Detecting Claude and ChatGPT using letter counting.

Last time I built a detector that counts four-letter runs (n-grams) and tells you whether Claude Opus 5 wrote something. It works well - about 91% on text it has never read. It has one obvious hole: it only knows one robot (Claude Opus 5). Paste ChatGPT output into it and it says “meatbag”. So I’ve fixed that. I’ve added a ChatGPT detector and you can try it here. Getting the ChatGPT data We did the same as last time, have the model rewrite the same 519 human passages, three ways. That’s 1,557 rewrites, plus 519 passages written from scratch that I keep out of training and use as a test. 2,076 generations in total. Last time this went through Anthropic’s batch API and cost about $30. This time I just ran the command line codex using the subscription I’m already paying. So it didn’t cost anything extra (I’m wishing I’d done that with claude last time!), There is a small issue with this - coodex is a coding agent, so it will have a system prompt that will influence the output, so our…