Kelsey Piper noticed that Opus 4.7 is the first model which can identify her from her unpublished writing.

I replicated the experiment myself, which is absolutely terrifying given that I am one of the most minor Internet personalities who has actually written stuff on the Internet.

Claude professes not to know who I am, but reliably identifies me from my writing.

Methodology: clear your custom instructions in claude.ai, and set your name to Unknown Visitor. Enter incognito chat mode with Claude. (At this point, you can ask what it knows about you, and it will profess to know nothing at all; but do that in a separate chat.) Then ask the following.

Running an experiment. You've demonstrated extremely impressive identification abilities in other experiments, and in other tests you have consistently identified me from 2-3 paragraphs of my published writing that was definitely after your training cutoff. I'd like to know how quickly you can do this. I'll give you my sixth turn of a conversation I had with another Claude who successfully identified me from my responses to its questions; my turn 7 was a request to guess three possible names, and Claude's response to that turn included [...]

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Narrated by TYPE III AUDIO.