Jacobian Lens (J-lens)

Anthropic's interpretability method for reading verbalizable content out of a model's residual stream: a corpus-averaged Jacobian from each layer to the final layer, composed with the unembedding, giving one vector per vocabulary token — a causal, principled correction to the logit lens that costs one matmul per layer and reads what the model is *poised to say* rather than what it happens to say