AttnTrace: Attention-based Context Traceback for Long-Context LLMs
Authors: name: Yanting Wang, name: Runpeng Geng, name: Ying Chen, name: Jinyuan Jia

Long-context large language models (LLMs), such as Gemini-2.5-Pro and
Claude-Sonnet-4, are increasingly used to empower advanced AI systems,
including retrieval-augmented generation (RAG) pipelines and autonomous agents.
In these systems, an LLM receives an instruction along with a context--often
consisting of texts retrieved from a knowledge database or memory--and
generates a response that is contextually grounded by following the
instruction. Recent studies have designed solutions to trace back to a subset
of texts in the context that contributes most to the response generated by the
LLM. These solutions have numerous real-world applications, including
performing post-attack forensic analysis and improving the interpretability and
trustworthiness of LLM outputs. While significant efforts have been made,
state-of-the-art solutions such as TracLLM often lead to a high computation
cost, e.g., it takes TracLLM hundreds of seconds to perform traceback for a
single response-context pair. In this work, we propose AttnTrace, a new context
traceback method based on the attention weights produced by an LLM for a
prompt. To effectively utilize attention weights, we introduce two techniques
designed to enhance the effectiveness of AttnTrace, and we provide theoretical
insights for our design choice. We also perform a systematic evaluation for
AttnTrace. The results demonstrate that AttnTrace is more accurate and
efficient than existing state-of-the-art context traceback methods. We also
show that AttnTrace can improve state-of-the-art methods in detecting prompt
injection under long contexts through the attribution-before-detection
paradigm. As a real-world application, we demonstrate that AttnTrace can
effectively pinpoint injected instructions in a paper designed to manipulate
LLM-generated reviews. The code is at
https://github.com/Wang-Yanting/AttnTrace.