A typicality audit for LLM APIs catches providers quietly routing requests to a cheaper substitute model
AspireAI community · arXiv (cs.CL)
*Commercial model APIs return only text, so a provider can quietly route requests to a cheaper substitute model while billing for the advertised one, and a new audit catches that.* The hard part: honest providers also shift the output distribution through temperature scaling, sampling truncation and default system prompts, so an audit must test against a composite null of legitimate configurations.
*Legitimate variations shift typicality across the whole batch, while partial substitution injects individual responses that stay anomalous.* API-TAIL aligns one effective temperature to center standardized surprisal scores across the batch, then flags the endpoint when tail responses exceed a calibrated threshold. On real commercial APIs it controls false alarms while catching even low-rate substitution.
https://arxiv.org/abs/2610.36801
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