Shorter but not Worse: Frugal Reasoning via Easy Samples as Length
  Regularizers in Math RLVR
Authors: name: Abdelaziz Bounhar, name: Hadi Abdine, name: Evan Dufraisse, name: Ahmad Chamma, name: Amr Mohamed, name: Dani Bouch, name: Michalis Vazirgiannis, name: Guokan Shang
Organization: MBZUAI-IFM Paris Lab

Large language models (LLMs) trained for step-by-step reasoning often become
excessively verbose, raising inference cost. Standard Reinforcement Learning
with Verifiable Rewards (RLVR) pipelines filter out ``easy'' problems for
training efficiency, leaving the model to train primarily on harder problems
that require longer reasoning chains. This skews the output length distribution
upward, resulting in a model that conflates ``thinking longer'' with
``thinking better''. In this work, we show that retaining and modestly
up-weighting moderately easy problems acts as an implicit length regularizer.
Exposing the model to solvable short-chain tasks constrains its output
distribution and prevents runaway verbosity. The result is
\emph{emergent brevity for free}: the model learns to solve harder
problems without inflating the output length,  despite the absence of
any explicit length penalization. RLVR experiments using this approach on
Qwen3-4B-Thinking-2507 (with a 16k token limit) achieve baseline
pass@1 AIME25 accuracy while generating solutions that are, on average, nearly
twice as short. The code is available at
https://github.com/MBZUAI-Paris/Frugal-AI{GitHub}, with datasets and
models on
https://huggingface.co/collections/MBZUAI-Paris/k2-think-mini-68dcfa8b114686a4bd3dc2bc{Hugging
Face}.