It depends on how much detail you want to understand. You can play Steins;Gate and Robotics;Notes on their own, but they are best understood after playing Chaos;Head (personally, I played C;H after them). Chaos;Child necessarily follows Chaos;Head. For Occultic;Nine, you can read it with no prior knowledge, as far as I’ve seen. For Anonymous;Code, you need Steins;Gate at the bare minimum, and at least a decent summary of Chaos;Head’s themes. Anything more is just added references that you’ll be able to get.

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