arXiv — NLP / Computation & Language · · 3 min read

Learning a Single Token to Replace Long System Prompts in LLMs

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Computer Science > Computation and Language

arXiv:2511.23271 (cs)
[Submitted on 28 Nov 2025 (v1), last revised 28 Aug 2026 (this version, v2)]

Title:Learning a Single Token to Replace Long System Prompts in LLMs

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Abstract:Long system prompts are widely used to steer Large Language Models (LLMs), but repeatedly processing them at inference time is inefficient and consumes valuable context budget. This motivates a central question: can the behavioral effect of a long system prompt be retained using only a minimal learned representation? To enable this, we propose a lightweight training framework that learns a single Behavior-Equivalent Token ([BE]). The framework first trains [BE] to encode the semantic content of the original system prompt via reconstruction, and then distills the prompt's downstream behavior into this single token. Importantly, our method requires no update to the pretrained LLM weights, no auxiliary compression models, and no labeled responses. Empirical evaluations on three datasets show that replacing long prompts with a single [BE] token yields up to a $3000\times$ prompt compression ratio, while retaining about 98% of the downstream performance of the original system prompts. This substantially reduces inference cost and frees nearly the entire context window for user inputs and model outputs.
Comments: 12 pages, 4 figures
Subjects: Computation and Language (cs.CL)
Cite as: arXiv:2511.23271 [cs.CL]
  (or arXiv:2511.23271v2 [cs.CL] for this version)
  https://doi.org/10.48550/arXiv.2511.23271
arXiv-issued DOI via DataCite

Submission history

From: Jiancheng Dong [view email]
[v1] Fri, 28 Nov 2025 15:22:52 UTC (6,625 KB)
[v2] Fri, 28 Aug 2026 06:59:02 UTC (4,612 KB)
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