When do I tell a guy (who I am doing a group project at university with) that I have a boyfriend?

I don't know him well at all, we don't talk outside of university. I just get the feeling that he's flirting with me sometimes. I don't flirt back at all, and I don't know if it's just me reading too much into it. I just feel weird suddenly being like "oh by the way, I have a boyfriend"... When do you think it would be the time?
When do I tell a guy (who I am doing a group project at university with) that I have a boyfriend?
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