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KBF: Knowledge Boundary as Fingerprint for Language Model and Black-Box API Auditing

arXiv Security Archived May 29, 2026 ✓ Full text saved

arXiv:2605.29524v1 Announce Type: new Abstract: Relay and reseller APIs increasingly intermediate access to large language models (LLMs), but users have no direct way to verify that a claimed endpoint is actually serving the advertised model. We introduce KBF, a low-cost black-box auditing protocol that fingerprints model APIs using stable numerical recall near the knowledge boundary. Across 16 production LLM endpoints, KBF flags all 155 economically relevant substitutions without rejecting any

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✦ AI Summary · Claude Sonnet

Computer Science > Cryptography and Security [Submitted on 28 May 2026] KBF: Knowledge Boundary as Fingerprint for Language Model and Black-Box API Auditing Yijia Fang, Yiqing Feng, Bingyu Li, Mingxun Zhou Relay and reseller APIs increasingly intermediate access to large language models (LLMs), but users have no direct way to verify that a claimed endpoint is actually serving the advertised model. We introduce KBF, a low-cost black-box auditing protocol that fingerprints model APIs using stable numerical recall near the knowledge boundary. Across 16 production LLM endpoints, KBF flags all 155 economically relevant substitutions without rejecting any same-model controls, remains stable under deployment variation, detects high-separation mixed-routing attacks when only 5-10% of traffic is substituted, and finds that 7 of 27 platform model cells in a six-platform shadow API audit are statistically inconsistent with their reference endpoints, with inconsistencies concentrated on premium Claude endpoints. Comments: 20 pages, 13 figures Subjects: Cryptography and Security (cs.CR); Artificial Intelligence (cs.AI) Cite as: arXiv:2605.29524 [cs.CR] (or arXiv:2605.29524v1 [cs.CR] for this version) https://doi.org/10.48550/arXiv.2605.29524 Focus to learn more Submission history From: Yijia Fang [view email] [v1] Thu, 28 May 2026 07:40:24 UTC (208 KB) Access Paper: HTML (experimental) view license Current browse context: cs.CR < prev | next > new | recent | 2026-05 Change to browse by: cs cs.AI References & Citations NASA ADS Google Scholar Semantic Scholar Export BibTeX Citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer (What is the Explorer?) Connected Papers Toggle Connected Papers (What is Connected Papers?) Litmaps Toggle Litmaps (What is Litmaps?) scite.ai Toggle scite Smart Citations (What are Smart Citations?) Code, Data, Media Demos Related Papers About arXivLabs Which authors of this paper are endorsers? | Disable MathJax (What is MathJax?)

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