Shouldn't this be catching missing commas before conjuctions?

[fdbryant3]

Grammarly and LanguageTool are sticklers for missing commas before conjunctions (and, but, etc.) when linking two ideas. Harper hasn't flagged it once. Is this because Harper is missing something or are Grammarly and LanguageTool just enforcing a convention that isn't a grammar rule? For example, Harper didn't highlight the missing comma after "something" in the previous sentence.

[hippietrail]

This is not something Harper does yet. You can flag a request for it. Linking to specific grammar rules can help a lot. English definitely doesn't want a comma before every single "and" and "but".

[ksandvik]

Does enabling Oxford comma fix this for a majority of cases (disabled by default)?

[hippietrail]

Nice observation. A concrete example would help. In fact all issues really should include illustrative concrete examples.

Oh, there's kind-of an "embedded" example:

Harper is missing something or are Grammarly and LanguageTool just enforcing a convention that isn't a grammar rule?

It's not clear if OP expects a missing comma between something and are, between Grammarly and LanguageTool, or both.