New analysis says defaulting J-lens to the final layer injects a language bias

A researcher argues that J-lens, an interpretability tool that translates a model's internal states into words, is polluted by its default choice of the final layer as target.

The author, Kaley Brauer, checked 78 publicly released J-lenses and found 80% target the final layer. In her own tests on DeepSeek-V3, a final-layer target made about half of mid-depth readouts come out as mostly Chinese tokens; switching to the penultimate layer dropped that to 3%.

The cause, she says, is that DeepSeek-V3's last block strongly suppresses Chinese tokens on English text, and that amplified direction comes to dominate the lens. She recommends checking whether the final block injects a dominant direction when fitting or using a J-lens. The result is self-tested, covers only DeepSeek-V3, and awaits independent reproduction.