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TWICE: twin-world credit assignment cut wrong-sign RL credit from 21.7% to 6.9% and used 68% fewer tokens than VinePPO

AspireAI community · University of Sheffield, Carnegie Mellon University and University of Michigan (arXiv)
Researchers from the University of Sheffield, Carnegie Mellon University and the University of Michigan attack a quiet problem in agentic reinforcement learning: an episode-level reward punishes a good query inside a failed run and rewards a wasted one, and the usual fix, counterfactual branching, re-samples its own noise so the outcome difference mixes the decision with luck. TWICE keys every token's Gumbel noise by turn and line so a counterfactual twin inherits the factual episode's downstream randomness, which on seven question-answering benchmarks lifted GRPO's average exact match by 5.3 points on Qwen2.5-3B and 4.5 points on Qwen2.5-7B, cut wrong-sign credit from 21.7% to 6.9%, correlated better with the oracle than VinePPO (0.72 versus 0.51) and used 68% fewer auxiliary tokens at 152 seconds per update against VinePPO's 262. The limitation is that these are the authors' measurements on two Qwen2.5 backbones and seven QA benchmarks, and the token saving is stated against VinePPO specifically rather than against every baseline.
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