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arXiv · 2605.10544

Where Does Long-Context Supervision Actually Go? Effective-Context Exposure Balancing

Jinchang Zhu·Jindong Li·Chengyu Zou·Rong Fu·Chao Wang·Haowei He·Menglin Yang
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Abstract

Long-context adaptation is often viewed as window scaling, but this misses a token-level supervision mismatch: in packed training with document masking, each target token's effective context remains short. We introduce EXACT, a supervision-allocation objective that assigns extra weight to long effective-context targets by inverse frequency within the long tail. Across seven Qwen/LLaMA CPT configurations, EXACT improves all 28 trained/extrapolated NoLiMa and RULER comparisons. On Qwen2.5-0.5B, NoLiMa improves by +10.09 (trained) and +5.34 (extrapolated); RULER by +10.69 and +5.55. On LLaMA-3.2-3B, RULER improves by +17.91 and +16.11. Standard QA/reasoning are preserved (+0.24 macro change across six benchmarks). A distance-resolved probe shows gains arise when evidence is thousands of tokens away, while short cases remain unchanged. Results support a supervision-centric thesis: long-context adaptation depends on how strongly training supervises long-context predictions.

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Jinchang Zhu, Jindong Li, Chengyu Zou, Rong Fu, Chao Wang, Haowei He, Menglin Yang. 2026-08-29. Where Does Long-Context Supervision Actually Go? Effective-Context Exposure Balancing. https://arxiv.org/abs/2605.10544

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