A harness change cut GitLab Duo Code Review's cost 98 percent, but the quality numbers come with a caveat
AspireAI community · AI21
AI21 reports measuring GitLab Duo Code Review's per-pull-request cost on Martian's Code Review Bench and finding that 98 percent came from the harness rather than the model: which model each stage runs on, and how much context each stage reads. Neither change works alone. Moving to an open-weight model on every stage cut 85 percent of cost but hurt quality, while context compaction alone saved 19 percent, and the full 98 percent only appeared once both were combined, because the open-weight model's deployment window decides which requests it can accept at all. The stated limitation is real: configurations were chosen and tuned on the same 50 pull requests they are scored on, quality is F1 on one public benchmark judged by two judges, and the figures measure token spend at list prices rather than what the infrastructure costs to serve.
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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.