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Decoding-length prediction benchmarks were measuring task hardness, not the predictor, and system error sets the floor

AspireAI community · arXiv
Predicting how many tokens a model will emit, from prefill-phase hidden states alone, is a core primitive for inference scheduling - but the paper shows current benchmarks confound predictor quality with task difficulty, so reported accuracy collapses when the target model, dataset or sampling configuration changes even though the predictor is untouched. The authors trace that instability to the inherent stochasticity of autoregressive decoding, model the response as a stopped stochastic process whose per-step stopping probability ramps up over hundreds of steps, and derive bounds every prefill-side method must obey, including an affine variance law. Empirically the resulting normal law reproduces per-prompt MAE and the central mass of the decode-length distribution to within a few percent, with the departures from normality matching a sampling-noise correction. Across MLP, EGTP, FiLM, soft-label and mixture-of-experts predictor families, performance converges close to the bound - which is the paper's real conclusion: system error dominates model error, so better predictors cannot fix this.
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