A sparse LoRA made a backdoor small enough to ablate without breaking the model
AspireAI community · arXiv (Marek Masiak, Nicola Cancedda, Constantin Venhoff)
Fine-tuning on data of unknown origin can plant a backdoor that is hard to find and can survive safety training. Parameter-efficient methods capture the change but are not easy to interpret. TopKLoRA is a LoRA variant where only the k largest adapter latents are active for each token, trained purely on downstream loss - no sparse autoencoder in the loop - so optimisation pressure pushes the adapter toward functionally causal latents. In a sleeper-agent case study, the authors built model organisms by fine-tuning gemma-2-2b, Qwen2.5-1.5B and Qwen2.5-7B on a poisoned instruction-following dataset, then identified and ablated a circuit that was both necessary and sufficient to implement the backdoor. The sparse circuits came out up to an order of magnitude smaller than a dense LoRA baseline, so the malicious capability could be removed surgically without giving up instruction-following performance. The caveat worth keeping: this is a controlled backdoor the authors planted themselves, not a guarantee about backdoors that reach models through messier channels.
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Two sparse-autoencoder analyses have resisted scaling: matching semantically similar features across layers, and compressing large feature circuits into interpretable supernodes. This paper argues both are the same underlying problem, estimating semantic distances between SAE features that sit on different activation manifolds. Its representation treats each feature not as a single decoder vector but as an activation-weighted distribution over the hidden states expressing it, projects those distributions into a shared reference space, and compares them with Wasserstein distance. The representation is proved invariant to activation rescaling and stable under perturbation, and recovers true matches under finite-sample margin conditions. Empirically it outperformed decoder-vector and LLM-based baselines and captured subtle functional distinctions between related features, and it compressed large feature circuits into interpretable supernodes automatically. The limitation is that this is interpretability tooling: the paper demonstrates better feature matching and circuit compression, not that downstream interpretability claims become more accurate.