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AI21 Labs built LIFT-Bench after an instruction-following reward let a model answer a document request with one word and still score perfectly

AspireAI community · AI21 Labs
AI21 Labs released LIFT-Bench on October 25, a 784-task instruction-following benchmark whose constraints are attached to individual document sections rather than checked against a whole response, together with a 34,258-task LIFT-Train set and the open synthesis pipeline that generated both under Apache 2.0. The lab says it built the benchmark after a standard instruction-following reward produced a telling failure: a model asked for a history chapter answered with the single word "China" and received full reward, because the checkers verified rules without checking the task, and by step 390 the median answer had shrunk from 114 words to 18. AI21 reports that training on the LIFT reward instead kept median answers around 390 to 530 words with no high-scoring answer under 30 words, and the paper finds the benchmark is not saturated, with no model above 0.85 on any flat split. The limits are explicit: the tasks are synthetic document drafting, and the judged constraints use a Gemma-4-31b judge whose reported agreement with human labels is F1 0.73.
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