[Submitted on 6 Oct 2026]
Title:Coverage, Not Difficulty, Sets How Much Synthetic Data an Activation Probe Needs
View a PDF of the paper titled Coverage, Not Difficulty, Sets How Much Synthetic Data an Activation Probe Needs, by Ankush Checkervarty
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Abstract:Activation probes that monitor deployed language models are trained on synthetic conversations, and how many a probe needs is open. We trace learning curves over 10-590 synthetic samples for three monitoring concepts, high-stakes situations, replies harmful to a person, and replies that do not follow the user's instruction, on fourteen held-out evaluation distributions and four probe models, varying the generator LLM and the prompt's detail. The need is set by what is monitored: probes for high-stakes and harmful are within a few hundredths of their plateau from 80 samples on Gemma-3-27B-IT, instruction probes need several times as many, and the ordering holds on three smaller probe models and on real samples (from dev set). Prior work advises spending a generation budget on breadth, more kinds of data, over depth, more of each kind. We read the depth a concept needs as the half-gain size of a fitted curve, the number of samples at which half the gain is in hand. Concept and distribution account for 42-45% of its variance, the generator, probe model, and prompt detail for under 10%. What sets the value of the half-gain size is coverage, not per-kind difficulty: the number of samples of its own kind a distribution needs to saturate. Every kind, one per evaluation distribution, has a median half-gain size of 7-11 own-kind synthetic samples under all three concepts alike. What differs is how far samples of one kind transfer to the concept's other kinds, almost fully under high-stakes, less under harmful, and least under instruction, which accounts for most of the gap between concepts on generated and real samples. Breadth therefore pays differently by concept: many kinds are necessary under instruction, where no kind covers another, and nearly redundant under high-stakes, where one kind covers the rest. We release the evaluation suites, dev sets, and generated sets.
Comments: 24 pages, 8 figures, 11 tables. Code and data: this https URL
Subjects:
Machine Learning (cs.LG)
Cite as: arXiv:2610.10594 [cs.LG]
(or arXiv:2610.10594v1 [cs.LG] for this version)
https://doi.org/10.48550/arXiv.2610.10594
arXiv-issued DOI via DataCite
Submission history
From: Ankush Checkervarty [view email] [v1] Tue, 6 Oct 2026 18:49:40 UTC (52 KB)
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