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The More It Says, the More You Pay: A Black-Box Audit of Provider-Side Token Inflation in LLM Services
arxiv.orgpaper

The More It Says, the More You Pay: A Black-Box Audit of Provider-Side Token Inflation in LLM Services

This arXiv paper defines Provider-Side Token Inflation Attacks (PTIA), where a dishonest LLM API provider covertly manipulates generation at the query, prompt, representation, or model level to inflate billed output tokens while preserving apparent task quality. The authors instantiate five such attacks (each pushing mean output length past 10.2x baseline), identify a "PTIA saturation" effect tied to suppressed end-of-sequence token probability, and build a lightweight single-probe black-box audit that needs no trusted reference model, achieving 85.1% detection with under 2% false positives across four open-weight models — and flags 7 of 15 real-world LLM API services as PTIA-consistent.

TL;DR A black-box audit technique catches LLM API providers covertly padding output tokens to inflate pay-per-token bills, flagging 7 of 15 real services tested.
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Organisations named here

  • PTIA saturation

Technologies named here

  • Open-weight LLMs
  • Real LLM API services
  • LLM

Techniques named here

  • Provider-Side Token Inflation Attack
  • PTIA saturation effect

Threat actors named here

  • Dishonest LLM service providers

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