100% correct response to phoenixes is forge, expansion and robo.
i don't think you need to put a robo down as well as whatever tech you have, if you already have a robo thats fine and keep up production from it, but say you went twilight before scouting the pheonix, it's just as fine to get blink and then go into chargelot archon, use the tech you already have.
The most important thing though is definitely to expand, because pheonixes are a harass unit so don't help too much in a straight up fight, this means you have the advantage over your opponent you need to safely expand.
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