CHIMERA: Compact Synthetic Data for Generalizable LLM Reasoning
This work addresses the problem of scalable and reproducible reasoning data for LLM developers, offering a synthetic solution to overcome annotation bottlenecks and domain limitations, though it is incremental in improving data generation methods.
The paper tackles the challenge of enabling generalizable reasoning in LLMs by addressing data-centric bottlenecks like the cold-start problem and limited domain coverage, introducing CHIMERA, a compact synthetic dataset of 9K samples that spans 8 scientific disciplines. The result shows that post-training a 4B Qwen3 model with CHIMERA achieves strong performance on benchmarks like GPQA-Diamond and AIME, approaching or matching larger models such as DeepSeek-R1 and Qwen3-235B.
Large Language Models (LLMs) have recently exhibited remarkable reasoning capabilities, largely enabled by supervised fine-tuning (SFT)- and reinforcement learning (RL)-based post-training on high-quality reasoning data. However, reproducing and extending these capabilities in open and scalable settings is hindered by three fundamental data-centric challenges: (1) the cold-start problem, arising from the lack of seed datasets with detailed, long Chain-of-Thought (CoT) trajectories needed to initialize reasoning policies; (2) limited domain coverage, as most existing open-source reasoning datasets are concentrated in mathematics, with limited coverage of broader scientific disciplines; and (3) the annotation bottleneck, where the difficulty of frontier-level reasoning tasks makes reliable human annotation prohibitively expensive or infeasible. To address these challenges, we introduce CHIMERA, a compact synthetic reasoning dataset comprising 9K samples for generalizable cross-domain reasoning. CHIMERA is constructed with three key properties: (1) it provides rich, long CoT reasoning trajectories synthesized by state-of-the-art reasoning models; (2) it has broad and structured coverage, spanning 8 major scientific disciplines and over 1K fine-grained topics organized via a model-generated hierarchical taxonomy; and (3) it employs a fully automated, scalable evaluation pipeline that uses strong reasoning models to cross-validate both problem validity and answer correctness. We use CHIMERA to post-train a 4B Qwen3 model. Despite the dataset's modest size, the resulting model achieves strong performance on a suite of challenging reasoning benchmarks, including GPQA-Diamond, AIME 24/25/26, HMMT 25, and Humanity's Last Exam, approaching or matching the reasoning performance of substantially larger models such as DeepSeek-R1 and Qwen3-235B.