How To Use Decision Models for Evals

Decision models have exploded onto the scene recently, starting with TypeSafe’s Jev, followed by Perplexity’s Decider, Liquid’s d1, Cloudflare’s Clef, and OpenAI’s Decisions API1. These models promise fast, inexpensive classification. Decision models are an appealing low-cost alternative to LLM judges2 for automated evals. But whether they’re the right choice depends on the cost, performance, and latency requirements of your use case. We’ve also noticed people using the default decision threshold of 0.50 without checking whether it works well for their task. This can leave a lot of performance on the table. In this post, we’ll start by showing how to tune thresholds to get the best results from decision models. With those thresholds tuned, we’ll compare cost and performance to help you assess which model fits your use case. Finally, we’ll explore how to interpret model scores when deciding whether to act, and how calibration can make them more useful as confidence estimates. What we tested We used 100 traces, or logs of conversations and actions, from an apartment leasing agent. The task is to identify traces containing an error, such as a failed booking or a missed handoff to a human.3 We used the same dataset and broad failure-finding task in our prior post on automated evals. Here, each decision model returns a failure probability for each trace. We tested Jev 1.13, Perplexity Decider 1.1 27B, Liquid d1 and d1-ultra, Cloudflare Clef Flash, and OpenAI Decisions (GPT-6 Luna)1. Each model received the same evaluation criteria. The appendix shows the prompt and the structured version used for Jev. We compare models on two dimensions: Performance: We compare each model’s predictions with traces that people have reviewed and labeled as passing or failing. This measures how well the model catches failures while avoiding false alarms. Cost: We calculate API cost from token usage and the provider’s prices.4 For brevity, we leave latency aside because it depends on factors outside this experiment’s control.5 Understanding decision thresholds We assign each trace one of two labels, pass or fail. This is a binary pass/fail eval, which we recommend because it encourages clear criteria and is easier to align with human preferences. Each decision model returns a score from 0 to 1 that estimates how likely a trace is to contain a failure. The APIs turn this score into a decision using a default decision threshold of 0.50. Traces with scores at or above the threshold …