In the 1920s, Ronald Knox came up with a list of "Ten Commandments" of Detective Fiction listing what wasn't fair cheating [1], getting at this exact point. So

In the 1920s, Ronald Knox came up with a list of "Ten Commandments" of Detective Fiction listing what wasn't fair cheating [1], getting at this exact point. So codifying what is and isn't fair cheating in detective stories is nearly as old as the genre. I'm 100% sure Rian Johnson was aware of these rules as the writer. I'd say Glass Onion technically violates 2 of them, but only for the first hour of the movie. Then you get let in on the cheating before the main climax of the movie. [1] https://agathachristie.fandom.com/wiki/The_%E2%80%9CRules%E2%80%9D_of_Detective_Fiction https://agathachristie.fandom.com/wiki/The_%E2%80%9CRules%E2...