Does "What did you get up to?" make sense?

Someone texted me the following

What did you get up to?

And when I asked what that meant, told me it meant What are you doing tonight?. Now to me that doesn't make any sense, but I'm not a native speaker. The other person is. Before I entrench myself in an argument, I want to make sure I'm right. Does that make sense?

"Get up to + something" is a British and informal expression. It just means "do something."

So "What did you get up to" means "What did you do?"

Perhaps your friend wanted to ask you about how you spent your free time and also implied about the rest of it.

"What did you get up to?" is asking what you've been doing, in the past.

"What are you doing tonight?" is asking what you will be doing, in the future.

They don't mean the same thing.

‘What have you been up to?’ can express a suspicion of bad behaviour, but not necessarily. 'What are you up to tonight?' can, depending on circumstances, be simply an inquiry about this evening's plans.