A good reaction to a fast expand build with cannons is to just fast expand yourself without cannons. It puts you ahead in tech, and you don't have to spend money on cannons to defend yourself. After all, what's the opponent going to do to you if they have 2-3 cannons at their natural? It's just never worth it to expand with cannons in pvp, you're open to die from so many things, or get behind with regular expansion builds.
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