Board Thread:Questions and Answers/@comment-188.166.87.191-20160722121104/@comment-294304-20160724104740

I think it's kinda nitpicky to say the story explicitly written as a prequel isn't a prequel because later material by a later author (who isn't Nasu mind you) contradicts it. Of course, Nasu's usual wishy washy "everything is canon except when it isn't" approach to continuity doesn't help. One could argue that literally every non-FSN product (including F/ha) isn't in continuity with FSN (or each other) due to the fact that they all contradict each other in terms of plot and story in line with the El Melloi thing somewhere.

(Side note: Seeing as Fate/Zero basically created the Waver/El Melloi II character, I find it ironic that his spinoff is being used as the thing to say Fate/Zero "isn't canon" to FSN.)

The best way to probably note it and make everyone happy is to note that Fate/Zero was written as a prequel to FSN and leave it at that. No need to confuse readers with obtuse continuity juggling based on later developments.