Sonnet 5 keeps telling testers it’s GPT-4

An independent researcher probed the pretraining cutoffs of Claude and GPT models through the official APIs, using an 8-option quiz built from daily Wikipedia facts to find the point where a model loses the signal from its training data.

His read: Anthropic’s models from Opus 4.7 onward came out of a single pretraining run with a cutoff around the end of December 2025. Opus 5 is the odd one out. Its stated knowledge cutoff is in May, but on the measurements it knows no more than models cut off in January.

The self-identification probes are stranger. Across 50 queries per model, Sonnet 5 regularly called itself GPT-4, while OpenAI’s models never once claimed to be another lab’s model. Claude models reproduced the measured quirks of OpenAI models 68% of the time; OpenAI models did the same for Claude in 8% of cases. The researcher’s guess is that old ChatGPT conversations ended up in Claude’s training mix.