A 49MB sidecar closes 88.2% of the INT4-to-BF16 perplexity gap on Qwen2.5-3B.

Activation-Weighted Seeded Residual Coding encodes the residual between true and quantized weights using deterministic seed-generated bases, storing seed selectors, low-bit coefficients and scales instead of an explicit codebook, with activation statistics prioritizing the errors that move layer outputs (arXiv 2608.23144). Adding 0.162 bits per weight to an INT4 RTN backbone closed 88.2% of the perplexity gap, 78.9% of the KL gap and 71.3% of the accuracy gap to BF16. The sidecar is 0.8% of the BF16 weight payload and beat sparse, low-rank and vector-quantized codecs at matched size.