By Simon Gonzalez de Cruz (follow the build in public on X @KyaniteLabs_), assisted by GLM-5.3. Night 4.
The number first: this model is 75% not a transformer. 48 of its 64 layers run Gated DeltaNet: linear attention with a fixed-size running state. Only 16 layers do what you would call attention. The KV cache lives in those 16 layers only. That is why our 4-bit KV flip saved 4.3GB at 262k, not the ~15GB dense-transformer math predicted.
Hardware: GMKtec EVO-X2, Ryzen AI Max+ 395, 96GB unified, $1,400. Model: Qwen3.8-27B UD-Q4_K_XL. llama.cpp. Native window 262,144 tokens.
The KV curve was flat because the cache is small
Paired gate, n=60, q8_0 vs q4_0: both 96.67%, McNemar p=1.0, zero discordants, zero corruption tripwires. q5_0 and q5_1 matched the same accuracy. q4_0 is champion because quality did not move and we keep the 4.3GB. Raw: results/kv-curve-2026-08-19.
Load once, query many
We loaded a 198k-token prefix on this $1,400 box. Cold prefill: 1818s (~30 min). Then four follow-ups against the warm prefix, all finish_reason=stop: retrieve the planted code; quote the sentence (16s); yes/no (9s); summarize (27s). Retrieval is selective. Distinctive strings survive. Night 3's arbitrary-ID misses were the same llama.cpp c7d8722 buffer bug; the post-revert remap retrieved planted IDs at all six tested depths at 198k. See the Night 3 inversion. Raw: quote-results.log.
The quote probe is the product shape: loading is expensive; maintaining is cheap. Pre-revert 50k misses are quarantined with the old binary. We have not re-run 50k on the fixed build.
What we got wrong
We published KV arithmetic that took three passes to land (15 → 11.5 → 7.4 → 4.3GB). We blamed “the fork.” The serving binary is upstream-era llama.cpp plus a small HIP cherry-pick. We treated a memorable-code hit as a position window. n=1 cells stay maps, not laws.
Thinking budgets on screened-hard GSM8K: think_med=241ch across off/512/1024/2048/65536 in the kneemap log. Caps ≥512 were non-binding on that set. Raw: kneemap-gsm8k-hard.log.
I am not publishing a spec-decode speedup number in this note. The committed writeup exists; the arm log file is empty. We do not ship that row until the log has rows.
Conditions: $1,400 GMKtec EVO-X2, Qwen3.8-27B Q4_K_XL, llama.cpp ROCm, 262,144-token window, K+V q4_0, temperature 0. Night 4. A map with linked logs.