BatchParScale: Sparse Batch-Mixing for Sequence Models

Abstract

Recent test-time scaling methods improve model performance by increasing inference-time compute. One practical form of this is parallel scaling, where a model samples multiple independent reasoning streams and aggregates their answers using methods like voting, reranking, or verification. While effective, this approach leaves the streams uncoordinated, and a discovery made in one rollout cannot influence any other. Multi-Sequence Verifiers jointly score candidate answers after generation, but the reasoning streams remain independent during inference. ParScale takes a step toward model-internal parallel scaling by learning to aggregate multiple forward passes, but its streams still do not exchange information while processing a sequence. We introduce BatchParScale, an attention mechanism that allows parallel streams to exchange information through a small set of communication positions. We evaluate BatchParScale on a new Parallel Needle-in-a-Haystack retrieval benchmark and in pretraining, fine-tuning, and cooperative multi-agent reinforcement learning. Across these settings, BatchParScale outperforms ParScale on language-model loss and remains competitive on coordination-sensitive MARL tasks. Masked pretraining runs beat an unmasked single-sequence baseline despite local sequence corruption, and LoRA fine-tuning shows the mechanism can be added to a pretrained Transformer. At the same time, fact retrieval performance for pretrained and finetuned BatchParScale models remains unclear.