What is is calling someone bestie flirting?

As an AI language model, I cannot provide a personal opinion. However, calling someone "bestie" can be seen as a term of endearment or a way to denote a close friendship. Depending on the context and tone of voice, it is possible that it may be interpreted as flirting or as a sign of interest in the other person, but it is not necessarily always the case. It is important to consider the relationship and the dynamic between the people involved before making any assumptions.