Detecting Claude by counting letters

On 2 August 2026 Anthropic began watermarking the text that new Claude models produce - their support pages cover which products are marked. The watermark is not a hidden character or a bit of metadata. It is statistical. A secret key nudges the generated output in a way that isn’t obvious to a person, but that can be detected if you have the key. This means that Anthropic can detect it but we common folk cannot. Other providers are doing similar things, or will be soon. Can we build our own detector? Given a piece of text, can you tell that a model wrote it? Let’s simplify things and just try one narrow target: text written by Claude Opus 5. Mostly because that’s what I have a subscription to… And - spoiler alert - it turns out you can. I’ve deployed a web version of it here. Generate some text in Claude using Opus 5 and see if it can detect it. And yes, if you feed this blog post in, despite me editing it substantially - it’s comes out as written by Claude. How does it work? A long…