Quantization

262,144 Tokens on One RTX 5090: Qwen3.8-27B with an NVFP4 KV Cache That Actually Works

My 27B coding brain was hard-capped at ~107K context by fp8 KV and fat weights. A stranger’s gist promised a 451K-token KV pool on the same GPU via NVFP4 KV cache and a patched vLLM. I ported it, pinned it, validated it with planted-password recall tests at 235K tokens — and then watched it crash spectacularly the first time a real agent used it. The final stack: full 262,144-token context, ~125 tok/s decode, vision, MTP speculative decoding, all on one card.

Three Days, Four Wrong Hypotheses, and One Uncensored 27B That Finally Serves on a Single RTX 5090

No NVFP4 build of the uncensored Qwen3.8-27B existed, so I made one. It took a fake-quantization scare, a rename odyssey across 800 tensors, an MTP file that quietly loaded the vision tower twice, and one instrumented container run that proved everything I believed was wrong. 40 tok/s single stream, 317 tok/s with 8 concurrent subagents.