DeepSeek-V4.1 Flash deep dive: pushing KV cache compression to the limit at 420 tok/s

teortaxesTex · x · 2026-09-17

A detailed architecture analysis of the DeepSeek-V4.1 Flash technical report, arguing it is substantial enough to be called "DeepSeek-V5 Flash".

Context: the model runs at nearly 420 tokens/s in practice, and DeepSeek took all V4 Pro models offline after release — signaling an architecture-level generational change, not a post-training iteration.

Key optimizations:

Prefill: borrowing from YOCO, the 40-layer model only passes 20 layers during prefill; activated params are 8B for prefill and 16B for decode;
KV cache compression: stacking GQA-style head compression, CSA-style block compression, new CSA2 cross-layer compression, plus optimized Sparse Attention indexer computation;
Precision: FP4 KV cache;
Motivation: long-horizon agent workflows keep growing context, and KV cache storage pressure in HBM/SSD was becoming a scaling bottleneck.

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DeepSeek-V4.1 Flash deep dive: pushing KV cache compression to the limit at 420 tok/s

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