PRISM: Prompt Representation Internalization via Selective Compression into LoRA Weights

Eshita Jha, Mingxian Wang, Andrei Barbu, Chetan Aggarwal


Abstract
Deploying large language models for specialized tasks requires lengthy prompts combining schema, demonstrations, and instructions — creating a fundamental tension between prompt richness and inference feasibility. We identify and characterize two distinct prompt regimes: pattern-dominant tasks, where demonstrations drive performance through implicit formatting and extraction conventions, and procedure-dominant tasks, where explicit instructions carry the critical signal. To surface this distinction, we design a two-inference diagnostic that compares model performance with instructions alone versus examples alone, revealing which component to target for compression. Guided by this diagnostic, we introduce PRISM, a self-distillation framework that generates silver training data from the model’s own full-prompt outputs and trains lightweight LoRA adapters to internalize the dominant component’s behavior. PRISM recovers 131% of example benefit on CUAD and 151% on MultiWOZ (pattern-dominant), and 39% of instruction benefit on GSM8K (procedure-dominant), eliminating 72–79% of prompt tokens at inference time. Silver data captures 80–94% of gold-labeled performance without human annotations. Beyond compression, PRISM frees context capacity: replacing a long prompt with LoRA weights enables 89.5% compliance with new runtime instructions versus 75.4% with recency prompting. The diagnostic suggests a practical pre-training heuristic to indicate when distillation is likely to succeed, helping practitioners avoid costly training on non-beneficial components.
Anthology ID:
2026.emnlp-main.2431
Volume:
Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Hung-Yi Lee, Sunipa Dev, Jessy Li, Desmond Elliott
Venue:
EMNLP
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
Note:
Pages:
55189–55201
Language:
URL:
https://aclanthology.org/2026.emnlp-main.2431/
DOI:
Bibkey:
Cite (ACL):
Eshita Jha, Mingxian Wang, Andrei Barbu, and Chetan Aggarwal. 2026. PRISM: Prompt Representation Internalization via Selective Compression into LoRA Weights. In Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing, pages 55189–55201, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
PRISM: Prompt Representation Internalization via Selective Compression into LoRA Weights (Jha et al., EMNLP 2026)
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PDF:
https://aclanthology.org/2026.emnlp-main.2431.pdf