In this case you should generate a GUID/hash for the user on your client side, cookie it, and use that to identify the user.

In terms of running tests against anonymous users, Flagsmith needs some sort of consistent ID that you store against the user on the client, otherwise it's impossible for us to serve a consistent test to that user. Generally people just create a random GUID and then store it on the client (e.g. using cookie on web) but it depends on your client implementation.
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