  Large language models (LLMs) are great at processing multiple natural
language processing tasks, but their abilities are constrained by inferior
performance with long context, slow inference speed, and the high cost of
computing the results. Deploying LLMs with precise and informative context
helps users process large-scale datasets more effectively and cost-efficiently.
Existing works rely on compressing long prompt contexts into soft prompts.
However, soft prompt compression encounters limitations in transferability
across different LLMs, especially API-based LLMs. To this end, this work aims
to compress lengthy prompts in the form of natural language with LLM
transferability. This poses two challenges: (i) Natural Language (NL) prompts
are incompatible with back-propagation, and (ii) NL prompts lack flexibility in
imposing length constraints. In this work, we propose a Natural Language Prompt
Encapsulation (Nano-Capsulator) framework compressing original prompts into NL
formatted Capsule Prompt while maintaining the prompt utility and
transferability. Specifically, to tackle the first challenge, the
Nano-Capsulator is optimized by a reward function that interacts with the
proposed semantics preserving loss. To address the second question, the
Nano-Capsulator is optimized by a reward function featuring length constraints.
Experimental results demonstrate that the Capsule Prompt can reduce 81.4% of
the original length, decrease inference latency up to 4.5x, and save 80.1% of
budget overheads while providing transferability across diverse LLMs and
different datasets.
