Decodability Is Not Computation: Examining Probing Claims

Kabir Singh Grewal


Abstract
High probe accuracy does not by itself establish that a language model computes and uses the decoded value. We examine this inference in a syntax-sensitive arithmetic testbed: Llama-3.1-8B often fails multiplication when an operand is wrapped in parentheses or an additive identity. In the primary operand band, an input-only linear baseline nearly matches the original hidden-state probe’s regression R2 ≈ 0.96. A classical variance decomposition gives an analytical headroom of only 0.031 for this baseline class. In wider bands, selection-separated reanalysis instead finds positive activation advantages, which increase with regularization tuning; joint input-and-activation probes also improve over the baseline. These predictive gains are distinct from exact integer recovery and causal use. In the primary band, tested answer-site steering and state transplants produce small average output shifts and no observed donor-answer emissions, whereas operand-site interventions produce large effects. Operand access is necessary, with a strong bottleneck at one attention layer. The causal tests do not cover the wider-band predictive gains. We advocate explicit input baselines, matched causal tests, and positive controls, with conclusions restricted to the distributions, representations, and interventions actually examined.
Anthology ID:
2026.blackboxnlp-1.14
Volume:
Proceedings of the 9th BlackboxNLP Workshop: Analyzing and Interpreting Neural Networks for NLP
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Dana Arad, Yonatan Belinkov, Hanjie Chen, Nils Feldhus, Aaron Mueller, Hosein Mohebbi, Gabriele Sarti, Martin Tutek
Venues:
BlackboxNLP | WS
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
201–219
Language:
URL:
https://aclanthology.org/2026.blackboxnlp-1.14/
DOI:
Bibkey:
Cite (ACL):
Kabir Singh Grewal. 2026. Decodability Is Not Computation: Examining Probing Claims. In Proceedings of the 9th BlackboxNLP Workshop: Analyzing and Interpreting Neural Networks for NLP, pages 201–219, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Decodability Is Not Computation: Examining Probing Claims (Grewal, BlackboxNLP 2026)
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PDF:
https://aclanthology.org/2026.blackboxnlp-1.14.pdf