both page and content fetches across many users, when Fusion processes a request on the server, the response will be sent to multiple users with differing user agents. In other words, there is no scenario in Fusion where processing user or client-specific information on the server will be useful, since producing a response that is specific to that client would mean it also applied to many other clients.
We recommend that any user or client specific calls be sent from the client-side via AJAX to a non-Fusion server which can fulfill these requests. This way, the HTML for the page can be cached by Fusion, and then any user-specific info can be processed and fulfilled from a separate server and fetched on the client. Alternatively if you are trying to do user segmentation, please discuss this with our Delivery team.