What’s the difference between 你去干嘛 and 你去吗
They don't mean the same.

&quot;你去干嘛?&quot; means &quot;What are you going to do?&quot; or maybe &quot;Why are you going?&quot; depending on the context.

&quot;你去吗?&quot; means &quot;Are you going?&quot;


Examples:

A: 我出去一会 = I'm leaving for a while. B: 你去干嘛? = What are you going to do?
A: 我去开会了 = I'm going to attend the meeting. B: 这会是给经理开的, 你去干嘛? = The meeting is intended for managers. Why are you going?
A: 我要去吃午饭, 你去吗 = I'm leaving for lunch. Are you going (with me)?