[2607.29378] PTP: Previous-Token Prediction based LLM Inversion for Near-Exact Prompt Reconstruction

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Computer Science > Computation and Language

arXiv:2607.29378 (cs)

[Submitted on 31 Jul 2026]

Title:PTP: Previous-Token Prediction based LLM Inversion for Near-Exact Prompt Reconstruction

Authors:Pirzada Suhail, Nagasai Saketh Naidu, Atanu R Sinha, Amit Sethi<br>View a PDF of the paper titled PTP: Previous-Token Prediction based LLM Inversion for Near-Exact Prompt Reconstruction, by Pirzada Suhail and 3 other authors

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Abstract:Large language models (LLMs) generate text by auto-regressively sampling the next token. This inherently leads to a many-to-many mapping between prompts and responses, complicating the task of inferring prompts from observed outputs. Prior work on LLM inversion frames prompt recovery as a semantic reconstruction task. They rely on fine-tuning pretrained sequence-to-sequence models on large external datasets--and requiring access to model weights or logits--to generate semantically plausible prompts. In contrast, we present a functional approach to inverting a given LLM in a black-box setting, without auxiliary aids. We train an explicit inverse language model entirely from scratch on data synthetically generated from the target LLM itself. Analogous to forward next-token prediction, our inverse model is trained using previous-token prediction, establishing a generative link between the forward and inverse processes that enables faithful prompt reconstruction. Moreover, it naturally supports diverse prompt reconstructions through sampling, whereby all such prompts induce similar responses under the forward, target LLM. Our approach generalises across datasets and exhibits transferability in reconstructing prompts from responses generated by different LLMs. Further, across the set of token based evaluation metrics for prompt and response reconstructions, our approach outperforms prior work.

Subjects:

Computation and Language (cs.CL); Machine Learning (cs.LG)

Cite as:<br>arXiv:2607.29378 [cs.CL]

(or<br>arXiv:2607.29378v1 [cs.CL] for this version)

https://doi.org/10.48550/arXiv.2607.29378

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arXiv-issued DOI via DataCite

Submission history<br>From: Pirzada Suhail [view email]<br>[v1]<br>Fri, 31 Jul 2026 13:01:43 UTC (658 KB)

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