punctuation - Is it mandatory to use a comma before a coordinating conjunction uniting the two independent clauses in a compound sentence?

My friend and I had an argument about whether this sentence required a comma:

I understand where you're coming from but I disagree.

My friend insisted that there should be a comma before "but":

I understand where you're coming from, but I disagree.

From my point of view, while grammatically correct, the addition of a comma is not necessary.

When I looked online for sources to answer our disagreement, I found no authoritative sources and many sources were contradicting one another. Some argued, like I believe, that it's not necessary because the addition of comma is of no value in that context (i.e. the addition of comma doesn't make the sentence easier to read) but others said it was mandatory.

So, which is it? Is a comma necessary before a conjunction?